Introduction to Phytoremediation

Introduction to Phytoremediation
EPA/600/R-99/107
February 2000
Introduction to Phytoremediation
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
i
Notice
The EPA through its ORD produced this document. It has been subjected to the Agency’s peer
and administrative review and has been approved for publication as an EPA document. Mention
of trade names or commercial products does not constitute endorsement or recommendation
for use.
ii
Foreword
The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation’s
land, air, and water resources. Under a mandate of national environmental laws, the Agency
strives to formulate and implement actions leading to a compatible balance between human
activities and the ability of natural systems to support and nurture life. To meet this mandate,
EPA’s research program is providing data and technical support for solving environmental problems today and building a science knowledge base necessary to manage our ecological resources wisely, understand how pollutants affect our health, and prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency’s center for investigation of
technicological and management approaches for reducing risks from threats to human health
and the environment. The focus of the Laboratory’s research program is on methods for the
prevention and control of pollution to air, land, water and subsurface resources; protection of
water quality in public water systems; remediation of contaminated sites and ground water; and
prevention and control of indoor air pollution. The goal of this research effort is to catalyze
development and implementation of innovative, cost-effective environmental technologies; develop scientific and engineering information needed by EPA to support regulatory and policy
decisions; and provide technical support and information transfer to ensure effective implementation of environmental regulations and strategies.
This publication has been produced as part of the Laboratory’s strategic long-term research
plan. It is published and made available by EPA’s Office of Research and Development to assist
the user community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
iii
Abstract
Phytoremediation is the name given to a set of technologies that use different plants as a
containment, destruction, or an extraction technique. Phytoremediation as a remediation technology that has been receiving attention lately as the results from field trials indicate a cost
savings compared to conventional treatments.
The U.S. EPA has a dual role in which it seeks to protect human health and the environment
associated with hazardous waste sites, while encouraging development of innovative technologies that might more efficiently clean up these sites.
This Introduction is intended to provide a tool for site regulators, owners, neighbors, and managers to evaluate the applicability of phytoremediation to a site. This document defines terms
and provides a framework to understand phytoremediation applications. It is a compilation of
research and remediation work that has been done to date. The format is intended to be accessible to EPA RPMs, state regulators, and others who need to choose between alternate technologies, as well for site owners, consultants, contractors, and students who are interested in
basic information. It is not a design manual, and is not intended to provide enough information
to choose, engineer, and install a phytoremediation application.
This work may also be used to help guide research, development, and regulation. Areas of
needed research have been identified. By compiling the published and unpublished work, research repetition can be avoided, and areas of opportunity that need attention should be clear.
iv
Table of Contents
Foreword .................................................................................................................................. iii
Acknowledgements ..................................................................................................................xi
Chapter 1
Introduction ........................................................................................................... 1
1.1 Objectives ...................................................................................................... 1
1.2 Approach ........................................................................................................ 1
1.3 Report Organization ....................................................................................... 1
Chapter 2
Overview of Phytoremediation ............................................................................... 3
2.1
2.2
2.3
2.4
2.5
2.6
Chapter 3
Evaluation of Phytoremediation Technologies ...................................................... 14
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
Chapter 4
Phytoextraction ............................................................................................. 14
Rhizofiltration ................................................................................................ 18
Phytostabilization .......................................................................................... 21
Rhizodegradation .......................................................................................... 23
Phytodegradation .......................................................................................... 28
Phytovolatilization ......................................................................................... 31
Hydraulic Control ........................................................................................... 34
Vegetative Cover Systems ............................................................................ 35
Riparian Corridors/Buffer Strips ..................................................................... 39
Phytoremediation System Selection and Design Considerations ......................... 41
4.1
4.2
4.3
4.4
4.5
4.6
Chapter 5
Background ..................................................................................................... 3
Technical Considerations ................................................................................ 7
Economic Considerations ............................................................................... 7
Regulatory Considerations .............................................................................. 9
Ecosystem Restoration ................................................................................. 10
Current Research .......................................................................................... 12
Contaminated Media Considerations .............................................................
Contaminant Considerations .........................................................................
Plant Considerations .....................................................................................
Site Considerations .......................................................................................
Treatment Trains ............................................................................................
Additional Information Sources .....................................................................
42
43
44
48
51
51
Remedial Objectives, Treatability, and Evaluation ................................................ 52
5.1 Remedial Objectives ..................................................................................... 52
5.2 Treatability Studies ....................................................................................... 53
5.3 Monitoring for Performance Evaluation .......................................................... 56
v
Contents (continued)
Chapter 6
Case Studies ...................................................................................................... 58
6.1 Edgewood Area J-Field Toxic Pits Site Aberdeen Proving Grounds
Edgewood, Maryland ..................................................................................... 58
6.2 Carswell Site Fort Worth, Texas .................................................................... 60
6.3 Edward Sears Properties Site New Gretna, New Jersey ............................... 64
6.4 Bioengineering Management: U.S. Nuclear Regulatory Commission
Beltsville, MD ............................................................................................... 67
6.5 Lakeside Reclamation Landfill Beaverton, Oregon ...................................... 68
6.6 Alternative Landfill Cover Demonstration Sandia National Laboratories
Albuquerque, NM .......................................................................................... 70
Appendix A
Appendix B
Appendix C
Appendix D
Glossary ............................................................................................................. A-1
Phytoremediation Database ................................................................................ B-1
References ......................................................................................................... C-1
Common and Scientific Names of Referenced Plants ........................................ D-1
vi
Figures
Figure 2-1.
Figure 2-2.
Figure 3-1.
Figure 3-2.
Figure 3-3.
Figure 3-4.
Figure 3-5.
Figure 3-6.
Figure 3-7.
Figure 6-1.
Figure 6-2.
Figure 6-3.
Figure 6-4.
Mechanisms for phytoremediation. ........................................................................ 4
Example root depths. ............................................................................................ 6
Phytoextraction. .................................................................................................. 16
Rhizodegradation. ................................................................................................ 24
Phytodegradation. ................................................................................................ 29
Phytovolatilization. .............................................................................................. 32
Hydraulic control of contaminated plume. ............................................................ 34
Illustration of an Evapotranspiration (ET) cover. .................................................. 36
Phytoremediation cover evolution. ....................................................................... 37
Experimental Design ........................................................................................... 61
TCE Concentrations ............................................................................................ 63
Site Map (Edward Sears Site) ............................................................................. 65
Sampling Grid (Edward Sears Site) ..................................................................... 66
vii
Tables
Table 2-1.
Table 2-2.
Table 2-3.
Table 3-1.
Table 4-1.
Table 4-2.
Table 5-1.
Table 5-2.
Table 5-3.
Table 5-4.
Table 6-1.
Table 6-3.
Table 6-2.
Table 6-4.
Table 6-5.
Table 6-6.
Table 6.7
Phytoremediation Applications .............................................................................. 5
Root Depth for Selected Phytoremediation Plants ................................................. 6
Phytoremediation at Superfund Sites .................................................................... 8
Phytoremediation Overview ................................................................................. 15
Phytoremediation Technologies Applicable to Different Contaminant Types1,2 ............ 43
Plant Selection Process ...................................................................................... 45
Summary of Phytoremediation Technologies and Method of Contaminant Control 52
Experimental Factors for Testing in Treatability Studies ....................................... 54
Information Needed for a Pilot Treatability Study ................................................. 55
Summary of Monitoring Parameters .................................................................... 57
Monitoring Approaches at the J-Field Site. .......................................................... 59
Estimated Cost of Phytoremediation at the Carswell Site. ................................... 63
Average Concentrations of TCE, cis-DCE, and trans-DCE at Carswell Site. ........ 63
Design Type and Completion Dates for the Experimental Covers. ....................... 67
Summary of Run-off, Evapotranspiration, and Deep Percolation From the
Bioengineered Plots. ........................................................................................... 68
Summary of Percolation and Precipitation Rates From
May 1997 Through March 1998 for the Six Cover Designs. ................................. 72
Construction Costs for the Final Landfill Covers .................................................. 72
viii
Acronyms
AAP
ACAP
ALCD
ANOVA
APG
ARARs
ASTM
BTEX
CERCLA
Co
DEPH
DNAPL
DOD
DOE
EPA
ERT
FFDCA
FIFRA
GC/MS
HCB
Kow
LNAPL
MCL
NPDES
NPL
NRMRL
OSC
ORD
OSWER
PAH
PCB
PCE
PCP
PRP
PVC
QA/QC
RCRA
RD
RPM
ROD
RTDF
SITE
TCA
TCAA
TCE
TIO
TNT
Army Ammunition Plant
Alternative Cover Assessment Program (U.S. EPA)
Alternative Landfill Cover Demonstration
Analysis of Variance
Aberdeen Proving Grounds
Applicable or Relevant and Appropriate Requirements
American Society for Testing and Materials
Benzene, Toluene, Ethylbenzene, Xylenes
Comprehensive Environmental Response, Compensation, and Liability Act
Original Concentration
Diethylhexylphthalate
Dense Nonaqueous Phase Liquid
Department of Defense
Department of Energy
Environmental Protection Agency
U.S. EPA Emergency Response Team
Federal Food, Drug, and Cosmetic Act
Federal Insecticide, Fungicide, and Rodenticide Act
Gas Chromatography/Mass Spectroscopy
Hexachlorobenzene
octanol-water partition coefficient
Light Nonaqueous Phase Liquid
Maximum Contaminant Level
National Pollutant Discharge Elimination System
National Priority List (Superfund)
National Risk Management Research Laboratory
On-Scene Coordinator
U.S. EPA Office of Research and Development
U.S. EPA Office of Solid Waste and Emergency Response
Polynuclear Aromatic Hydrocarbons
Polychlorinated Biphenyls
Perchloroethylene, tetrachloroethene
Pentachlorophenol
Potentially Responsible Party
Polyvinyl Chloride
Quality Assurance/Quality Control
Resource Conservation and Recovery Act
Remedial Design
Remedial Project Manager
Record of Decision
Remediation Technologies Development Forum
Superfund Innovative Technology Evaluation Program (EPA)
Tetrachloroethane
Trichloroacetic acid
Trichloroethylene
U.S. EPA Technology Innovation Office
Trinitrotoluene
ix
Acronyms (continued)
TPH
TSCA
USDA
UXO
VOC
Total Petroleum Hydrocarbons
Toxic Substances Control Act
U.S. Department of Agriculture
Unexploded Ordinance
Volatile Organic Compounds
x
Acknowledgements
This Introduction to Phytoremediation was written by an EPA Team consisting of Nancy Adams,
Dawn Carroll, Kelly Madalinski, Steve Rock, and Tom Wilson as well as Bruce Pivetz of ManTech
Environmental Research Services Corporation. Additional assistance was also provided by Todd
Anderson, Jon Chappell, Scott Huling, Jessica Palmiotti, and Phil Sayre. The team would like to
thank Ed Barth, Michelle Laur, Ken Lovelace, Andrea McLaughlin, Linda Fiedler, Susan Thornloe,
and Albert Venosa for their extensive review.
xi
Chapter 1
Introduction
Phytoremediation is an emerging technology that uses
various plants to degrade, extract, contain, or immobilize
contaminants from soil and water. This technology has been
receiving attention lately as an innovative, cost-effective
alternative to the more established treatment methods used
at hazardous waste sites.
• Provide a detailed bibliography of additional resources for
those interested in learning more about phytoremediation.
• Provide access to general information on various resource applications. However, it should be noted that
this document is not a design manual and is not intended to provide enough information to engineer and
install any phytoremediation application.
The U.S. Environmental Protection Agency (EPA) seeks
to protect human health and the environment from risks
associated with hazardous waste sites, while encouraging
development of innovative technologies such as
phytoremediation to more efficiently clean up these sites.
• Provide a guide for research, development, and regulation, and identify areas of needed research. Through
the compilation of published and unpublished work, research repetition can be avoided, and areas of opportunity that need attention should be clear.
This document reports the results of phytoremediation
efforts as originally reported by researchers. No attempts
were made to validate data obtained from the literature.
1.2 Approach
1.1 Objectives
The following approach was used to compile and summarize information on phytoremediation processes:
The objectives of this report are as follows:
• Conduct a comprehensive literature search.
• Provide an educational tool for site regulators, owners,
neighbors, and managers to evaluate the applicability
of phytoremediation to a site. Phytoremediation projects
have been proposed or applied to ecosystem restoration and soil, surface water, groundwater, and sediment
remediation. This document identifies and defines
phytoremediation technologies, and provides a guide
to current research to aid in evaluation of proposed
phytoremediation applications.
• Contact contractors and researchers to obtain information on phytoremediation applications and cost.
• Review and evaluate existing research and field applications of current phytoremediation projects.
• Assemble a compilation of research and remedial work
that has been performed to date.
• Use the resources of the Internet to both gather and
disseminate information. The creators of this document
have written their sections so that they can be regularly
updated to keep them relevant as the technology
changes. This document may be accessed on the
Internet “www.clu-in.org”.
• Develop a format that is accessible to EPA and state
regulators and others who need to evaluate alternate
remedial technologies, as well as to site owners, project
managers, consultants, contractors, and students who
are interested in basic information.
• Evaluate the various phytoremediation processes (e.g.,
phytodegradation, rhizofiltration, hydraulic control).
1.3 Report Organization
This report has been designed to provide quick access to
information on the various phytoremediation processes and
associated information as follows:
• Present phytoremediation system characteristics that
site managers and others might find useful in assessing the potential applicability of phytoremediation to a
specific site.
• Chapter 2 provides an overview of phytoremediation including applications, limits, cost information, and regulatory concerns. Ecosystem restoration as it applies to
phytoremediation processes is also discussed.
• Present case studies illustrating field applications of
phytoremediation.
1
• Chapter 5 presents the remedial objectives for
phytoremediation as well as associated monitoring
needed to evaluate system performance. Information
on conducting treatability studies is also included in
this chapter.
• Chapter 3 provides a literature review and evaluation of
the major phytoremediation processes. This chapter is
divided into subsections that present definitions, mechanisms, site characteristics, applicable media, contaminants amenable to each phytoremediation process, and
the associated concentrations where available. The
advantages, disadvantages, and current status of each
process are also discussed. Finally, an annotated reference list is included at the end of the discussion of
each phytoremediation process to provide more detailed,
specific information. The purpose of this chapter is to
provide site managers with an overview of the various
phytoremediation processes as well as what can be
expected from each process and its limitations.
• Chapter 6 presents six case studies where
phytoremediation has been applied. The six case studies presented in this chapter illustrate specific field applications of phytoremediation. This chapter includes
site descriptions, design considerations, monitoring recommendations, status, and costs of various
phytoremediation processes.
• Included as appendices are a glossary of
phytoremediation terms, references, common and scientific names of referenced plants, and part of the database that provides information on phytoremediation
projects available on the Internet or through EPA.
• Chapter 4 discusses considerations involved in the selection, design, and implementation of phytoremediation
systems. It presents information that will help a site
manager to identify whether phytoremediation may be
appropriate for a site and to select a particular
phytoremediation technology, based on the conditions
occurring at, or applicable to, a site. This chapter introduces issues and concepts that should be considered
in the design and implementation of a phytoremediation
system.
Please note that because phytoremediation is an emerging technology, standard performance criteria for
phytoremediation systems have not been developed. Data
are being gathered and assessed to develop performance
measures that can be used to predict the function and efficacy of an individual system.
2
Chapter 2
Overview of Phytoremediation
2.1 Background
control of runoff, erosion, and infiltration by vegetative covers. A brief explanation of these application categories follows, with more detailed explanations in following chapters.
Phytoremediation is the name given to a set of technologies that use plants to clean contaminated sites. Many techniques and applications have been called phytoremediation,
possibly leading to confusion. This document uses the term
phytoremediation to refer to a set of plant-contaminant interactions, and not to any specific application. Many of the
phytoremediation techniques involve applying information that
has been known for years in agriculture, silviculture, and
horticulture to environmental problems.
2.1.1.1
Plants may enhance degradation in the rhizosphere (root
zone of influence). Microbial counts in rhizosphere soils can
be 1 or 2 orders of magnitude greater than in nonrhizosphere
soils. It is not known whether this is due to microbial or fungal
symbiosis with the plant, plant exudates including enzymes,
or other physical/chemical effects in the root zone. There are,
however, measurable effects on certain contaminants in the
root zone of planted areas. Several projects examine the interaction between plants and such contaminants as trinitrotoluene (TNT), total petroleum hydrocarbons (TPH), pentachlorophenol (PCP), and polynuclear aromatic hydrocarbons (PAH).
The term phytoremediation (phyto = plant and remediation
= correct evil) is relatively new, coined in 1991. Basic information for what is now called phytoremediation comes from
a variety of research areas including constructed wetlands,
oil spills, and agricultural plant accumulation of heavy metals. The term has been used widely since its inception, with
a variety of specific meanings. In this document
phytoremediation is used to mean the overall idea of using
plant-based environmental technologies, not any specific
application.
Another possible mechanism for contaminant degradation
is metabolism within the plant. Some plants may be able to
take in toxic compounds and in the process of metabolizing
the available nutrients, detoxify them. Trichloroethylene (TCE)
is possibly degraded in poplar trees and the carbon used for
tissue growth while the chloride is expelled through the roots.
EPA has three projects underway in the field using populus
species to remediate TCE. Tests at the University of Washington are being developed to verify this degradation mechanism under controlled conditions.
Research efforts into remediation can be roughly categorized into two sets: exploration of mechanisms and evaluation
of claims. Mechanism work has centered on finding theoretical limits, and explanations for results observed in the field.
Pilot-scale field work has both preceded and followed explanatory laboratory research, and early successes have piqued
interest. Long-term, objective field evaluation is critical to understanding how well phytoremediation may work, what the
real cost of application will be, and how to build models to
predict the interaction between plants and contaminants. Most
of the projects are ongoing and thus provide only preliminary
data.
2.1.1
Degradation
2.1.1.2
Extraction
Phytoextraction, or phytomining, is the process of planting
a crop of a species that is known to accumulate contaminants in the shoots and leaves of the plants, and then harvesting the crop and removing the contaminant from the site.
Unlike the destructive degradation mechanisms, this technique yields a mass of plant and contaminant (typically metals) that must be transported for disposal or recycling. This is
a concentration technology that leaves a much smaller mass
to be disposed of when compared to excavation and landfilling.
This technology is being evaluated in a Superfund Innovative
Technology Evaluation (SITE) demonstration, and may also
be a technology amenable to contaminant recovery and recycling.
Applications
Phytoremediation applications (as shown in Figure 2-1 and
Table 2-1) can be classified based on the contaminant fate:
degradation, extraction, containment, or a combination of these.
Phytoremediation applications can also be classified based
on the mechanisms involved. Such mechanisms include extraction of contaminants from soil or groundwater; concentration of contaminants in plant tissue; degradation of contaminants by various biotic or abiotic processes; volatilization or
transpiration of volatile contaminants from plants to the air;
immobilization of contaminants in the root zone; hydraulic
control of contaminated groundwater (plume control); and
Rhizofiltration is similar to phytoextraction in that it
is also a concentration technology. It differs from phytoextraction
3
Figure 2-1. Mechanisms for phytoremediation.
2.1.1.3
in that the mechanism is root accumulation and harvest
using hydroponic (soil-less) growing techniques. This is useful for separating metal contaminants from water.
Rhizofiltration has been demonstrated on U.S. Department
of Energy (DOE) sites for radionuclides.
Containment and Immobilization
Containment using plants either binds the contaminants
to the soil, renders them nonbioavailable, or immobilizes
them by removing the means of transport.
Physical containment of contaminants by plants can take
the form of binding the contaminants within a humic molecule (humification), physical sequestration of metals as
occurs in some wetlands, or by root accumulation in
nonharvestable plants. Certain trees sequester large concentrations of metals in their roots, and although harvesting
and removal is difficult or impractical, the contaminants
present a reduced human or environmental risk while they
are bound in the roots.
Volatilization or transpiration through plants into the atmosphere is another possible mechanism for removing a
contaminant from the soil or water of a site. It is often raised
as a concern in response to a proposed phytoremediation
project, but has not been shown to be an actual pathway for
many contaminants. Mercury (Hg) has been shown to move
through a plant and into the air in a plant that was genetically altered to allow it to do so. The thought behind this
media switching is that elemental Hg in the air poses less
risk than other Hg forms in the soil. However, the technology or the associated risk has not been evaluated.
Risk reduction may also be achieved by transforming the
contaminant into a form that is not hazardous, or by render-
4
Table 2-1. Phytoremediation Applications
Mechanism
Contaminant
Degradation
Atrazine, nitrates
Surface Water
Poplar
Applied
Schnoor 1995a
Degradation
Landfill leachate
Groundwater
Poplar
Applied
Licht 1990
Degradation
TCE
Groundwater
Poplar, cottonwood
Field demo
Rock 1997
Degradation
TNT
Wetlands
Various
Field demo
Bader 1996
Carreira 1996
McCutcheon 1995
Degradation
TPH
Soil
Grasses, crops
Field demo
Banks 1997
Drake 1997
Extraction-Concentration Lead
in shoot
Soil
Indian mustard
Field demo
Blaylock 1997
Extraction-Concentration Uranium
in root
Surface water
Sunflower
Field demo
Dushenkov 1997
Extraction, Volatilization
Soil, Surface Water
Various
Applied
Bañuelos 1996
Terry 1996
Selenium
Media
Plant
Status
Reference
tions will be monitored carefully and consistently to gather
data to both evaluate performance and to build and verify
models to predict the performance of other proposed installations. Data from a national network of sites that have
similar measurement regimes will be a powerful tool for
evaluating the appropriateness of a proposed installation,
and help develop the tools for predicting the efficacy of
similar cover systems.
ing the contaminant nonbioavailable. EPA and the U.S. Department of Agriculture (USDA) have ongoing research in
this area.
Hydraulic control is another form of containment. Groundwater contaminant plume control may be achieved by water consumption, using plants to increase the evaporation
and transpiration from a site. Some species of plants use
tremendous quantities of water, and can extend roots to
draw from the saturated zone. EPA is pursuing research in
this area at a number of sites, including the SITE demonstrations at Ogden, UT and Ft. Worth, TX, and the Emergency Response Team (ERT) lead projects at Aberdeen
Proving Grounds (Edgewood, MD) and the Edward Sears
Properties Site (New Gretna, NJ). Private companies have
installed trees as a hydraulic control at many sites.
2.1.2
Limits of Phytoremediation at
Hazardous Waste Sites
As a result of the early information provided by some
research and reported by the media, site owners and citizen groups are interested in phytoremediation as possibly
the cleanest and cheapest technology that may be employed in the remediation of selected hazardous sites. Although current research continues to explore and push
the boundaries of phytoremediation applications, there are
certain limitations to plant-based remediation systems.
Vegetative cover (evapotranspiration or water-balance
cover) systems are another remediation application utilizing the natural mechanisms of plants for minimizing infiltrating water. Originally proposed in arid and semi-arid regions, vegetative covers are currently being evaluated for
all geographic regions. The effectiveness in all regions and
climates needs to be assessed on a site-specific basis.
2.1.2.1
Root System
Root contact is a primary limitation on phytoremediation
applicability. Remediation with plants requires that the contaminants be in contact with the root zone of the plants.
Either the plants must be able to extend roots to the contaminants, or the contaminated media must be moved to
within range of the plants. This movement can be accomplished with standard agricultural equipment and practices,
such as deep plowing to bring soil from 2 or 3 feet deep to
within 8 to 10 inches of the surface for shallow-rooted crops
and grasses, or by irrigating trees and grasses with contaminated groundwater or wastewater. Because these activities can generate fugitive dust and volatile organic compound emissions, potential risks may need to be evaluated. As shown in Table 2-2 and illustrated in Figure 2-2,
the effective root depth of plants varies by species and
depends on soil and climate condition.
If there is potential for gas generation a vegetative cover
may not be an option. For example, a municipal solid waste
landfill can produce landfill gas that may be of concern to
human health and the environment. Sites with requirements
to collect and control landfill gas may not meet Federal
requirements under the Clean Air Act if a vegetative cover
is used.
Hydraulic control for groundwater plumes and water balance covers are two technologies that are being applied in
the field prior to model development predicting their behavior. Under an EPA initiative called Alternative Cover Assessment Program (ACAP), several of these field installa-
5
clean the high volumes of lower contaminant concentrations.
Table 2-2. Root Depth for Selected Phytoremediation Plants
Plant
Maximum Root Depth
Indian mustard
Grasses
Poplar trees
2.1.2.2
To 12 inches
To 48 inches
To 15 feet
Target Contaminants
2.1.2.4
Metals
Organics
Metals, organics,
chlorinated solvents
Some ecological exposure may occur whenever plants
are used to interact with contaminants from the soil. The
fate of the metals in the biomass is a concern. At one site,
sunflower plants that extracted cesium (Cs) and strontium
(Sr) from surface water were disposed of as radioactive
waste (Adler 1996).
Growth Rate
Phytoremediation is also limited by the growth rate of the
plants. More time may be required to phytoremediate a site
as compared with other more traditional cleanup technologies. Excavation and disposal or incineration takes weeks
to months to accomplish, while phytoextraction or degradation may need several years. Therefore, for sites that pose
acute risks for human and other ecological receptors,
phytoremediation may not be the remediation technique of
choice.
2.1.2.3
Impacts of Contaminated Vegetation
Although some forms of phytoremediation involve accumulation of metals and require handling of plant material
embedded with metals, most plants do not accumulate significant levels of organic contaminants. While metal accumulating plants will need to be harvested and either recycled or disposed of in compliance with applicable regulations, most phytoremediative plants do not require further treatment or disposal.
Contaminant Concentration
Often overlooked, however, is the possibility that natural
vegetation on the site is already creating very similar (but
often unrecognized) food chain exposures. In addition, even
on currently unvegetated sites, contaminants will be entering the food chain through soil organisms.
Sites with widespread, low to medium level contamination within the root zone are the best candidates for
phytoremediative processes. High concentrations of contaminants may inhibit plant growth and thus may limit application on some sites or some parts of sites. This phytotoxicity could lead to a tiered remedial approach in which
high concentration waste is handled with expensive ex situ
techniques that quickly reduce acute risk, while in situ
phytoremediation is used over a longer period of time to
The remediation plan should identify and, if possible, quantify potential avenues of ecological exposure, and determine if and where any accumulation of toxics in the selected plants will occur. Accumulation in fruits, seeds, and
Poplar Trees 15 ft.
Alfalfa 4-6 ft.
Grasses 2 ft.
Indian
Mustard 1 ft.
Figure 2-2. Example root depths.
6
leaves typically creates more exposure than accumulation
in stems and roots. Most organic contaminants do not accumulate in significant amounts in plant tissue.
for a more accepted technology. Monitoring needs to address both the decrease in the concentration of the contaminants in the media of concern, and examine the fate
of the contaminants. The monitoring plan must be tailored
to the site and plants.
Some plant-eating animals have been shown to avoid
eating plants with elevated metal levels (Pollard 1996). In
addition, the increased habitat provided by the plants may
in some cases offset any potential localized impacts.
2.2.1
Prior Applications of
Phytoremediation
One indication of acceptability of a technique is previous successful applications on similar sites. Because it is
a relatively new technology, phytoremediation does not have
a long history of completed cleanups. Table 2-3 lists 12
Superfund sites where phytoremediation has been accepted or is being field-tested for possible remediation of
soil or groundwater contamination. Appendix B lists approximately 180 sites where the technology has been applied or is being field-tested. The peer-reviewed field data
that are available on these projects are limited. More data
should become available in the next few years through the
efforts of programs such as the Superfund Innovative Technology Evaluation (SITE) program, the Remediation Technologies Development Forum (RTDF), and others.
If some organisms (e.g., caterpillars, rodents, deer, etc.)
seem likely to ingest significant amounts of the vegetation, and if harmful bioconcentration up the food chain is a
concern during the life of the remediation effort, appropriate exposure control measures should be implemented
including perimeter fencing, overhead netting, and pre-flowering harvesting. Phytoextraction techniques aim to harvest metal-laden crops just as the plants translocate metals into shoots, thereby limiting availability of contaminants
for consumption.
Transfer of the contaminants or metabolites to the atmosphere might be the greatest regulatory concern. Transpiration of TCE into the atmosphere has been measured
(Newman et al. 1997a), but little information is available
that would indicate any release of vinyl chloride.
Results of studies done in greenhouses and on field test
plots can be used to show proof of concept, and some of
that data may be directly applicable to site-specific consideration. If time and funding permit, soil or water from the
site should be used in lab or greenhouse studies. Such
treatability studies can confirm the effectiveness of the sitespecific treatment. Chapter 5 provides more information
on treatability studies.
Research being done on the bioavailability of contaminants and on human health and environmental risk assessment is directly related to phytoremediation. Studies
are underway to determine if contaminants that are not
available to plants for uptake or that are not vulnerable to
plant remediation are less of a risk to human health and
the environment.
2.3 Economic Considerations
2.2 Technical Considerations
Because phytoremediation is an emerging technology,
standard cost information is not readily available. Subsequently, the ability to develop cost comparisons and to
estimate project costs will need to be determined on a sitespecific basis. Two considerations influence the economics of phytoremediation: the potential for application, and
the cost comparison to conventional treatments. Care must
be taken to compare whole system costs, which may include:
Several key factors to consider when evaluating whether
phytoremediation is a potential site remedy are described
below.
1. Determine whether evidence of the potential effectiveness of phytoremediation is specific to the site
matrix and contaminants. If laboratory studies on the
plants and contaminants of interest are the primary
evidence used to support the use of phytoremediation
at the site, the studies should at least show that the
plants to be used at the site are capable of remediating
site contaminants.
Design costs:
Site characterization
Work plan and report preparation
Treatability and pilot testing
Installation costs
Site preparation
Facilities removal
Debris removal
Utility line removal/relocation
Soil preparation
Physical modification: tilling
Chelating agents
pH control
Drainage
Infrastructure
Irrigation system
2. Consider the protectiveness of the remedy during the
time it takes the plants associated with phytoremediation
to establish themselves at the site to a point where
they are containing/degrading the contaminants of interest.
3. Consider whether phytoremediation is likely to clean
up the site in an acceptable time frame.
4. An adequate backup or contingency technology
should be identified in the event that phytoremediation
is attempted and does not succeed.
Fencing
Planting
Seeds, plants
Labor
Protection
Additionally, monitoring the efficacy of any innovative
treatment may be more extensive than would be required
7
Operating costs:
Maintenance
Irrigation water
Fertilizer
pH control
Chelating agent
Drainage water disposal
Pesticides
Fencing/pest control
Replanting
Monitoring
Soil nutrients
Soil pH
Soil water
Plant nutrient status
Plant contaminant status
roots, shoots, stems,
leaves)
Tree sap flow monitoring
Air monitoring (leaves,
branches, whole tree, area)
Weather monitoring
Table 2-3. Phytoremediation at Superfund Sites
Site Name, State
Date Planted
Plant
Contaminant/Matrix
Carswell Site, TX
Spring 1996
Eastern cottonwood tree
TCE/groundwater at 4-12 feet
Aberdeen Proving Grounds, MD
Spring 1996
Hybrid poplar trees
TCE/groundwater
Edward Sears Site, NJ
Fall 1996
Hybrid poplar trees
TCE/groundwater at 8 feet
Iowa Army Ammunition Depot, IA
Spring 1997
Wetland and terrestrial plants
TNT/soil and pond water
Fort Wainwright, AK
Spring 1997
Felt leaf willow
Pesticides/soil and groundwater
Kaufman & Minteer, NJ
Spring 1997
Hybrid poplar trees
PCE/groundwater
Calhoun Park, SC
Fall 1998
Local landscaping plants
PAH/groundwater at 1-4 feet
Solvent Recovery Systems of
New England, CT
Spring 1998
Hybrid poplar trees
Mixed solvents/groundwater
Twin Cities Army Ammunition
Plant, MN
Spring 1998
Corn, Indian mustard
Metals/soil
Bofors-Nobel, MI
Planting scheduled
Various trees and wetland plants
Residual sludge in waste lagoons
Del Monte, HI
Spring 1998
Koa haole
Pesticides/soil and groundwater
INEEL, ID
Spring 1999
Kochia, willow
Cesium, mercury in soil
The national potential for phytoremediation could be estimated by first totaling the number of sites that contain
organics and metals suitable for phytoremediation, i.e.,
those sites that contain contaminants in moderate concentrations in near-surface groundwater or in shallow soils.
Currently, such specific information about hazardous waste
sites in the United States is not available. Kidney (1997)
has estimated the current domestic market for
phytoremediation at only $2 to $3 million for organics removal from groundwater, and $1 to $2 million for removal
of heavy metals from soils. The same study indicates that
by the year 2005, however, the market for phytoremediation
of organics in groundwater will be $20 to $45 million, of
metals in soils will be $40 to $80 million, and of radionuclides will be $25 to $50 million.
(B) Cost estimates made for remediation of a hypothetical case of a 20-in.-thick layer of sediments contaminated
with Cd, Zn, and 137Cs from a 1.2-acre chemical waste
disposal pond indicated that phytoextraction would cost
about one-third the amount of soil washing (Cornish et al.
1995).
(C) Costs were estimated to be $60,000 to $100,000
using phytoextraction for remediation of one acre of 20in.-thick sandy loam compared to a minimum of $400,000
for just excavation and storage of this soil (Salt et al. 1995).
Rhizofiltration Costs
The cost of removing radionuclides from water with sunflowers has been estimated to be $2 to $6 per thousand
gallons of water (Dushenkov et al. 1997).
David Glass (1998) and others have estimated that total
system costs for some phytoremediation applications will
be 50 to 80% lower than alternatives. Each application of
plants will yield a separate performance evaluation including rate and extent of cleanup and cost. Three actual cost
estimates of applications are compared to conventional
treatments in Table 2-4.
Phytostabilization Costs
Cropping system costs have been estimated at $200 to
$10,000 per hectare, equivalent to $0.02 to $1.00 per cubic meter of soil, assuming a 1-meter root depth
(Cunningham et al. 1995b).
For some of phytoremediation applications, hypothetical
cost comparisons have been projected. These are estimates based on laboratory and pilot scale work and tend
to reflect projected total project costs.
Hydraulic Control Costs
Estimated costs for remediation of an unspecified contaminant in a 20-foot-deep aquifer at a 1-acre site were
$660,00 for conventional pump-and-treat, and $250,000
for phytoremediation using trees (Gatliff 1994).
Phytoextraction Costs
(A) The estimated 30-year costs (1998 dollars) for
remediating a 12-acre lead site were $12,000,000 for excavation and disposal, $6,300,000 for soil washing,
$600,000 for a soil cap, and $200,000 for phytoextraction
(Cunningham 1996).
Vegetative Cover Costs
Cost estimates indicate savings for an evapotranspiration cover compared to a traditional cover design to be 20
8
Table 2-4. Example Cost Comparisons
Phytoremediation
Application
Problem
Lead in soil, 1 acrea
Extraction, harvest
disposal
Cost
($ thousand)
Conventional
Treatment
Cost
($ thousand)
Projected
Savings
$150-250
Excavate and landfill
$500
50-65%
Solvents in groundwater, Degradation and
2.5 acresb
hydraulic control
$200 install and
initial maintenance
Pump and treat
$700 annual running
cost
50% cost saving
by third year
TPH in soil, 1 acrec
$50-100
Excavate and landfill
incinerate
$500
80%
In situ degradation
a
Phytotech estimate for Magic Marker site (Blaylock et al. 1997).
PRP estimate for Solvent Recovery Systems of New England site.
c
PERF estimate (Drake 1997)
b
to 50%, depending on availability of suitable soil (RTDF
1998).
The Corrective Action Program, under RCRA, requires
corrective action, as necessary, to protect human health
and the environment for releases from solid waste management units at facilities seeking RCRA permits. This program is implemented primarily through a series of policy
directives, and is similar in nature to the Superfund program’s
remedy selection process contained in the National Contingency Plan (NCP). EPA also delegates the Corrective Action Program to the states. Policy directives pertinent to the
Corrective Action Program are available at http://
www.epa.gov/correctiveaction.
2.4 Regulatory Considerations
While Federal regulations specific to phytoremediation
have not been developed, a range of existing Federal and
state regulatory programs may pertain to site-specific decisions regarding the use of this technology. These programs include those established under the: Resource Conservation and Recovery Act (RCRA); Comprehensive Environmental Response, Compensation, and Liability Act
(CERCLA) referred to as “Superfund”; Clean Air Act (CAA);
Toxic Substances Control Act (TSCA); Federal Insecticide,
Fungicide, and Rodenticide Act (FIFRA); Federal Food,
Drug, and Cosmetic Act (FFDCA); and statutes enforced
by the U.S. Department of Agriculture. These programs
are discussed in the following sections.
2.4.1
2.4.2
CERLCA (Superfund)
Remedial actions taken under the Superfund program must
attain a general standard of cleanup that assures protection
of human health and the environment, must be cost effective, and must use permanent solutions and alternative treatment technologies or resource recovery technologies to the
maximum extent practicable. The regulatory framework for
response actions under CERCLA is contained in 40 CFR
Part 300, the National Oil and Hazardous Substances Pollution Contingency Plan, referred to as the NCP. The remedy selection process outlined in the NCP includes a Feasibility Study (FS), in which alternatives that represent viable approaches are assessed against nine criteria. With
respect to phytoremediation, data collected from treatability studies or other means will provide the necessary scientific documentation to allow an objective evaluation (using
the nine criteria) of whether phytoremediation is the most
appropriate remedial option for a given site.
RCRA
RCRA has nine sections (Subtitles) that deal with specific waste management activities. Two of these Subtitles
are most likely to pertain to the use of phytoremediation:
Subtitle C (Hazardous Waste Management), and Subtitle
D (Solid Waste Management).
EPA issued closure requirements for Subtitles C and D
treatment, storage, or disposal (TSD) units, which may be
closed by removal or decontamination (“clean closure”) or
closed with waste in place (“landfill closure”) (see 40 CFR
Parts 257, 258, 264, and 265). The regulations include
general closure requirements for all RCRA units and specific closure requirements for each type of TSD unit. The
requirements are performance-based, and therefore do not
stipulate any design standards. EPA delegates these regulatory programs to the states, which are responsible for
their implementation. The Federal requirements are minimum requirements that must be incorporated into state
regulatory programs; however, states may promulgate closure requirements that are more stringent than those of
the Federal program. Site-specific evaluation of the use of
alternative covers at TSDs that close as a landfill will need
to include consideration of these requirements.
An important component in Superfund response actions
is the requirement that for any material remaining on-site,
EPA will attain or exceed any Federal or state limitation,
standard, or criteria that is considered to be applicable or
relevant and appropriate (ARAR) given the circumstances
of the site (for off-site actions, all applicable requirements
must be met). Further, on-site remedial actions must attain promulgated state ARARs that are more stringent than
Federal ARARs. A requirement is applicable if the specific
terms of the law or regulations directly address the circumstances at the site. If not applicable, a requirement
may nevertheless be relevant and appropriate if circumstances at the site are sufficiently similar to the problems
9
view by EPA under its authority to regulate pesticides. EPA
regulates pesticides under two statutes: the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), and
the Federal Food, Drug, and Cosmetic Act (FFDCA). Substances that plants produce to protect themselves against
pests and disease are pesticides under the definition of
FIFRA Section 2 (i.e., if they are “...intended for preventing, destroying, repelling, mitigating any pest....”) regardless of whether the pesticidal capabilities evolved in the
plants or were introduced by breeding or through the techniques of modern biotechnology. These substances, along
with the genetic material necessary to produce them, are
designated “plant-pesticides”(EPA, 1994). Additional details about EPA plant pesticide regulations can be found at
http://www.epa.gov/fedrstr/EPA-PEST/1994/November/
Day-23/. The U.S. Food and Drug Administration has responsibility for food safety under the FFDCA. It is unlikely
FDA would be involved in the review of phytoremediation
plants since use of phytoremediation plants as food or as
components of food is an unlikely scenario.
regulated by the requirement. The Maximum Contaminant
Levels (MCLs) under the Safe Drinking Water Act are an
example of relevant and appropriate requirements.
In order for phytoremediation to be selected as a remedy at a CERLCA site, it will be necessary to meet or waive
the ARARs identified for the site. ARARs can be waived
under six specific circumstances described in the NCP.
2.4.3
CAA
Sections 111 and 112 of the Clean Air Act (CAA) contain
the statutory basis for regulation of criteria and hazardous
air pollutant emissions from source categories. The New
Source Performance Standards and Emission Guidelines for
Municipal Solid Waste Landfills, 40 CFR Part 60 Subparts
WWW and Cc, are statutorily based on section 111 of the
CAA. These standards were promulgated in March 1996 and
regulate air emissions of non-methane organic compounds
(NMOCs) from municipal solid waste landfills. Specifically,
these standards require municipal solid waste landfills with a
waste capacity of at least 2.5 million megagrams and with
the potentially to emit at least 50 megagrams of NMOC to
collect and control landfill gas. A second standard, 40 CFR
Part 63 Subpart AAAA, statutorily based on section 112 of
the CAA is currently under development. This regulation will
regulate hazardous air pollutant emissions from municipal
solid waste landfills. These regulations set performance based
standards that allow owners and operators a number of options to achieve compliance. EPA delegates the implementation of these regulations to Federal, State and local governments. These Federal standards establish minimum requirements that must be implemented. However, State and
local governments may choose to increase the stringency of
these requirements. Any site contemplating the use of
Phytoremediation needs to consider the requirements established by Federal, State and local government programs
to regulate air emissions from municipal solid waste landfills.
2.4.4
2.4.6
2.5 Ecosystem Restoration
TSCA
Vegetation is not only an aid to ecosystem restoration, it
is a key indicator. Plant species that are present on a site,
as well as their quantities and condition, describe a
watershed’s health and resilience. Loss of vegetation
through clearing, building, and human land use has severe ecosystem effects. After a human disturbance such
as mining, dumping, industrial, agricultural, or residential
use, revegetation may occur slowly. Recolonization of contaminated or disturbed ground by plants typically starts at
the edges of an impacted area. Natural revegetation may
take decades or hundreds of years because it is dependent on animal and windborne seeding. If replanting is
designed and carried out in a way that includes the perspectives of engineers, botanists, ecologists, landscape
architects, and others, the environmental systems can
begin to be restored in a few years. In some cases
phytoremediation can help restore wild species diversity
through habitat growth in addition to aiding in remediation
of soil and water.
Although the EPA does not currently regulate plants intended for commercial bioremediation, EPA believes the
Toxic Substances Control Act (TSCA) gives it authority to
do so if such action is necessary to prevent unreasonable
risk to human health or the environment. TSCA gives EPA
authority to regulate “chemical substances.” TSCA defines
chemical substances broadly to mean all chemicals and
mixtures of chemical substances. Living organisms such
as plants are mixtures of chemical substances and thus
are subject to TSCA. Although TSCA could potentially be
applied to plants used in bioremediation, EPA has not yet
made a determination of whether such action is necessary to protect the environment and human health. EPA
to date has only issued regulations for microorganisms
under Section 5 of TSCA (EPA, 1997). Further information on TSCA and biotechnology products can be found at
http://www.epa.gov/opptintr/biotech/.
2.4.5
Department of Agriculture Statutes
Plants used for phytoremediation could be potentially
regulated under several U.S. statutes. The U.S. Department of Agriculture (USDA) administers several statutes
that could be used to regulate such plants: e.g., the Federal Plant Pest Act (7 U.S.C. 150aa et seq.), the Plant Quarantine Act (7 U.S.C. 151 et seq.), and the Federal Noxious
Weed Act (7 U.S.C. 2801 et seq.). Pertinent regulations
are found at 7 CFR Parts 319, 321, 330, 340, and 360,
respectively. Under USDA authority, one type of plant
(transgenic or naturally-occurring) potentially subject to
review would be a plant considered to be a plant pest. For
additional guidance on USDA regulations pertaining to
plants, refer to http://www.aphis.usda.gov/bbep/bp/.
FIFRA/FFDCA
Despite the general ecological advantages of
phytoremediation, adverse ecological effects are possible
to both on- and off-site biological communities. In evaluat-
Certain plants engineered to contain sequences that afford the plant resistance to pests to enhance the
remediation efficacy of the plant could be subject to re-
10
Public Uses
ing such concerns, it is important to compare the relative
ecological risks posed by phytoremediation to those risks
already occurring on site or those risks posed by alternative cleanup methods. Actions needed to protect ecosystems should be clearly specified in the site cleanup plan.
A number of contaminated sites are being converted to
parks and other low-intensity public uses. These sites,
particularly with their greater flexibility in the timing and
design of cleanup, frequently offer significant ecological
opportunities. Trees and shrubs do not have to be planted
in straight rows to be effective in remediation.
Overall ecological risks associated with remediation of
a site are often overlooked, even by interested parties who
may be familiar with the human health and ecological risks
associated with current site conditions or with the general
risks posed by feasible alternative cleanup methods. These
overall ecological concerns may be expressed in a limited
context that does not help in the selection of an alternative. Often, however, a site visit will broaden the understanding of interested parties and thus enable them to better
assist in identifying options with the greatest overall ecological benefits.
Commercial Uses
Businesses traditionally landscape for aesthetic reasons
and storm run-off control. These functions may be combined with phytoremediation to offer significant opportunities. Properly designed and located, such landscaping could
also provide long-term treatment and enhanced ecological habitats. A site owner may be willing to significantly
expand the land committed to phytoremediative landscaping if that commitment would reduce overall cleanup costs
and allow quicker site redevelopment. A phased approach,
with intensive short-term treatment by one plant species
followed by permanent plantings with more beneficial vegetation, may maximize ecosystem benefits.
Listed below are issues that typically arise in such discussions. Also included are frequently overlooked factors
that should be considered in identifying the relative risks.
2.5.1
Introduction of Non-Native Plants
Wood Lot Uses
History is rife with disastrous examples of newly introduced plant species spreading quickly to damage native
ecosystems (e.g., kudzu, Eurasian watermilfoil, etc.). Plants
that work best in remediating a particular contaminant may
or may not be native to a particular area. Although native
plants are most desirable, non-native species may be acceptable under the following circumstances:
Short-rotation woody crops for pulp, fuel, or timber may
be grown on land with nonaccumulating organic contaminants. The trees could be grown and harvested while recalcitrant compounds slowly degrade.
• The plants have been previously introduced, and are
now so common that their proposed use would not
create a new ecological risk.
Phytoremediation is most suitable for remediating sites
or portions of sites with widespread, low-mid level contaminants that are often too expensive to remediate by
traditional means. Absent a cost-effective remediation
method, contaminants are often left in place, and contact
with the waste attempted to be minimized by fences, institutional controls and deed restrictions. Too often no one is
required or able to cleanup the site, and many sites have
been abandoned with no clean up or controls. Occasionally a phytoremediation system may be inexpensive enough
that it could be installed during considerations and debate
regarding a permanent solution, and removed once a final
remedy is implemented. The interim ecosystem benefits,
coupled with improved aesthetics, and some containment
and/or degradation may combine to make even temporary
revegetation worthwhile.
2.5.3
• The plants are unable to propagate effectively in the
wild (e.g., sterility, dependence on human cultivation,
etc.).
• Genetically altered plants have been introduced. Mankind has been using selective breeding to obtain desired plant characteristics for at least 10,000 years.
Now, researchers in many fields are using new genetic engineering techniques to replace selective breeding, allowing them to achieve their desired results more
quickly and selectively. Decisions on the desirability of
using genetically engineered plants must be site specific.
2.5.2
Phytoremediation as an Interim
Solution
Although phytoremediation may not be the selected final technique, the benefits of a well-designed, properly installed, and capably managed phytoremediation system
may be preferable to the risks posed by leaving a waste
site completely untreated during delays in implementing a
final remedy.
Integration of Phytoremediation
Into the Site’s Long-Term
Landscaping Objectives
Long-term phytoremediation-based treatment can be designed into future site landscaping plans, e.g., tree borders
used for shading and visual screening can also provide ongoing groundwater remediation, etc. Such vegetation can
often create valuable ecological niches, particularly in urban industrial areas.
2.5.4
Ecosystem Restoration at
Phytoremediation Sites
Many sites with contamination support complex ecosystems, primarily due to the low level of human activity on the
11
site. Plants and animals recolonize some areas where decreased human traffic allows vegetation to take root. Phytoremediation could aid the natural revegetation underway
on such site. Phytoremediation can make use of various
types of plants, and it is possible to consider local or native
plants as part of a remediation system.
on which parameters are crucial to measure, and very few
projects can afford to sample, analyze, or monitor very
many parameters over the years needed for most
phytoremediation projects.
The EPA’s Office of Research and Development (ORD)
and the Office of Solid Waste and Emergency Response
(OSWER) have several programs that investigate the efficacy, risk, and cost of phytoremediation. These EPA activities include EPA in-house laboratory research efforts,
support given to universities that are centers for
phytoremediation research, and joint EPA-private cooperative efforts to field-test phytoremediation.
In order to consider using native plants, the remediation
potential of plants already growing on the site must be
carefully assessed. It must be determined whether these
plants are just tolerating the contaminant or whether they
are already actively remediating it. A field or greenhouse
study may be needed to make this determination.
If non-native plants must be utilized, appropriate control
techniques (e.g., sterile plants) should be used to ensure
that genetic contamination or invasive spread does not
occur.
EPA’s Office of Research and Development manages
various in-house research projects, and several EPA laboratories have work underway to determine the fate of contaminants in phytoremediation applications. Steve
McCutcheon and Lee Wolfe at the EPA National Exposure
Research Laboratory (NERL) in Athens, GA, have explored
the degradation of TNT by wetland plants, and continue to
investigate plant enzyme and contaminant interactions.
Albert Venosa at the EPA National Risk Management Research Laboratory (NRMRL) in Cincinnati, OH, is researching the effect of plants on oil spills in salt and fresh water
wetlands. James Ryan, who is also at EPA-NRMRL, is
working with Rufus Chaney of USDA on using plants to
immobilize metals in soil. Richard Brenner, also at EPANRMRL, is leading a team comparing the use of land farming and phytoremediation on the site of a former manufactured gas facility. Harry Compton and George Prince of
the ERT are monitoring poplar tree plantings at Superfund
sites in MD and NJ (see Table 2-3). Larry Erickson’s EPAsupported Hazardous Substance Research Center at Kansas State University has for many years sponsored research and symposia on the interaction of plants and contaminants. Tom Wilson at EPA Region 10 continues to explore and encourage innovative applications and interactions between phytoremediation and ecosystem restoration.
Phytoremediative plants with desirable ecological values could
provide diversified habitat where appropriate. A combination of
trees, understory shrubs, and grasses may provide shelter and
food for numerous species. Nonphytoremediative plants can be
added to supplement ecological values such as soil stabilization
or to provide a food source.
Evaluating the ecological recovery of a site is important,
but such an evaluation does not have to be expensive or
complex. Neighborhood environmental and school groups
could “adopt” a brownfield or similar distressed site and
provide data of mutual interest (e.g., the local Audubon
Society could assess bird habitat utilization; a biology class
could track plant species survival and growth, etc.). Properly done, such collaborative monitoring can build community understanding and support, while also providing data
that would be otherwise unaffordable, and is seldom collected during remediation. Many sites will not allow access
by untrained personnel. Personal protection must always
be a foremost concern in any such collaboration.
As noted earlier, phytoremediation offers significant ecological promise, but it is not a perfect solution. Ecological
benefits in one area may create ecological impacts in others. Negative impacts must be avoided. Some stakeholders may disagree with this definition of ecosystem restoration because it does not attempt to recreate a pristine ecosystem. Although it may not be possible or feasible to return a site to its condition before human impact,
phytoremediation may provide realistic opportunities to improve the overall ecological health of a site.
The Superfund Innovative Technology Evaluation Program (SITE) demonstrates field-ready technologies that
are initiated and installed by the developer of the technology. SITE began evaluating phytoremediation projects in
1994. Currently four full demonstrations (including two at
Superfund sites), and one Emerging Program project have
been done or are underway using phytoremediation, coordinated by Steve Rock at EPA-NRMRL in Cincinnati, OH.
Reports detailing the performance of the demonstrations
will be published at the conclusion of the field work. Information on the SITE program or individual projects can be
found at http://www.epa.gov/ORD/SITE.
2.6 Current Research
To assess the appropriateness of any phytoremediation
application, media- and contaminant-specific field data must
be obtained that can show the rate and extent of degradation or extraction. The existing knowledge base is limited,
and specific data are needed on more plants, contaminants, and climate conditions.
EPA’s Office of Research and Development and the Office of Solid Waste and Emergency Response (OSWER)
jointly support the Remediation Technologies Development
Forum (RTDF). The RTDF was established in 1992 by the
EPA to foster collaboration between the public and private
sectors in developing innovative solutions to mutual haz-
In addition, monitoring systems need to be standardized. Currently there is no industry or research consensus
12
In addition to EPA efforts, other Federal agencies, universities, consultants, and remediation contractors have
research underway in phytoremediation. All these projects
expand the knowledge base of what plants can be expected
to do consistently, and make the application of innovative
technologies more acceptable to regulators and consumers.
ardous waste problems. The RTDF has grown to include
partners from industry, several government agencies, and
academia who share the common goal of developing more
effective, less-costly hazardous waste characterization and
treatment technologies. There are currently seven RTDF
Action Teams, including the “Phytoremediation of Organics
Action Team.” This Action Team was formed in early 1997,
and is currently comprised of three working groups that
are concerned with phytoremediation of three separate
pollution/matrix situations: petroleum compounds in shallow soils, chlorinated solvents in near-surface groundwater, and the use of vegetation with high transpiration
rates as an alternative cap for hydraulic containment and/
or degradation of various pollutants. The Action Team
has held several meetings, and has regular conference
calls to select and implement field testing projects. Current co-chairs in the subcommittees include representatives from Chevron, Exxon, the Air Force, and Union
Carbide.
Continuing research and policy discussions in the related areas of determining possible risk-based alternative
endpoints for cleanups, and measuring the intrinsic
remediative capacity of a site (natural attenuation) will impact the applicability of many biological-based technologies, including plant-based systems.
Enhancements to the various phytoremediation processes are continuing. Some applied research is directed
at selecting and breeding plants that have more of an attractive quality such as hyperaccumulation of metal, production of certain enzymes, and affinity or tolerance for
contaminants. Research continues in genetic engineering
of plants to combine positive traits, alter enzyme systems,
or increase a plant’s natural range.
To access meeting and teleconference minutes, bibliographic information on phytoremediation, and other information, refer to http://www.rtdf.org. The Technology Innovation Office (TIO) within OSWER supports RTDF activities,
as well as other efforts aimed at bringing innovative site
characterization and treatment technologies to commercialization. Further information on the Technology Innovation
Office and resources generated by TIO can be found at
http://www.clu-in.org.
An engineering approach could be pursued by using existing plant traits as only a part of a remediation system of
combined planted systems and mechanical, thermal, or
chemical systems in treatment trains. Suggested combinations include electrokinetics, bioventing, and surfactant
addition.
13
Chapter 3
Evaluation of Phytoremediation Technologies
This chapter presents a literature review and evaluation
of the major phytoremediation processes or technologies.
The technologies presented represent the major, significant,
or widely studied forms of phytoremediation.
• Metal hyperaccumulators are generally slow-growing with
a small biomass and shallow root systems.
• Plant biomass must be harvested and removed, followed by metal reclamation or proper disposal of the
biomass. Hyperaccumulators may accumulate significant metal concentrations — e.g., Thlaspi rotundifolium
grown in a lead-zinc mine area contained 8,200 g/g Pb
(0.82%) and 17,300 g/g zinc (Zn) (1.73%), and Armeria
maritima var. halleri contained 1,300 g/g Pb, dry weight
basis (Reeves and Brooks 1983).
This chapter is divided into subsections that present definitions, mechanisms, site characteristics, applicable media, contaminants amenable to each process, and the associated concentrations where available. The advantages,
disadvantages, and current status of each process are also
discussed. Finally, an annotated reference list is included
at the end of the discussion of each process to provide
more detailed, specific information.
• Metals may have a phytotoxic effect (Nanda Kumar et
al. 1995).
The purpose of this chapter is to provide site managers
with an overview of the various phytoremediation processes
as well as what can be expected from the process and its
limitations. Therefore, information on applicable contaminants/concentrations is included even though the information may not be complete. Table 3-1 presents a summary of
the various phytoremediation processes.
• Phytoextraction studies conducted using hydroponicallygrown plants, with the contaminant added in solution,
may not reflect actual conditions and results occurring
in soil. Phytoextraction coefficients measured under field
conditions are likely to be less than those determined
in the laboratory (Nanda Kumar et al. 1995).
3.1 Phytoextraction
3.1.1
Definition/Mechanism
Phytoextraction is the uptake of contaminants by plant
roots and translocation within the plants. Contaminants are
generally removed by harvesting the plants. This concentration technology leaves a much smaller mass to be disposed of than does excavation of the soil or other media.
This technology is most often applied to metal-contaminated soil as shown in Figure 3-1.
3.1.2
3.1.5.1
Applicable Contaminants
• Metals: Ag, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Zn
The relative degree of uptake of different metals will
vary. Experimentally-determined phytoextraction coefficients [ratio of g metal/g dry weight (DW) of shoot to g
metal/g DW of soil] for B. juncea (Nanda Kumar et al.
1995) indicate, for example, that lead was much more
difficult to take up than cadmium:
Media
Metal
Cr6+
Cd2+
Ni2+
Cu2+
Pb2+
Cr3+
Zn2+
Advantages
The plant biomass containing the extracted contaminant
can be a resource. For example, biomass that contains selenium (Se), an essential nutrient, has been transported to
areas that are deficient in Se and used for animal feed
(Bañuelos 1997a).
3.1.4
Applicable Contaminants/
Concentrations
Constituents amenable to phytoextraction include:
Phytoextraction is primarily used in the treatment of soil,
sediments, and sludges. It can be used to a lesser extent
for treatment of contaminated water.
3.1.3
3.1.5
Disadvantages
• Metalloids: As, Se
Phytoextraction has the following disadvantages:
14
Phytoextraction Coefficient
58
52
31
7
1.7
0.1
17
Table 3-1. Phytoremediation Overview
Mechanism
Process Goal
Media
Contaminants
Plants
Status
Phytoextraction
Contaminant extraction
and capture
Soil, sediment,
sludges
Metals: Ag, Cd, Co,
Cr, Cu, Hg, Mn, Mo, Ni,
Pb, Zn; Radionuclides:
90
Sr, 137Cs, 239Pu, 238,234U
Indian mustard,
pennycress, alyssum
sunflowers, hybrid
poplars
Laboratory, pilot, and
field applications
Rhizofiltration
Contaminant extraction
and capture
Groundwater,
surface water
Metals, radionuclides
Sunflowers, Indian
mustard, water
hyacinth
Laboratory and pilotscale
Phytostabilization
Contaminant
containment
Soil, sediment,
sludges
As, Cd, Cr, Cu, Hs, Pb,
Zn
Indian mustard,
hybrid poplars,
grasses
Field application
Rhizodegradation
Contaminant
destruction
Soil, sediment,
sludges,
groundwater,
Organic compounds
(TPH, PAHs, pesticides
chlorinated solvents,
PCBs)
Red mulberry,
grasses, hybrid
poplar, cattail, rice
Field application
Phytodegradation
Contaminant destruction
Soil, sediment,
sludges,
groundwater
surface water
Organic compounds,
chlorinated solvents,
phenols, herbicides,
munitions
Algae, stonewort,
hybrid poplar,
black willow, bald
cypress
Field demonstration
Phytovolatilization
Contaminant extraction
from media and release
to air
Groundwater,
soil, sediment,
sludges
Chlorinated solvents,
some inorganics (Se,
Hg, and As)
Poplars, alfalfa
black locust,
Indian mustard
Laboratory and field
application
Hydraulic control
(plume control)
Contaminant degradation
or containment
Groundwater,
surface water
Water-soluble organics
and inorganics
Hybrid poplar,
cottonwood, willow
Field demonstration
Vegetative cover
(evapotranspiration
cover)
Contaminant containment, Soil, sludge,
erosion control
sediments
Organic and inorganic
compounds
Poplars, grasses
Field application
Riparian corridors
(non-point source
control)
Contaminant destruction
Water-soluble organics
and inorganics
Poplars
Field application
Surface water,
groundwater
• Radionuclides: 90Sr, 137Cs, 239Pu, 238U, 234U
• 2000 mg/kg Cd was used in studies of Cd uptake in
vegetables (Azadpour and Matthews, 1996).
• Nonmetals: B
• 110 mg/kg Pb (Pierzynski and Schwab 1992).
• Organics: The accumulation of organics and subsequent
removal of biomass generally has not been examined
as a remedial strategy.
3.1.5.2
• 625 mg/kg Pb (Nanda Kumar et al. 1995).
• 40 mg/kg Se (Bañuelos et al. 1997b).
Contaminant Concentrations
• 444 mg/kg Zn (Thlaspi caerulescens) (Baker et al. 1995).
Contaminated soil concentrations used in research
studies or found in field investigations are given below.
These are total metal concentrations; the mobile or available concentrations would be less.
• 1,165 mg/kg Zn was suspected to have phytotoxic effects (Pierzynski and Schwab 1992).
Nanda Kumar et al. (1995) reported that the following
concentrations were not phytotoxic to Brassica juncea
when added to soil mixtures:
• 1,250 mg/kg As (Pierzynski et al. 1994).
• 9.4 mg/kg Cd (Pierzynski et al. 1994).
2 mg/L Cd2+
50 mg/L Cr3+
3.5 mg/L Cr6+
10 mg/L Cu2+
• 11 mg/kg Cd (Pierzynski and Schwab 1992).
• 13.6 mg/kg Cd (Thlaspi caerulescens) (Baker et al.
1995).
15
100 mg/L Ni2+
500 mg/L Pb2+
100 mg/L Zn2+
Physical Effects - Plant transpiration results in
contaminant being concentrated in plant
Contaminant uptake
Contaminant uptake
Figure 3-1. Phytoextraction.
3.1.7
The following solution concentrations were reported in
studies of phytoextraction that used hydroponically-grown
plants:
Hyperaccumulator plants are found in the Brassicaceae,
Euphorbiaceae,Asteraceae, Lamiaceae, or Scrophulariaceae plant
families (Baker 1995). Examples include:
• Cd: Thlaspi caerulescens survived 63.2 M Cd without
evidence of chlorosis at 21 days in hydroponic solution, but was severely affected at 200 M (22 mg/L)
(Brown et al. 1995).
• Brassica juncea (Indian mustard) - a high-biomass plant
that can accumulate Pb, Cr (VI), Cd, Cu, Ni, Zn, 90Sr,
B, and Se (Nanda Kumar et al. 1995; Salt et al. 1995;
Raskin et al. 1994). It has over 20 times the biomass of
Thlaspi caerulescens (Salt et al. 1995). Brassicas can
also accumulate metals. Of the different plant species
screened, B. juncea had the best ability to transport
lead to the shoots, accumulating >1.8% lead in the
shoots (dry weight). The plant species screened had
0.82 to 10.9% Pb in roots (with Brassica spp. having
the highest), with the shoots having less Pb. Except for
sunflower (Helianthus annuus) and tobacco (Nicotiana
tabacum), other non-Brassica plants had phytoextraction
coefficients less than one. 106 B. juncea cultivars varied widely in their ability to accumulate Pb, with different cultivars ranging from 0.04% to 3.5% Pb accumulation in the shoots and 7 to 19% in the roots (Nanda
Kumar et al. 1995).
• Pb: 6, 22, 47, 98, 188 mg/L. Root uptake of Pb became
saturated at Pb solution concentrations above 188 mg/
L (Nanda Kumar et al. 1995).
• Zn: Thlaspi caerulescens survived 3,160 M Zn without
evidence of chlorosis at 21 days in hydroponic solution, but was severely affected at 10,000 M (650 mg/L)
(Brown et al. 1995).
3.1.6
Plants
Root Depth
Phytoextraction is generally limited to the immediate zone
of influence of the roots; thus, root depth determines the
depth of effective phytoextraction. The root zones of most
metal accumulators are limited to the top foot of soil.
16
• Thlaspi caerulescens (Alpine pennycress) for Ni and
Zn (Brown et al. 1994).
browsing animals, and harvested plant material must be
properly disposed of.
• Thlaspi rotundifolium ssp. cepaeifolium, a noncrop Brassica and one of the few Pb accumulators mentioned in
the literature (Nanda Kumar et al. 1995).
3.1.8.1
Soil conditions must be appropriate for plant growth and
contaminant migration to the plant, yet not allow leaching of
the metals. The pH of the soil may need to be adjusted and/
or chelating agents may need to be added to increase plant
bioavailability and uptake of metals.
• Alyssum wulfenianum for Ni (Reeves and Brooks 1983).
• Baker (1995) found 80 species of nickel-accumulating
plants in the Buxaceae (including boxwood) and
Euphoribiaceae (including cactus-like succulents) families. Some euphorbs can accumulate up to 5% of their
dry weight in nickel.
3.1.8.2
3.1.8.3
3.1.9
Current Status
Both laboratory and field experiments have been conducted. The first controlled field trial of Thlaspi caerulescens
in the UK was in 1994 (Moffat 1995). In this study, Thlaspi
caerulescens accumulated Zn and Cd to several percent
dry weight. A commercial operation, Phytotech, Inc., also
conducted field tests and small-scale field applications (including the “Magic Marker” site in Trenton, NJ) with some
degree of success using Indian mustard (Brassica juncea)
to remove lead from soil.
• Lambsquarter leaves had relatively higher As concentrations (14 mg/kg As) than other native plant or poplar
leaves (8 mg/kg) in mine-tailing wastes (Pierzynski et
al. 1994).
• Sunflowers took up Cs and Sr, with Cs remaining in the
roots and Sr moving into the shoots (Adler 1996).
• Metal accumulator plants such as the crop plants corn,
sorghum, and alfalfa may be more effective than
hyperaccumulators and remove a greater mass of metals due to their faster growth rate and larger biomass.
Additional study is needed to quantify contaminant removal.
Plant selection, breeding, and genetic engineering for fastgrowing, high-biomass hyperaccumulators are active areas
of research. Information on uptake and translocation of metals has been assessed by Nellessen and Fletcher (1993a).
3.1.10 System Cost
The number of taxonomic groups (taxa) of
hyperaccumulators varies according to which metal is
hyperaccumulated:
3.1.8
Climatic Conditions
Hyperaccumulators are often found in specific geographic
locations and might not grow under other climatic conditions.
• Hybrid poplar trees were used in a field study in minetailing wastes contaminated with As and Cd (Pierzynski
et al. 1994).
Ni
Co
Cu
Zn
Mn
Pb
Cd
Ground and Surface Water
The primary considerations for phytoremediation in groundwater are depth to groundwater and depth to contamination
zone. Groundwater phytoremediation is essentially limited
to unconfined aquifers in which the water table depths are
within reach of plant roots.
• Indian mustard (Brassica juncea) and canola (Brassica
napus) have been shown to accumulate Se and B. Kenaf
(Hibiscus cannabinus L. cv. Indian) and tall fescue
(Festuca arundinacea Schreb cv. Alta) also take up Se,
but to a lesser degree than canola (Bañuelos et al.
1997b).
Metal
Soil Conditions
The estimated 30-year costs (1998 dollars) for remediating
a 12-acre lead site are $12,000,000 for excavation and disposal, $6,300,000 for soil washing, $600,000 for a soil cap,
and $200,000 for phytoextraction (Cunningham 1996).
Number of Taxonomic Groups of Hyperaccumulators
In a hypothetical study involving the remediation of a 20in.-thick layer of sediments contaminated with Cd, Zn, and
137
Cs from a 1.2-acre chemical waste disposal pond,
phytoextraction cost was estimated to be about one-third
the cost of soil washing (Cornish et al. 1995).
>300
26
24
18
8
5
1
Phytoextraction costs were estimated to be $60,000 to
$100,000 for remediation of one acre of 20-in.-thick sandy
loam, compared to a minimum of $400,000 for just excavation and storage of this soil (Salt et al. 1995).
Site Considerations
Because potentially toxic levels of metals can accumulate in the aboveground portion of the plant, access to the
plants must be controlled and plant debris must be monitored more closely than with other phytoremediation technologies. Thus, care must be taken to restrict access of
3.1.11 Selected References
Azadpour, A., and J. E. Matthews. 1996. Remediation of
Metal-Contaminated Sites Using Plants. Remed. Summer.
6(3):1-19.
17
In this literature review of research conducted on the
uptake of metals by plants, factors that affect metals
uptake are provided along with examples of plants examined for phytoextraction.
sediments, or sludges because the contaminant needs to
be in solution in order to be sorbed to the plant system.
3.2.3
Rhizofiltration has the following advantages:
Chaney, R. L. 1983. Plant Uptake of Inorganic Waste Constituents. pp. 50-76. In J. F. Parr, P. B. Marsh, and J. M. Kla
(eds.), Land Treatment of Hazardous Waste. Noyes Data
Corporation, Park Ridge, NJ.
• Either terrestrial or aquatic plants can be used. Although
terrestrial plants require support, such as a floating platform, they generally remove more contaminants than
aquatic plants.
This literature review of factors affecting metals uptake
by plants, metals tolerance, and metals impacts on plant
growth is written from the viewpoint of the impact of
land-applied waste on plants. It is also an early proposal for the use of plants to remediate contaminated
sites.
• This system can be either in situ (floating rafts on ponds)
or ex situ (an engineered tank system).
• An ex situ system can be placed anywhere because
the treatment does not have to be at the original location of contamination.
Cornish, J. E., W. C. Goldberg, R. S. Levine, and J. R.
Benemann. 1995. Phytoremediation of Soils Contaminated
with Toxic Elements and Radionuclides. pp. 55-63. In R. E.
Hinchee, J. L. Means, and D. R. Burris (eds.), Bioremediation
of Inorganics. Battelle Press, Columbus, OH.
3.2.4
Disadvantages
Rhizofiltration has the following disadvantages:
• The pH of the influent solution may have to be continually adjusted to obtain optimum metals uptake.
This review focuses on the application of plants for
remediating U.S. Department of Energy sites contaminated with metals and radionuclides. It lists the contaminant ranges found at these sites, the phytotoxicity
threshold for the contaminants, and the number of
hyperaccumulating species for the contaminants. It
develops a hypothetical example to show the potential
cost savings of phytoextraction.
• The chemical speciation and interaction of all species
in the influent have to be understood and accounted
for.
• A well-engineered system is required to control influent
concentration and flow rate.
• The plants (especially terrestrial plants) may have to
be grown in a greenhouse or nursery and then placed in
the rhizofiltration system.
Nanda Kumar, P. B. A., V. Dushenkov, H. Motto, and I.
Raskin. 1995. Phytoextraction: The Use of Plants to Remove Heavy Metals from Soils. Environ. Sci. Technol.
29(5):1232-1238.
• Periodic harvesting and plant disposal are required.
Experimental studies are described examining metals
uptake by a variety of plant species and different Brassica juncea cultivars. The experiments focused on lead
but also included other metals.
• Metal immobilization and uptake results from laboratory and greenhouse studies might not be achievable in
the field.
3.2.5
3.2 Rhizofiltration
3.2.1
Definition/Mechanism
Applicable Contaminants/
Concentrations
Constituents amenable to phytoremediation include:
Rhizofiltration is the adsorption or precipitation onto plant
roots, or absorption into the roots of contaminants that are
in solution surrounding the root zone, due to biotic or abiotic
processes. Plant uptake, concentration, and translocation
might occur, depending on the contaminant. Exudates from
the plant roots might cause precipitation of some metals.
Rhizofiltration first results in contaminant containment, in
which the contaminants are immobilized or accumulated on
or within the plant. Contaminants are then removed by physically removing the plant.
3.2.2
Advantages
• Metals:
- Lead
(i) Pb2+ at a solution concentration of 2 mg/L, was
accumulated in Indian mustard roots with a bioaccumulation coefficient of 563 after 24 hours. Pb2+
(at solution concentrations of 35, 70, 150, 300, and
500 mg/L) was accumulated in Indian mustard roots, although root adsorption of Pb saturated
at 92 to 114 mg Pb/g DW root. Pb disappeared
from the 300- and 500-mg/L solutions due to precipitation of lead phosphate. Pb absorption by
roots was found to be rapid, although the amount
of time required to remove 50% of the Pb from
solution increased as the Pb concentration increased
(Dushenkov et al. 1995).
Media
Extracted groundwater, surface water, and waste water
can be treated using this technology. Rhizofiltration is generally applicable to low-concentration, high-water-content
conditions. This technology does not work well with soil,
18
concentration of approximately 0.1 mg/L (Wang et al.
1996).
(ii) Pb was accumulated in the roots of Indian mustard
(Brassica juncea) in water concentrations of approximately 20 to 2,000 g/L, with bioaccumulation
coefficients of 500 to 2,000 (Salt et al. 1997).
- Chromium
(i) Cr6+ (4 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 179
after 24 hours. The roots contained Cr3+, indicating reduction of Cr6+ (Dushenkov et al. 1995).
(iii) Pb at concentrations of 1 to 16 mg/L was accumulated by water milfoil (Myriophyllum spicatum)
with a minimum residual concentration below 0.004
mg/L (Wang et al. 1996).
(ii) Cr (VI) was accumulated by the roots of Indian
mustard (Brassica juncea) in water concentrations
of about 20 to 2000 g/L, with bioaccumulation coefficients of 100 to 250 (Salt et al. 1997).
- Cadmium
Cd2+ (2 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 134 after
24 hours (Dushenkov et al. 1995). Cd was accumulated by the roots of Indian mustard (Brassica juncea)
in water concentrations of about 20 to 2,000 g/L, with
bioaccumulation coefficients of 500 to 2,000. The
seedlings removed 40 to 50% of the Cd within 24
hours at a biomass loading of 0.8 g dry weight/L solution. The Cd went from 20 g/L to 9 g/L within 24
hours. After 45 hours, the Cd reached 1.4% in the
roots and 0.45% in the shoots. Cd saturation was
reached in the roots in 12 hours and in the shoots in
45 hours. Removal of competing ions in the solution
increased the uptake 47-fold (Salt et al. 1997). Cd at
concentrations of 1 to 16 mg/L was accumulated by
water milfoil (Myriophyllum spicatum) with a minimum
residual concentration of approximately 0.01 mg/L
(Wang et al. 1996).
• Radionuclides:
- Uranium
U was studied using sunflowers in bench-scale and
pilot-scale engineered systems (Dushenkov et al.
1997).
Co = 56 g/L, reduced by >95% in 24 hours.
Co = 600 g/L, to 63 g/L in 1 hour, then down to 10 g/
L after 48 hours.
Co = 10, 30, 90, 810, or 2430 g/L with no signs of
phytotoxicity, and doubled their biomass.
Co = several hundred g/L, went to below regulatory
goal of 20 g/L.
- Copper
Co = >1,000 g/L, could not reach 20 g/L goal; went
down to 40 to 70 g/L.
Cu2+ (6 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 490 after
24 hours (Dushenkov et al. 1995). Cu at concentrations of 1 to 16 mg/L was accumulated by water milfoil (Myriophyllum spicatum) with a minimum residual
concentration of approximately 0.01 mg/L (Wang et
al. 1996).
Average Co = 207 g/L, went to <20 g/L.
Influent concentrations at the field site were 21 to
874 g/L.
- Cesium
- Nickel
(i) Cs was used with sunflowers in bench-scale and
pilot-scale engineered systems (Dushenkov et
al. 1997). Co = 200 g/L, decreased noticeably after
6 hours, then went below 3 g/L after 24 hours.
Ni2+ (10 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 208 after
24 hours (Dushenkov et al. 1995). Ni was accumulated by the roots of Indian mustard (Brassica juncea)
in water concentrations of about 20 to 2,000 g/L, with
bioaccumulation coefficients of 500 to 2,000 (Salt et
al. 1997). Ni at concentrations of 1 to 16 mg/L was
accumulated by water milfoil (Myriophyllum spicatum)
with a minimum residual concentration of approximately 0.01 mg/L (Wang et al. 1996).
(ii) Cs was accumulated in the roots of Indian mustard
(Brassica juncea) in water concentrations of approximately 20 to 2,000 g/L, with bioaccumulation
coefficients of 100 to 250 (Salt et al. 1997).
- Strontium
(i) Sr was used with sunflowers (Dushenkov et al.
1997). Co = 200 g/L, went to 35 g/L within 48 hours,
then down to 1 g/L by 96 hours.
- Zinc
Zn2+ (100 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 131 after
24 hours (Dushenkov et al. 1995). Zn at concentrations of 1 to 16 mg/L was accumulated by water milfoil (Myriophyllum spicatum) with a minimum residual
(ii) Sr was accumulated in the roots of Indian mustard (Brassica juncea) in water concentrations
of approximately 20 to 2,000 g/L (Salt et al. 1997).
19
Rhizofiltration has not been evaluated for use with nutrients or organics.
3.2.6
systems than terrestrial plants (Dushenkov et al. 1995).
Floating aquatic plants include water hyacinth
(Eichhornia crassipes ), pennyworth ( Hydrocotyle
umbellata), duckweed (Lemna minor), and water velvet
(Azolla pinnata) (Salt et al. 1995).
Root Depth
Rhizofiltration occurs within the root zone in water. For
rhizofiltration to occur, the water must come into contact
with the roots. Engineered systems can be designed to maximize this contact zone by matching the depth of the unit to
the depth of the roots. Groundwater may be extracted from
any depth and piped to an engineered hydroponic system
for ex-situ treatment. The depth of treatable groundwater is
a function of the extraction system, not the rhizofiltration
treatment system.
• The floating aquatic plant water milfoil (Myriophyllum
spicatum), at a biomass density of 0.02 kg/L, rapidly
accumulated Ni, Cd, Cu, Zn, and Pb. The plant accumulated up to 0.5% Ni, 0.8% Cd, 1.3% Cu, 1.3% Zn,
and 5.5% Pb by weight (Wang et al. 1996).
• Wetland plants can be used in engineered or constructed
beds to take up or degrade contaminants. Hydroponically-grown plants concentrated Pb, Cr(VI), Cd, Ni, Zn,
and Cu onto their roots from wastewater. Lead had the
highest bioaccumulation coefficient, and zinc the lowest (Raskin et al. 1994).
For in situ technologies, such as natural water bodies,
the depth of the roots might not be the same as the depth of
the water body. The water must be adequately circulated in
such cases to ensure complete treatment, which is likely to
become more difficult as the depth of the water increases.
3.2.7
3.2.8
Site Considerations
In situ applications in water bodies are not likely to represent a disturbance or limitation to the use of a site because
site activities generally do not occur in water.
Plants
The following are examples of plants used in rhizofiltration
systems:
3.2.8.1
• Terrestrial plants can be grown and used hydroponically
in rhizofiltration systems. These plants generally have
a greater biomass and longer, faster-growing root systems than aquatic plants (Dushenkov et al. 1995). Seedlings have been proposed for use instead of mature
plants because seedings do not require light or nutrients for germination and growth for up to 2 weeks (Salt
et al. 1997).
Soil Conditions
Because this technology involves the hydroponic or
aquatic use of plants, soil use may be limited to raising
plants prior to installation. A layer of soil may be required on
a floating platform.
3.2.8.2
Ground and Surface Water
An ex-situ engineered system using rhizofiltration needs
to accommodate the predicted volume and discharge rate
of groundwater or surface water. Groundwater and surfacewater chemistry must be assessed to determine the interactions of the constituents in the water.
• Under hydroponic conditions, 5 dicots (broadleaf crops),
3 monocots (cereals), 11 cool season grasses, and 6
warm season grasses were each effective in accumulating Pb in their roots after three days of exposure to
300 mg/L Pb. The maximum lead concentration on a
dry weight basis was 17% in a cool season grass (colonial bentgrass), and the minimum was 6% in a warm
season grass (Japanese lawngrass). The dicot Indian
mustard (Brassica juncea) was also effective in taking
up other metals (Dushenkov et al. 1995).
Groundwater must be extracted prior to rhizofiltration. Ex
situ rhizofiltration of groundwater or surface water in an engineered system might also require pretreatment of the influent. Pretreatment could include pH adjustment, removal
or settling out of particulate matter, or other modification of
the water chemistry to improve the efficiency of rhizofiltration.
In situ applications such as the treatment of water bodies
might also require pretreatment, although this is likely to be
more difficult than with an engineered system due to the
potentially larger water volume and more complex configuration.
• Sunflowers (Helianthus annuus L.) removed concentrated Cr6+, Mn, Cd, Ni, Cu, U, Pb, Zn, and Sr in laboratory greenhouse studies (Salt et al. 1995). Sunflowers
also were more effective than Indian mustard (Brassica
juncea) and bean (Phaseolus coccineus) in removing
uranium. Bioaccumulation coefficients for uranium in
the sunflowers were much higher for the roots than for
the shoots (Dushenkov et al. 1997).
3.2.8.3
Climatic Conditions
The amount of precipitation is not important in this technology because the plants are grown in water and often in
greenhouses. The treated media (water) supplies the water
requirements of the plants.
• At a field site in Chernobyl, Ukraine, sunflowers were
grown for 4 to 8 weeks in a floating raft on a pond.
Bioaccumulation results indicated that sunflowers could
remove 137Cs and 90Sr from the pond.
3.2.9
Current Status
Rhizofiltration applications are currently at the pilot-scale
stage.
• Aquatic plants have been used in water treatment, but
they are smaller and have smaller, slower-growing root
20
Scientists from Rutgers University and Phytotech, Inc.,
have conducted laboratory, greenhouse, and field pilot-scale
rhizofiltration studies. Phytotech tested a pilot-scale
rhizofiltration system in a greenhouse at a DOE uraniumprocessing facility in Ashtabula, Ohio (Dushenkov et al.
1997). This engineered ex situ system used sunflowers to
remove uranium from contaminated groundwater and/or process water. Phytotech also conducted a small-scale field
test of rhizofiltration to remove radionuclides from a small
pond near the Chernobyl reactor, Ukraine, using sunflowers
floating on a raft.
or impact the dissociation of organic compounds. The plantaffected soil environment can convert metals from a soluble
to an insoluble oxidation state (Salt et al. 1995).
Phytostabilization can occur through sorption, precipitation,
complexation, or metal valence reduction (EPA 1997a).
Plants can also be used to reduce the erosion of metalcontaminated soil.
The term phytolignification has been used to refer to a
form of phytostabilization in which organic compounds are
incorporated into plant lignin (Cunningham et al. 1995b). Compounds can also be incorporated into humic material in soils
in a process likely related to phytostabilization in its use of
plant material.
The use of constructed wetlands for wastewater treatment and/or acid mine drainage is a related technology that
has a significant history of research and application. Ex
situ rhizofiltration in engineered systems might be the
phytoremediation technology that most often uses traditional
engineering methods.
3.3.2
Media
Phytostabilization is used in the treatment of soil, sediments, and sludges.
3.2.10 System Cost
3.3.3
Advantages
The cost of removing radionuclides from water by using
sunflowers has been estimated to be $2 to $6 per thousand
gallons of water.
Phytostabilization has the following advantages:
3.2.11 Selected References
• It has a lower cost and is less disruptive than other
more-vigorous soil remedial technologies.
• Soil removal is unnecessary.
Dushenkov, V., P. B. A. Nanda Kumar, H. Motto, and I.
Raskin. 1995. Rhizofiltration: The Use of Plants to Remove
Heavy Metals from Aqueous Streams. Environ. Sci. Technol.
29:1239-1245.
• Revegetation enhances ecosystem restoration.
• Disposal of hazardous materials or biomass is not required.
This study examined metals removal by roots of a variety of plant species. It provides bioaccumulation coefficents
and discusses the mechanisms of uptake. The study focuses on lead, but also provides information on other metals.
3.3.4
Disadvantages
Phytostabilization has the following disadvantages:
• The contaminants remain in place. The vegetation and
soil may require long-term maintenance to prevent rerelease of the contaminants and future leaching.
Dushenkov, S., D. Vasudev, Y. Kapulnik, D. Gleba, D.
Fleisher, K. C. Ting, and B. Ensley. 1997. Removal of Uranium from Water Using Terrestrial Plants. Environ. Sci.
Technol. 31(12):3468-3474.
• Vegetation may require extensive fertilization or soil
modification using amendments.
This research included growth chamber, greenhouse, and
field-scale studies for remediation of uranium-contaminated
water. Continuous operation and optimization of an ex-situ
system were examined.
• Plant uptake of metals and translocation to the
aboveground portion must be avoided.
• The root zone, root exudates, contaminants, and soil
amendments must be monitored to prevent an increase
in metal solubility and leaching.
3.3 Phytostabilization
3.3.1
Definition/Mechanism
• Phytostabilization might be considered to only be an
interim measure.
Phytostabilization is defined as (1) immobilization of a
contaminant in soil through absorption and accumulation
by roots, adsorption onto roots, or precipitation within the
root zone of plants, and (2) the use of plants and plant roots
to prevent contaminant migration via wind and water erosion, leaching, and soil dispersion.
• Contaminant stabilization might be due primarily to the
effects of soil amendments, with plants only contributing to stabilization by decreasing the amount of water
moving through the soil and by physically stabilizing
the soil against erosion.
Phytostabilization occurs through root-zone microbiology
and chemistry, and/or alteration of the soil environment or
contaminant chemistry. Soil pH may be changed by plant
root exudates or through the production of CO 2 .
Phytostabilization can change metal solubility and mobility
3.3.5
Applicable Contaminants/
Concentrations
Phytostabilization has not generally been examined in
terms of organic contaminants. The following is a discus-
21
sion of metals and metal concentrations, with implications
for phytostabilization:
The following grasses have been used to reduce metals
leaching (Salt et al. 1995):
• Arsenic: As (as arsenate) might be taken up by plants
because it is similar to the plant nutrient phosphate,
although poplar leaves in a field study did not accumulate significant amounts of As (Pierzynski et al. 1994).
Poplars were grown in soil containing an average of
1250 mg/kg As (Pierzynski et al. 1994).
• Colonial bentgrass (Agrostis tenuis cv Goginan) for acid
lead and zinc mine wastes.
• Colonial bentgrass (Agrostis tenuis cv Parys) for copper mine wastes.
• Red fescue (Festuca rubra cv Merlin) for calcareous
lead and zinc mine wastes.
• Cadmium: Cd might be taken up by plants because it
is similar to the plant nutrients Ca, Zn, although poplar
leaves in a field study did not accumulate significant
amounts of Cd (Pierzynski et al. 1994). Poplars were
grown in soil containing an average of 9.4 mg/kg Cd.
Plants were grown in mine waste containing up to 160
mg/kg Cd (Pierzynski et al. 1994).
Native and tame grasses and leguminous forbs including
big bluestem (Andropogon gerardi Vit.), tall fescue (Festuca
arundinacea Schreb.), and soybean [Glycine max (L.) Merr.]
were studied to determine their effectiveness in remediating
mine wastes (Pierzynski et al. 1994). In addition, hybrid
poplars were evaluated in a field study at a Superfund site
to determine their metal tolerance (Pierzynski et al. 1994).
• Chromium: Indian mustard (Brassica juncea) might be
able to reduce Cr6+ to Cr3+.
3.3.8
Site Considerations
• Copper: Mine wastes containing copper were stabilized by grasses (Salt et al. 1995).
Plants used will require long-term maintenance if sitespecific constraints prohibit reversal of the stabilization process.
• Mercury: Mercury might be one of the leading candidates for the phytostabilization of metals, although additional study is required (EPA 1997b).
3.3.8.1
Phytostabilization might be most appropriate for heavytextured soils and soils with high organic matter content
(Cunningham et al. 1995a). Phytostabilization can be performed after more active soil treatment technologies have
been tried. “Hot spots” of higher contaminant concentrations can be excavated and treated using other technologies, or landfilled. Soil amendments can also be used to
stabilize metals in soils. Amendments should be selected
that will maximize the growth of vegetation, which then also
helps to phytostabilize the soil (Berti and Cunningham, 1997).
• Lead: Pb in leachate was 22 g/mL in soil containing
Indian mustard (Brassica juncea) compared to 740 µg/
mL in soil without plants (Salt et al. 1995). Mine wastes
containing lead were stabilized by grasses (Salt et al.
1995). 625 µg/g Pb was used in a sand-Perlite mixture
that supported Indian mustard (Brassica juncea) (Salt
et al. 1995). Soil with 1660 mg/kg Pb had less than
50% plant cover. Plants in soil with 323 mg/kg Pb exhibited heavy chlorosis. Plants were grown in mine
waste containing up to 4500 mg/kg (Pierzynski et al.
1994).
3.3.8.2
3.3.8.3
Climatic Conditions
As discussed in Chapter 4, plans for remedial activities
must take into account the fact that phytoremediation systems can be severely impacted by weather conditions.
3.3.9
Root Depth
Current Status
The following are examples of typical phytostabilization
studies:
The root zone is the primary area affecting chemicallymoderated immobilization or root precipitation. Plants can
be selected for their root depth; for example, poplars can be
used for remediation of soil to a depth of 5 to 10 feet. The
impact of the roots may extend deeper into the soil, depending on the transport of root exudates to lower soil depths.
3.3.7
Ground and Surface Water
Soil water content, which can affect redox conditions in
the soil, must be appropriate for plant growth.
• Zinc: Mine wastes containing zinc were stabilized by
grasses (Salt et al. 1995). Soil with 4230 mg/kg Zn
had less than 50% plant cover. Plants in soil with 676
mg/kg Zn exhibited heavy chlorosis. Plants were grown
in mine waste containing up to 43,750 mg/kg Zn
(Pierzynski et al. 1994).
3.3.6
Soil Conditions
• Land affected by mining activities has been revegetated
with potentially useful plants. For example, a stabilizing
cover of vegetation was successfully established on
metalliferous mine wastes in the United Kingdom (Salt
et al. 1995).
Plants
Metal-tolerant plants are required for heavy-metal-contaminated soils. Brassica juncea has been shown to reduce
leaching of metals from soil by over 98% (Raskin et al.
1994).
• Phytostabilization using metal-tolerant grasses is being investigated for large areas of Cd- and Zn-contaminated soils at a Superfund site in Palmerton, PA. Experimental plots of poplars have been studied at the
22
Whitewood Creek Superfund site, SD, and vegetative
remediation has been proposed as part of the
remediation at the Galena Superfund site in southeastern KS (Pierzynski et al. 1994).
toremediation: A Novel Strategy for the Removal of Toxic
Metals from the Environment Using Plants. Biotechnol.
13:468-474.
This article is an introduction to the use of
phytoremediation technologies for reducing metals contamination. Field research is presented on the use of
plants to immobilize metals in soils. Bioavailability issues and mechanisms of plant accumulation are discussed in detail.
• The IINERT (In-Place Inactivation and Natural Ecological Restoration Technologies) Soil-Metals Action
team coordinated by EPA’s Jim Ryan and Dupont’s
Bill Berti under the RTDF program has used plants to
physically stabilize metal-contaminated soil in order to
decrease the off-site movement of contaminants.
3.4 Rhizodegradation
3.4.1
Definition/Mechanism
• Researchers at Kansas State University and Montana
State University, among others, are actively examining the use of vegetation in reclaiming sites contaminated by mining wastes.
Rhizodegradation is the breakdown of an organic contaminant in soil through microbial activity that is enhanced by
the presence of the root zone (Figure 3-2). Rhizodegradation
is also known as plant-assisted degradation, plant-assisted
bioremediation, plant-aided in situ biodegradation, and enhanced rhizosphere biodegradation.
3.3.10 System Cost
Cropping system costs have been estimated at $200 to
$10,000 per hectare, equivalent to $0.02 to $1.00 per cubic
meter of soil, based on a 1-meter root depth (Cunningham
et al. 1995b).
Root-zone biodegradation is the mechanism for implementing rhizodegradation. Root exudates are compounds produced by plants and released from plant roots. They include
sugars, amino acids, organic acids, fatty acids, sterols,
growth factors, nucleotides, flavanones, enzymes, and other
compounds (Shimp et al. 1993; Schnoor et al. 1995a). The
microbial populations and activity in the rhizosphere can be
increased due to the presence of these exudates, and can
result in increased organic contaminant biodegradation in
the soil. Additionally, the rhizosphere substantially increases
the surface area where active microbial degradation can be
stimulated. Degradation of the exudates can lead to
cometabolism of contaminants in the rhizosphere.
3.3.11 Selected References
Azadpour, A., and J. E. Matthews. 1996. Remediation of
Metal-Contaminated Sites Using Plants. Remed. Summer.
6(3):1-19.
This is a literature review of factors that affect metals
uptake by plants. It discusses plant tolerance to heavy
metals and summarizes work done on the use of plants
in soils that contain high levels of metal.
Cunningham, S. D., W. R. Berti, and J. W. Huang. 1995b.
Remediation of Contaminated Soils and Sludges by Green
Plants. pp. 33-54. In R.E. Hinchee, J. L. Means, and D. R.
Burris (eds.), Bioremediation of Inorganics. Battelle Press,
Columbus, OH.
Plant roots can affect soil conditions by increasing soil
aeration and moderating soil moisture content, thereby creating conditions more favorable for biodegradation by indigenous microorganisms. Thus, increased biodegradation
could occur even in the absence of root exudates. One study
raised the possibility that transpiration due to alfalfa plants
drew methane from a saturated methanogenic zone up into
the vadose zone where the methane was used by
methanotrophs that cometabolically degraded TCE
(Narayanan et al. 1995).
The chemistry of metals is discussed in this paper,
with a focus on lead. The article examines the stabilization and bioavailability of lead using sequential extractions. Phytoextraction of metals and
phytoremediation of organic contaminants are also discussed.
The chemical and physical effects of the exudates and
any associated increase in microbial populations might
change the soil pH or affect the contaminants in other ways.
Pierzynski, G. M., J. L. Schnoor, M. K. Banks, J. C. Tracy,
L. A. Licht, and L. E. Erickson. 1994. Vegetative Remediation at Superfund Sites. Mining and Its Environ. Impact
(Royal Soc. Chem. Issues in Environ. Sci. Technol. 1). pp.
49-69.
3.4.2
3.4.3
Media
Advantages
Rhizodegradation has the following advantages:
This paper discusses in detail the chemical and microbiological aspects of metal-contaminated soils. Two
case studies of the phytoremediation of mine waste
sites are presented along with a modeling discussion
of the fate of heavy metal in vegetated soils.
• Contaminant destruction occurs in situ.
• Translocation of the compound to the plant or atmosphere is less likely than with other phytoremediation
technologies since degradation occurs at the source of
the contamination.
Salt, D. E., M. Blaylock, P. B. A. Nanda Kumar, V.
Dushenkov, B. D. Ensley, I. Chet, and I. Raskin. 1995. Phy-
• Mineralization of the contaminant can occur.
23
Figure 3-2. Rhizodegradation.
• Low installation and maintenance cost as compared
to other remedial options.
3.4.4
• Organic matter from the plants may be used as a carbon source instead of the contaminant, which could
decrease the amount of contaminant biodegradation. In
laboratory sediment columns, debris from the salt marsh
plant Spartina alterniflora decreased the amount of oil
biodegradation. This could have been due to competition for limited oxygen and nutrients between the indigenous oil-degrading microorganisms and the microorganisms degrading plant organic matter (Molina et al.
1995).
Disadvantages
Rhizodegradation has the following disadvantages:
• Development of an extensive root zone is likely to require substantial time.
• Root depth can be limited due to the physical structure
or moisture conditions of the soil.
3.4.5
• The rhizosphere might effect an increase in the initial
rate of degradation compared to a nonrhizosphere soil,
but the final extent or degree of degradation might be
similar in both rhizosphere and nonrhizosphere soil.
Applicable Contaminants/
Concentrations
The following contaminants are amenable to
rhizodegradation:
• TPH (total petroleum hydrocarbons)
• Plant uptake can occur for many of the contaminants
that have been studied. Laboratory and field studies
need to account for other loss and phytoremediation
mechanisms that might complicate the interpretation
of rhizodegradation. For example, if plant uptake occurs, phytodegradation or phytovolatilization could occur in addition to rhizodegradation.
• The plants need additional fertilization because of microbial competition for nutrients.
- Several field sites contaminated with crude oil, diesel,
a heavier oil, and other petroleum products were studied for phytoremediation by examining TPH disappearance. Rhizodegradation and humification were the most
important disappearance mechanisms, with little plant
uptake occurring. Phytoremediation was able to bring
TPH levels to below the plateau level found with normal
(non-plant-influenced) bioremediation (Schwab 1998).
• The exudates might stimulate microorganisms that are
not degraders, at the expense of degraders.
- High initial petroleum hydrocarbon contents (2,000
to 40,000 mg/kg TPH) were studied at several field
24
- Propanil herbicide: An increased number of gramnegative bacteria were found in rhizosphere soil. It
was hypothesized that the best propanil degraders
would benefit from the proximity to plant roots and
exudates (Hoagland et al. 1994).
sites. Plant growth varied by species, but the presence of some species led to significantly greater TPH
disappearance than with other species or in
unvegetated soil (Schwab 1998).
• PAHs (polycyclic aromatic hydrocarbons)
- 2,4-D herbicide: Microorganisms capable of degrading 2,4-D occurred in elevated numbers in the rhizo
sphere of sugar cane, compared to nonrhizosphere
soil (Sandmann and Loos 1984). The rate constants
for 2,4-D biodegradation were higher in rhizosphere
soil than in nonrhizosphere soil (Boyle and Shann
1995).
- Chrysene, benzo(a)anthracene, benzo(a)pyrene, and
dibenzo(a,h)anthracene had greater disappearance
in vegetated soil than in nonvegetated soil (Aprill and
Sims 1990).
- Anthracene and pyrene had greater disappearance
in vegetated soils than in unvegetated soil (Reilley
et al. 1996).
- 2,4,5-T herbicide: The rate constants for 2,4,5-T biodegradation were higher in rhizosphere soil than in
nonrhizosphere soil (Boyle and Shann 1995).
- Pyrene was mineralized at a greater rate in a planted
system than in an unplanted system (Ferro et al.
1994a).
- Pyrene at 150 mg/kg was used in an experiment
with crested wheatgrass (Ferro et al. 1994b).
- Increased degradation of 0.3 g/g trifluralin, 0.5 g/g
atrazine, and 9.6 g/g metolachlor occurred in rhizosphere soil compared to nonrhizosphere soil (Anderson et al. 1994).
- Anthracene and pyrene at 100 mg/kg were used in
a study with grasses and a legume (Reilley et al.
1996).
- Parathion and diazinon at 5 g/g had greater mineralization in rhizosphere soil than in nonrhizosphere soil
(Hsu and Bartha 1979).
- 10 mg/kg PAH (chrysene, benzo(a)anthracene,
benzo(a)pyrene, dibenzo(a,h)anthracene) had
greater disappearance in vegetated soil than in
nonvegetated soil (Aprill and Sims 1990).
- Rhizosphere soil with 3 g/g propanil had increased
numbers of gram-negative bacteria that could rapidly transform propanil (Hoagland et al. 1994).
• Chlorinated solvents
- PAHs at 1,450 to 16,700 mg/kg (in soil also contaminated with PCP) strongly inhibited germination
and growth of eight species of grasses (Pivetz et al.
1997).
- Greater TCE mineralization was measured in vegetated soil as compared to nonvegetated soil (Anderson and Walton 1995).
- TCE and TCA dissipation was possibly aided by
rhizosphere biodegradation enhanced by the plant
roots (Narayanan et al. 1995).
• BTEX (Benzene, toluene, ethylbenzene, and xylenes)
- Soil from the rhizosphere of poplar trees had higher
populations of benzene-, toluene-, and o-xylene-de
grading bacteria than did nonrhizosphere soil. Root
exudates contained readily biodegradable organic
macromolecules (Jordahl et al. 1997).
- TCE at 100 and 200 g/L in groundwater was used in
a soil and groundwater system (Narayanan et al.
1995).
- TCA at 50 and 100 g/L in groundwater was used in
a soil and groundwater system (Narayanan et al. 1995).
• Pesticides
- Atrazine, metolachlor, and trifluralin herbicides: Soil
from the rhizosphere had increased degradation
rates compared to nonrhizosphere soil. The experiments were conducted in the absence of plants to
minimize effects of root uptake (Anderson et al.
1994).
• PCP (pentachlorophenol)
- PCP was mineralized at a greater rate in a planted
system than in an unplanted system (Ferro et al.
1994b).
- 100 mg PCP/kg soil was used in an experiment with
hycrest crested wheatgrass [Agropyron desertorum
(Fisher ex Link) Schultes] (Ferro et al. 1994b).
- Parathion and diazinon organophosphate insecticides: Mineralization rates of the radiolabeled compounds were higher in rhizosphere soil (soil with
roots) than in nonrhizosphere soil (soil without roots).
Diazinon mineralization in soil without roots did not
increase when an exudate solution was added, but
parathion mineralization did increase (Hsu and
Bartha 1979).
- Proso millet (Panicum miliaceum L.) seeds treated
with a PCP-degrading bacterium germinated and
grew well in soil containing 175 mg/L PCP, compared
to untreated seeds (Pfender 1996).
25
• Alfalfa (Medicago sativa) appears to have contributed
to the dissipation of TCE and TCA through exudates on
soil bacteria (Narayanan et al. 1995).
- PCP at 400 to 4100 mg/kg (in soil also contaminated
with PAHs) strongly inhibited germination and growth
of eight species of grasses (Pivetz et al. 1997).
• A legume [Lespedeza cuneata (Dumont)], Loblolly pine
[Pinus taeda (L.)], and soybean [Glycine max (L.) Merr.,
cv Davis] increased TCE mineralization compared to
nonvegetated soil (Anderson and Walton 1995).
• PCBs (polychlorinated biphenyls)
- Compounds (such as flavonoids and coumarins)
found in leachate from roots of specific plants stimulated the growth of PCB-degrading bacteria (Donnelly
et al. 1994; Gilbert and Crowley 1997).
• At a Gulf Coast field site, the use of annual rye and St.
Augustine grass led to greater TPH disappearance after 21 months than that experienced with the use of
sorghum or an unvegetated plot (Schwab 1998).
• Surfactants
- Linear alkylbenzene sulfonate (LAS) and linear alcohol ethoxylate (LAE) had greater mineralization
rates in the presence of root microorganisms than
in nonrhizosphere sediments (Federle and Schwab
1989).
• At one field site, although white clover did not survive
the second winter, concentrations of TPH were reduced
more than with tall fescue or bermudagrass with annual
rye, or bare field (Schwab 1998).
• PAH degradation occurred through the use of the following mix of prairie grasses: big bluestem (Andropogon
gerardi), little bluestem (Schizachyrium scoparius),
Indiangrass (Sorghastrum nutans), switchgrass (Panicum virgatum), Canada wild rye (Elymus canadensis),
western wheatgrass (Agropyron smithii), side oats grama
(Bouteloua curtipendula), and blue grama (Bouteloua
gracilis) (Aprill and Sims 1990).
- LAS and LAE at 1 mg/L had greater mineralization
rates in the presence of root microorganisms than
in nonrhizosphere sediments (Federle and Schwab
1989).
3.4.6
Root Depth
Because the rhizosphere extends only about 1 mm from
the root and initially the volume of soil occupied by roots is
a small fraction of the total soil volume, the soil volume
initially affected by the rhizosphere is limited. With time,
however, new roots will penetrate more of the soil volume
and other roots will decompose, resulting in additional exudates to the rhizosphere. Thus, the extent of rhizodegradation
will increase with time and with additional root growth. The
effect of rhizodegradation might extend slightly deeper than
the root zone. If the exudates are water soluble, not strongly
sorbed, and not quickly degraded, they may move deeper
into the soil. Contaminated groundwater can be affected if it
is within the influence of roots.
3.4.7
• Fescue (Festuca arundinacea Schreb), a cool-season
grass; sudangrass (Sorghum vulgare L.) and switchgrass (Panicum virgatum L.), warm-season grasses; and
alfalfa (Medicago sativa L.), a legume, were used to
study PAH disappearance; greater disappearance was
seen in the vegetated soils than in unvegetated soils
(Reilley et al. 1996).
• Hycrest crested wheatgrass [Agropyron desertorum
(Fischer ex Link) Schultes] increased mineralization
rates of PCP and pyrene relative to unplanted controls
(Ferro et al. 1994a, 1994b).
• In PAH- and PCP-contaminated soil, a mix of fescues
[hard fescue (Festuca ovina var. duriuscula), tall fescue (Festuca arundinacea), and red fescue (Festuca
rubra)] had higher germination rates and greater biomass relative to controls than did a mix of wheatgrasses
[western wheatgrass (Agropyron smithii) and slender
wheatgrass (Agropyron trachycaulum)] and a mix of little
bluestem ( Andropogon scoparius ), Indiangrass
(Sorghastrum nutans), and switchgrass (Panicum
virgatum) (Pivetz et al. 1997).
Plants
Plants that produce exudates that have been shown to
stimulate growth of degrading microorganisms or stimulate
cometabolism will be of more benefit than plants without
such directly useful exudates. The type, amount, and effectiveness of exudates and enzymes produced by a plant’s
roots will vary between species and even within subspecies or varieties of one species.
The following are examples of plants capable of
rhizodegradation:
• Bush bean (Phaseolus vulgaris cv. “Tender Green”) rhizosphere soil had higher parathion and diazinon mineralization rates than nonrhizosphere soil (Hsu and Bartha
1979).
• Red mulberry (Morus rubra L.), crabapple [Malus fusca
(Raf.) Schneid], and osage orange [Maclura pomifera
(Raf.) Schneid] produced exudates containing relatively
high levels of phenolic compounds, at concentrations
capable of stimulating growth of PCB-degrading bacteria (Fletcher and Hegde 1995).
• Rice (Oryza sativa L.) rhizosphere soil had increased
numbers of gram-negative bacteria, which were able to
rapidly transform propanil (Hoagland et al. 1994).
• Spearmint (Mentha spicata) extracts contained a compound that induced cometabolism of a PCB (Gilbert
and Crowley 1997).
• Kochia sp. rhizosphere soil increased the degradation
of herbicides relative to nonrhizosphere soil (Anderson
et al. 1994).
26
• Cattail (Typha latifolia) root microorganisms produced
greater mineralization rates of LAS and LAE than did
nonrhizosphere sediments (Federle and Schwab
1989).
Volume 563. American Chemical Society, Washington, DC.
249 pp.
This is a collection of 17 articles examining
rhizodegradation. The papers introduce the concepts
involved in rhizodegradation; discuss interactions between microorganisms, plants, and chemicals; and provide examples of rhizodegradation of industrial chemicals and pesticides.
• Hybrid poplar tree (Populus deltoides X nigra DN-34,
Imperial Carolina) rhizosphere soil contained significantly higher populations of total heterotrophs,
denitrifiers, pseudomonads, BTX degraders, and atrazine degraders than did nonrhizosphere soil (Jordahl et
al. 1997).
3.4.8
Site Considerations
3.4.8.1
Soil Conditions
Anderson, T. A., E. A. Guthrie, and B. T. Walton. 1993.
Bioremediation in the Rhizosphere. Environ. Sci. Technol.
27:2630-2636.
This literature review summarizes research work conducted on a variety of contaminants (pesticides, chlorinated solvents, petroleum products, and surfactants).
The physical and chemical soil conditions must allow for
significant root penetration and growth.
3.4.8.2
Ground and Surface Water
Anderson, T. A., and B. T. Walton. 1995. Comparative Fate
of [14c]trichloroethylene in the Root Zone of Plants from a
Former Solvent Disposal Site. Environ. Toxicol. Chem.
14:2041-2047.
Although rhizodegradation is primarily soil-based, groundwater movement can be induced by the transpiration of plants
bringing contaminants from the groundwater into the root
zone.
3.4.8.3
Exposure chambers within an environmental chamber
were used with a variety of plant types and with radiolabeled TCE. Mineralization rates were greater in vegetated
soils than in unvegetated soils.
Climatic Conditions
Field studies that include rhizodegradation as a component have been conducted under in a wide variety of climates including the humid south, arid west, and the cold
north.
3.4.9
Aprill, W., and R. C. Sims. 1990. Evaluation of the Use of
Prairie Grasses for Stimulating Polycyclic Aromatic Hydrocarbon Treatment in Soil. Chemosphere. 20:253-265.
Current Status
The following list provides information on the status or application of rhizodegradation studies:
Eight prairie grasses were examined using chambers
constructed of 25-cm-diameter PVC pipe. PAH-spiked
soil at 10 mg PAH/kg soil was added to the chambers
prior to seeding. Soil, leachate, and plant tissue samples
were collected during the study. PAH disappearance was
greater in planted chambers compared to unplanted
chambers.
• Rhizodegradation was first extensively studied in relation
to the biodegradation of pesticides in agricultural soils.
• Numerous laboratory and greenhouse studies and several field studies have been conducted, including a field
study conducted at the McCormick & Baxter Superfund
Site.
Ferro, A. M., R. C. Sims, and B. Bugbee. 1994a. Hycrest
Crested Wheatgrass Accelerates the Degradation of Pentachlorophenol in Soil. J. Environ. Qual. 23:272-279.
• “Hot spots” of higher contaminant concentrations can be
excavated and treated using other technologies, or
landfilled. Rhizodegradation could be applied as a polishing or final step after active land treatment bioremediation
has ended.
A growth-chamber study conducted using radiolabeled
pentachlorophenol indicated that mineralization was
greater in planted systems than in unplanted systems.
• A TPH/PAH subgroup has been established as part of
the RTDF Phytoremediation of Organics Action Team to
examine rhizodegradation. The Petroleum Environmental
Research Forum is also examining rhizodegradation in
the phytoremediation of petroleum hydrocarbons.
Fletcher, J. S., and R. S. Hegde. 1995. Release of Phenols by Perennial Plant Roots and their Potential Importance in Bioremediation. Chemosphere. 31:3009-3016.
Greenhouse studies identified chemical and microbiological evidence for the occurrence of rhizodegradation.
The potential for biodegradation within the root zone
was determined to be dependent on the particular plant
species and exudates produced by the plant.
3.4.10 System Cost
Cost information for rhizodegradation is incomplete at this
time.
3.4.11 Selected References
Schnoor, J. L., L. A. Licht, S. C. McCutcheon, N. L. Wolfe,
and L. H. Carreira. 1995a. Phytoremediation of Organic and
Nutrient Contaminants. Environ. Sci. Technol. 29:318A-323A.
Anderson, T. A., and J. R. Coats (eds.). 1994. Bioremediation
Through Rhizosphere Technology, ACS Symposium Series,
27
This paper introduces the important concepts for
rhizodegradation and phytodegradation, including the
role of plant enzymes. Laboratory and field research for
TNT, pesticides, and nutrient contaminants is summarized. Applications and limitations of phytoremediation
are discussed, and field applications of phytoremediation
are tabulated.
Plant uptake of organic compounds can also depend on
type of plant, age of contaminant, and many other physical and chemical characteristics of the soil. Definitive conclusions cannot always be made about a particular chemical. For example, when PCP was spiked into soil, 21%
was found in roots and 15% in shoots after 155 days in the
presence of grass (Qiu et al. 1994); in another study, several plants showed minimal uptake of PCP (Bellin and
O’Connor 1990).
Schwab, A. P. 1998. Phytoremediation of Soils Contaminated with PAHs and Other Petroleum Compounds. Presented at: Beneficial Effects of Vegetation in Contaminated
Soils Workshop, Kansas State University, Manhattan, KS,
January 7-9, 1998. Sponsored by Great Plains/Rocky
Mountain Hazardous Substance Research Center.
3.5.1.2
Metabolism
Metabolism within plants has been identified for a diverse
group of organic compounds, including the herbicide atrazine (Burken and Schnoor 1997), the chlorinated solvent TCE
(Newman et al. 1997a), and the munition TNT (Thompson et
al. 1998). Other metabolized compounds include the insecticide DDT, the fungicide hexachlorobenzene (HCB), PCP, the
plasticizer diethylhexylphthalate (DEHP), and PCBs in plant
cell cultures (Komossa et al. 1995).
This presentation summarizes the methods and results of field test plots at a variety of geographic and
climatic regions. Dissipation of TPH was greater in
planted plots than in unplanted plots, and differences
were seen in the growth and effectiveness of different
plant species.
3.5.1.3 Plant-Formed Enzymes
3.5 Phytodegradation
3.5.1
Definition/Mechanism
Plant-formed enzymes have been identified for their potential use in degrading contaminants such as munitions,
herbicides, and chlorinated solvents. Immunoassay tests
have been used to identify plants that produce these enzymes (McCutcheon 1996).
Phytodegradation (also known as phytotransformation)
is the breakdown of contaminants taken up by plants
through metabolic processes within the plant, or the breakdown of contaminants external to the plant through the
effect of compounds (such as enzymes) produced by the
plants. As shown in Figure 3-3, the main mechanism is
plant uptake and metabolism. Additionally, degradation may
occur outside the plant, due to the release of compounds
that cause transformation. Any degradation caused by microorganisms associated with or affected by the plant root
is considered rhizodegradation.
3.5.2
Media
Phytodegradation is used in the treatment of soil, sediments, sludges, and groundwater. Surface water can also
be remediated using phytodegradation.
3.5.3
Advantages
Contaminant degradation due to enzymes produced by a
plant can occur in an environment free of microorganisms
(for example, an environment in which the microorganisms
have been killed by high contaminant levels). Plants are
able to grow in sterile soil and also in soil that has concentration levels that are toxic to microorganisms. Thus,
phytodegradation potentially could occur in soils where biodegradation cannot.
3.5.1.1 Uptake
For phytodegradation to occur within the plant, the compounds must be taken up by the plant. One study identified more than 70 organic chemicals representing many
classes of compounds that were taken up and accumulated by 88 species of plants and trees (Paterson et al.
1990). A database has been established to review the
classes of chemicals and types of plants that have been
investigated in regard to their uptake of organic compounds
(Nellessen and Fletcher 1993b).
3.5.4
Disadvantages
Phytodegradation has the following disadvantages:
• Toxic intermediates or degradation products may form.
In a study unrelated to phytoremediation research, PCP
was metabolized to the potential mutagen
tetrachlorocatechol in wheat plants and cell cultures
(Komossa et al. 1995).
Uptake is dependent on hydrophobicity, solubility, and
polarity. Moderately hydrophobic organic compounds (with
log kow between 0.5 and 3.0) are most readily taken up by
and translocated within plants. Very soluble compounds
(with low sorption) will not be sorbed onto roots or translocated within the plant (Schnoor et al. 1995a). Hydrophobic
(lipophilic) compounds can be bound to root surfaces or
partitioned into roots, but cannot be further translocated
within the plant (Schnoor et al. 1995a; Cunningham et al.
1997). Nonpolar molecules with molecular weights <500
will sorb to the root surfaces, whereas polar molecules will
enter the root and be translocated (Bell 1992).
• The presence or identity of metabolites within a
plant might be difficult to determine; thus contaminant destruction could be difficult to confirm.
3.5.5
Applicable Contaminants/
Concentrations
Organic compounds are the main category of contaminants subject to phytodegradation. In general,
28
Figure 3-3. Phytodegradation.
ter containing about 50 ppm TCE and metabolized the TCE within the tree (Newman et al. 1997a).
organic compounds with a log kow between 0.5 and 3.0 can
be subject to phytodegradation within the plant. Inorganic
nutrients are also remediated through plant uptake and metabolism. Phytodegradation outside the plant does not depend on log kow and plant uptake.
3.5.5.1
- Minced horseradish roots successfully treated wastewater containing up to 850 ppm of 2,4-dichlorophenol
(Dec and Bollag 1994).
Organics
• Herbicides
• Chlorinated solvents
- Atrazine in soil was taken up by trees and then hydrolyzed and dealkylated within the roots, stems, and
leaves. Metabolites were identified within the plant
tissue, and a review of atrazine metabolite toxicity
studies indicated that the metabolites were less toxic
than atrazine (Burken and Schnoor 1997).
- The plant-formed enzyme dehalogenase, which
can dechlorinate chlorinated compounds, has
been discovered in sediments (McCutcheon
1996).
- TCE was metabolized to trichloroethanol, trichloroacetic acid, and dichloroacetic acid within hybrid
poplar trees (Newman et al. 1997a). In a similar
study, hybrid poplar trees were exposed to wa-
- The plant-formed enzyme nitrilase, which can degrade herbicides, has been discovered in sediments (Carreira 1996).
29
3.5.7
- A qualitative study indicated that the herbicide
bentazon was degraded within black willow
trees, as indicated by bentazon loss during a nurs
ery study and by identification of metabolites
within the tree. Bentazon was phytotoxic to six tree
species at concentrations of 1000 and 2000 mg/L.
At 150 mg/kg, bentazon metabolites were detected
within tree trunk and canopy tissue samples (Conger and Portier 1997).
The aquatic plant parrot feather ( Myriophyllum
aquaticum) and the algae stonewort (Nitella) have been
used for the degradation of TNT. The nitroreductase enzyme has also been identified in other algae, ferns, monocots, dicots, and trees (McCutcheon 1996).
Degradation of TCE has been detected in hybrid poplars and in poplar cell cultures, resulting in production of
metabolites and in complete mineralization of a small portion of the applied TCE (Gordon et al. 1997; Newman et
al. 1997a). Atrazine degradation has also been confirmed
in hybrid poplars (Populus deltoides x nigra DN34, Imperial Carolina) (Burken and Schnoor 1997). Poplars have
also been used to remove nutrients from groundwater (Licht
and Schnoor 1993).
- Atrazine at 60.4 g/kg (equivalent to about 3 times
field application rates) was used to study phytodegradation in hybrid poplars (Burken and Schnoor
1997).
- The herbicide bentazon was phytotoxic at concentrations of 1,000 to 2,000 mg/L, but allowed growth
at 150 mg/L (Conger and Portier 1997).
Black willow (Salix nigra), yellow poplar (Liriodendron
tulipifera), bald cypress (Taxodium distichum), river birch
(Betula nigra), cherry bark oak (Quercus falcata), and live
oak (Quercus viginiana) were able to support some degradation of the herbicide bentazon (Conger and Portier
1997).
• Insecticides
- The isolation from plants of the enzyme phosphatase, which can degrade organophosphate
insecticides,may have phyotodegradation applications (McCutcheon 1996).
• Munitions
Site Considerations
3.5.8.1
Soil Conditions
3.5.8.2
Ground and Surface Water
Groundwater that can be extracted by tree roots or that
is pumped to the surface may be treated by this system.
Phytodegradation can also occur in surface water, if the
water is able to support the growth of appropriate plants.
- Hybrid poplar trees metabolized TNT to 4-amino2,6-dinitrotoluene (4-ADNT), 2-amino-4,6dinitrotoluene (2-ADNT), and other unidentified
compounds (Thompson et al. 1998).
3.5.8.3
Climatic Conditions
Phytoremediation studies involving phytodegradation
have been conducted under a wide variety of climatic conditions.
- TNT concentrations in flooded soil decreased from
128 to 10 ppm with parrot feather (Schnoor et
al. 1995b).
3.5.9
• Phenols
Current Status
Research and pilot-scale studies have been conducted
primarily at Army Ammunition Plants (AAPs). These demonstrations include field studies at the Iowa AAP, Volunteer AAP, and Milan AAP (McCutcheon 1996).
- Chlorinated phenolic concentrations in wastewater decreased in the presence of oxidoreductase
enzymes in minced horseradish roots (Dec and
Bollag 1994).
3.5.10 System Costs
Inorganics
Cost information has not been reported.
• Nutrients
3.5.11
- Nitrate will be taken up by plants and transformed to
proteins and nitrogen gas (Licht and Schnoor 1993).
3.5.6
3.5.8
Phytodegradation is most appropriate for large areas of
soil having shallow contamination.
- The plant-formed enzyme nitroreductase, which
can degrade munitions, has been discovered in
sediments; this enzyme, from parrot feather, degraded TNT (McCutcheon 1996).
3.5.5.2
Plants
Selected References
Bell, R. M. 1992. Higher Plant Accumulation of Organic
Pollutants from Soils. Risk Reduction Engineering Laboratory, Cincinnati, OH. EPA/600/R-92/138.
Root Depth
This paper includes an extensive literature review of the
behavior of organic contaminants in plant-soil systems
and the uptake of contaminants by plant. A wide variety
of plant species and contaminant types are covered in
Phytodegradation is generally limited to the root zone,
and possibly below the root zone if root exudates are
soluble, nonsorbed, and transported below the root zone.
The degree to which this occurs is uncertain.
30
cesses, Correlations and Models. Chemosphere. 21:297331.
the paper. Tables and graphs in the reviewed literature
provide quantitative information on plant uptake. Experiments conducted on plant uptake of hexachlorobenzene,
phenol, toluene, and TCE are described in depth.
The routes of entry (root uptake and foliar uptake) of
organic compounds into plants are discussed. Equations
are presented that correlate the concentration in various parts of a plant to the octanol-water partition coefficient, molecular weight, or Henry’s Law constant. A review of plant uptake models is also included. Crossedreferenced tables are included that identify the literature
citations for plant uptake research conducted on different plant species and on different chemical compounds.
Burken, J. G., and J. L. Schnoor. 1997. Uptake and Metabolism of Atrazine by Poplar Trees. Environ. Sci. Technol.
31:1399-1406.
This presentation describes poplar trees grown in soil or
sand that took up, hydrolyzed, and dealkylated radiolabeled atrazine to less-toxic compounds. Metabolism was
found to occur in roots, stems, and leaves, and the
amount of metabolism increased with increased time in
plant tissue. In leaves, the atrazine parent compound
was found to be 21% of the radiolabel at 50 days, and
10% at 80 days. In the sand planting, uptake of the radiolabel was 27.8% at 52 days and 29.2% at 80 days.
Less than 20% of radiolabel remained as bound residue
in plant tissue. Atrazine degradation in unplanted soil
was similar to degradation in planted soil. A model for
atrazine metabolism was also presented.
Thompson, P. L., L. A. Ramer, and J. L. Schnoor. 1998.
Uptake and Transformation of TNT by Hybrid Poplar Trees.
Environ. Sci. Technol. 32:975-980.
In these laboratory experiments, hybrid poplars and radiolabeled TNT were used in hydroponic and soil systems. Much of the TNT was bound in the roots, with relative little (<10%) translocation within the tree. Metabolites of TNT were found within the plant tissue.
3.6 Phytovolatilization
3.6.1 Definition/Mechanism
McCutcheon, S. 1996. Phytoremediation of Organic Compounds: Science Validation and Field Testing. In W. W.
Kovalick and R. Olexsey (eds.), Workshop on
Phytoremediation of Organic Wastes, December 17-19,
1996, Ft. Worth, TX. An EPA unpublished meeting summary.
Phytovolatilization (Figure 3-4) is the uptake and transpiration of a contaminant by a plant, with release of the
contaminant or a modified form of the contaminant to the
atmosphere from the plant through contaminant uptake,
plant metabolism, and plant transpiration.
Phytodegradation is a related phytoremediation process
that can occur along with phytovolatilization.
An overview of the uses, advantages, and disadvantages
of phytoremediation are presented along with the identification and use of plant-derived enzymes for
photodegradation. Field demonstrations at several Army
Ammunition Plants are also discussed.
3.6.2
Media
Phytovolatilization has mainly been applied to groundwater, but it can be applied to soil, sediments, and sludges.
Newman, L. A., S. E. Strand, N. Choe, J. Duffy, G. Ekuan,
M. Ruszaj, B. B. Shurtleff, J. Wilmoth, P. Heilman, and M.
P. Gordon. 1997a. Uptake and Biotransformation of Trichloroethylene by Hybrid Poplars. Environ. Sci. Technol.
31:1062-1067.
3.6.3
Advantages
Phytovolatilization has the following advantages:
Discussions are presented of axenic poplar tumor cell
cultures dosed with TCE and samples that were analyzed for degradation products and 14CO2. The cells studied metabolized TCE to trichloroethanol and di- and
trichloracetic acid. The cell cultures oxidized 1 to 2% of
the TCE to CO2 in 4 days. Whole trees were exposed to
50 ppm TCE. Leaves were bagged and the entrapped
air sampled for TCE. Plant parts were harvested and
analyzed for TCE and metabolites. TCE-exposed trees
had significant TCE in stems but minimal amounts in
leaves. Equal concentrations of trichloroethanol and TCE
were found in leaves, but a smaller concentration of
trichloroethanol than TCE was found in stems. Trichloroacetic acid appeared in stems and leaves. Roots contained TCE, trichloroacetic acid, dichloroacetic acid, and
trichloroethanol. TCE was transpired from the trees.
• Contaminants could be transformed to less-toxic forms,
such as elemental mercury and dimethyl selenite gas.
• Contaminants or metabolites released to the atmosphere
might be subject to more effective or rapid natural degradation processes such as photodegradation.
3.6.4
Disadvantages
Phytovolatilization has the following disadvantages:
• The contaminant or a hazardous metabolite (such as
vinyl chloride formed from TCE) might be released into
the atmosphere. One study indicated TCE transpiration, but other studies found no transpiration.
• The contaminant or a hazardous metabolite might accumulate in vegetation and be passed on in later products such as fruit or lumber. Low levels of metabolites
have been found in plant tissue (Newman et al. 1997a).
Paterson, S., D. Mackay, D. Tam, and W. Y. Shiu. 1990.
Uptake of Organic Chemicals by Plants: A Review of Pro-
31
Figure 3-4. Phytovolatilization.
3.6.5
3.6.5.1
3.6.5.2
Applicable Contaminants/
Concentrations
Inorganics
The inorganic contaminants Se and Hg, along with
As, can form volatile methylated species (Pierzynski et
al. 1994). Selenium has been taken up and transpired
at groundwater concentrations of 100 to 500 µg/L
(Bañuelos et al. 1997a) and at soil concentrations of 40
mg/L (Bañuelos et al. 1997b). Genetically engineered
plants were able to germinate and grow in 20-ppm Hg++
and then volatilize the Hg; 5 to 20 ppm Hg++ was phytotoxic to unaltered plants (Meagher and Rugh 1996).
Organics
Chlorinated solvents include TCE, 1,1,1-trichloroethane
(TCA) and carbon tetrachloride (Newman et al. 1997a,
1997b; Narayanan et al. 1995). In two years, hybrid poplars removed >97% of the 50-ppm TCE from the water
(Newman et al. 1997b). 100 and 200 µg/L TCE in groundwater was studied using alfalfa (Narayanan et al. 1995).
50 and 100 µg/L TCE in groundwater were studied using
alfalfa (Narayanan et al. 1995). In one year, 95% of 50ppm carbon tetrachloride was removed by hybrid poplars
(Newman et al. 1997b).
3.6.6
Root Depth
The contaminant has to be within the influence of the
root of the plant. Since groundwater is the target me32
dia, contaminated groundwater upgradient of the plants
may flow into the area of influence of the plants. Contaminated water may also be pumped and watered on
plants.
3.6.7
chlorinated solvents. A SITE demonstration project has
been started at the Carswell Site, Fort Worth, TX using
poplars to phytoremediate TCE-contaminated groundwater and to examine the possible fate of the TCE, including
volatilization.
Plants
A significant amount of research, including field testing
and application, has been conducted on selenium volatilization.
Plants used for phytovolatilization include:
• University of Washington researchers have extensively
studied the use of poplars in the phytoremediation of
chlorinated solvents. In these studies, transformation
of TCE was found to occur within the trees (Newman
et al. 1997a).
3.6.10 System Costs
Cost information is being collected as part of the SITE
demonstration project at the Carswell Site.
• Alfalfa (Medicago sativa) has been studied by Kansas
State University researchers for its role in the
phytovolatilization of TCE.
3.6.11 Selected References
Bañuelos, G. S., H. A. Ajwa, N. Terry, and S. Downey.
1997a. Abstract: Phytoremediation of Selenium-Laden Effluent. Fourth International In Situ and On-Site
Bioremediation Symposium, April 28 - May 1, 1997, New
Orleans, LA. 3:303.
• Black locust species were studied for use in
remediating TCE in groundwater (Newman et al.
1997b).
This abstract summarizes the methods used in field investigations of the use of Brassica napus (canola) to
remediate water contaminated with selenium. These field
studies included an investigation of the volatilization of
selenium by the plants.
• Indian mustard (Brassica juncea) and canola (Brassica napus) have been used in the phytovolatilization
of Se. Selenium (as selenate) was converted to lesstoxic dimethyl selenite gas and released to the atmosphere (Adler 1996). Kenaf (Hibiscus cannabinus L.
cv. Indian) and tall fescue (Festuca arundinacea
Schreb cv. Alta) have also been used to take up Se,
but to a lesser degree than canola (Bañuelos et al.
1997b).
Bañuelos, G. S., H. A. Ajwa, B. Mackey, L. L. Wu, C.
Cook, S. Akohoue, and S. Zambrzuski. 1997b. Evaluation
of Different Plant Species Used for Phytoremediation of
High Soil Selenium. J. Environ. Qual. 26:639-646.
• A weed from the mustard family (Arabidopsis thaliana)
genetically modified to include a gene for mercuric reductase converted mercuric salts to metallic mercury
and released it to the atmosphere (Meagher and Rugh
1996).
3.6.8
This evaluation discusses three plant species (canola,
kenaf, and tall fescue) grown in seleniferous soil under greenhouse conditions. Total soil selenium was
significantly reduced by each species. A partial mass
balance indicated that some selenium was lost by a
mechanism that was not measured. Selenium volatilization was hypothesized as the cause of the decrease
in soil concentration.
Site Considerations
Because phytovolatilization involves the transfer of contaminants to the atmosphere, the impact of this contaminant transfer on the ecosystem and on human health needs
to be addressed.
3.6.8.1
Meagher, R. B., and C. Rugh. 1996. Abstract: Phytoremediation of Mercury Pollution Using a Modified Bacterial
Mercuric Ion Reductase Gene. International Phytoremediation
Conference, May 8-10, 1996, Arlington, VA. International
Business Communications, Southborough, MA.
Soil Conditions
For significant transpiration to occur, the soil must be
able to transmit sufficient water to the plant.
3.6.8.2
This abstract describes transgenic plants developed
to reduce mercuric ion to metallic mercury, which was
then volatilized, and additional plants developed to
process methyl mercury to metallic mercury.
Ground and Surface Water
Groundwater must be within the influence of the plant
(usually a tree) roots.
3.6.8.3
Newman, L. A., S. E. Strand, N. Choe, J. Duffy, G. Ekuan,
M. Ruszaj, B. B. Shurtleff, J. Wilmoth, P. Heilman, and M.
P. Gordon. 1997a. Uptake and Biotransformation of Trichloroethylene by Hybrid Poplars. Environ. Sci. Technol.
31:1062-1067.
Climatic Conditions
Climatic factors such as temperature, precipitation, humidity, insolation, and wind velocity can affect transpiration
rates.
3.6.9
Current Status
Whole trees were exposed to 50 ppm TCE and bags
were placed around leaves. Analysis of the entrapped
air indicated that TCE was transpired from the trees.
Several research groups are performing active laboratory and field studies of TCE phytovolatilization and other
33
Newman, L. A., C. Bod, N. Choe, R. Crampton, R.
Cortellucci, D. Domroes, S. Doty, J. Duffy, G. Ekuan, D.
Fogel, R. Hashmonay, P. Heilman, D. Martin, I.A.
Muiznieks, T. Newman, M. Ruszaj, T. Shang, B. Shurtleff,
S. Stanley, S. E. Strand, X. Wang, J. Wilmoth, M. Yost,
and M. P. Gordon. 1997b. Abstract: Phytoremediation of
Trichloroethylene and Carbon Tetrachloride: Results from
Bench to Field. Presentation 55. In 12th Annual Conference on Hazardous Waste Research - Abstracts Book, May
19-22, 1997, Kansas City, MO.
3.7.2
Media
Hydraulic control is used in the treatment of groundwater, surface water, and soil water.
3.7.3
Advantages
Hydraulic control has the following advantages:
• An engineered pump-and-treat system does not need
to be installed.
Axenic poplar cell culture, metabolic chamber-grown
rooted cuttings, and pilot-scale systems for the
phytoremediation of TCE and carbon tetrachloride are
briefly described. Oxidation of TCE, update and transpiration of TCE and carbon tetrachloride, and removal of
TCE and carbon tetrachloride from water under field
conditions are discussed.
• Costs will be lower.
• Roots will penetrate into and be in contact with a much
greater volume of soil than if a pumping well is used.
3.7.4
Disadvantages
Hydraulic control has the following disadvantages:
3.7 Hydraulic Control
3.7.1 Definition/Mechanism
• Water uptake by plants is affected by climatic and seasonal conditions; thus, the rate of water uptake will not
be constant. Water uptake by deciduous trees will slow
considerably during winter.
Hydraulic control is the use of plants to remove groundwater through uptake and consumption in order to contain
or control the migration of contaminants (Figure 3-5). Hydraulic control is also known as phytohydraulics or hydraulic
plume control.
• Groundwater removal is limited by the root depth of
the vegetation.
Figure 3-5. Hydraulic control of contaminated plume.
34
3.7.5
mates of the rate of water withdrawal by plants are given
below.
Applicable Contaminants/
Concentrations
Water-soluble leachable organics and inorganics are
used at concentrations that are not phytotoxic. Poplar trees
were used to form a barrier to groundwater movement at a
site contaminated with gasoline and diesel (Nelson 1996).
3.7.6
• Poplar trees on a landfill in Oregon transpired 70 acreinches of water per acre of trees (Wright and Roe
1996).
• Two 40-foot-tall cottonwood trees in southwestern Ohio
pumped 50 to 350 gallons per day (gpd) per tree, based
on calculations using observed water-table drawdown
(Gatliff 1994).
Root Depth
Hydraulic control by plants occurs within the root zone
or within a depth influenced by roots, for example:
• A 5-year-old poplar tree can transpire between 100
and 200 L water per day (Newman et al. 1997a).
• The effective rooting depth of most crops is 1 to 4 feet.
Trees and other vegetation can be used to remediate
groundwater in water table depths of 30 feet or less
(Gatliff 1994).
• Young poplars were estimated to transpire about 8 gpd
per tree, based on the observed water table drawdown
(Nelson 1996).
• Plant roots above the water table can influence contaminants in the groundwater by interfacing through
the capillary fringe. Fe, Tc, U, and P diffused upward
from the water table and were absorbed by barley roots
that were 10 cm (3.9 in) above the water table interface (Sheppard and Evenden 1985).
• Mature phreatophyte trees were estimated to use 200
to 400 gpd (Sloan and Woodward 1996).
3.7.8.3
The amount of precipitation, temperature, and wind may
affect the transpiration rate of vegetation.
• The placement depth of roots during planting can be
varied. Root depth, early tree growth, and nitrogen accumulation were enhanced by placing poplar tree root
balls closer to shallow groundwater during planting
(Gatliff 1994).
3.7.7
Climatic Conditions
3.7.9
Current Status
Several U.S. companies have installed phytoremediation
systems that have successfully incorporated hydraulic control.
Plants
The following plants are used in hydraulic control:
3.7.10 System Cost
• Cottonwood and hybrid poplar trees were used at
seven sites in the East and Midwest to contain and
treat shallow groundwater contaminated with heavy
metals, nutrients, or pesticides (Gatliff 1994). Poplars
were used at a site in Utah to contain groundwater
contaminated with gasoline and diesel (Nelson 1996).
Passive gradient control was studied at the French
Limited Superfund site using a variety of phreatophyte
trees; native nondeciduous trees were found to perform the best (Sloan and Woodward 1996).
3.7.8
Estimated costs for remediating an unspecified contaminant in a 20-foot-deep aquifer at a 1-acre site were $660,00
for conventional pump-and-treat, and $250,000 for
phytoremediation using trees (Gatliff 1994).
3.7.11
Gatliff, E. G. 1994. Vegetative Remediation Process Offers Advantages Over Traditional Pump-and-Treat Technologies. Remed. Summer. 4(3):343-352.
Site Considerations
A summary is presented of the impact of poplar or cottonwood trees to influence a shallow water table at sites
along the East Coast and in the Midwest that were contaminated with pesticides, nutrients, or heavy metals. The
contribution of the trees to water table drawdown was
measured at some sites. Information is presented on the
decrease in contaminant concentrations at some of the
sites.
The establishment of trees or other vegetation is likely
to require a larger area than would be required for the installation of a pumping well.
3.7.8.1
Soil Conditions
The primary considerations for selecting hydraulic control as the method of choice are the depth and concentration of contaminants that affect plant growth. Soil texture
and degree of saturation are influential factors. Planting technique and materials can extend the influence of plants
through non-saturated zones to water-bearing layers.
3.7.8.2
Selected References
Wright, A. G., and A. Roe. 1996. It’s Back to Nature for
Waste Cleanup. ENR. July 15. pp. 28-29.
A poplar tree system for landfill leachate collection and
treatment is described. The trees use up to 70 inches of
water per acre per year. A proposed project at another
landfill is presented.
Ground and Surface Water
The amount of water transpired by a tree depends on
many factors, especially the size of the tree. Some esti-
35
3.8 Vegetative Cover Systems
3.8.1 Definition/Mechanism
degradation of underlying waste. Risk reduction relies on
the degradation of contaminants, the isolation of contaminants to prevent human or wildlife exposure, and the reduction of leachate formation or movement. Mechanisms
include water uptake, root-zone microbiology, and plant
metabolism. The phytoremediation cover incorporates certain aspects of hydraulic control, phytodegradation,
rhizodegradation, phytovolatilization, and perhaps
phytoextraction. Figure 3-7 presents the evolution of a
phytoremediation cover as it moves from a remediation function to a water exclusion function.
A vegetative cover is a long-term, self-sustaining system of plants growing in and/or over materials that pose
environmental risk; a vegetative cover may reduce that
risk to an acceptable level and, generally, requires minimal maintenance. There are two types of vegetative covers: the Evapotranspiration (ET) Cover and the
Phytoremediation Cover.
• Evapotranspiration Cover: A cover composed of soil
and plants engineered to maximize the available storage capacity of soil, evaporation rates, and transpiration processes of plants to minimize water infiltration.
The evapotranspiration cap is a form of hydraulic control by plants. Risk reduction relies on the isolation of
contaminants to prevent human or wildlife exposure
and the reduction of leachate formation or movement.
Fundamentally, an ET cover is a layer of monolithic
soil with adequate soil thickness to retain infiltrated
water until it is removed by evaporation and transpiration mechanisms. Mechanisms include the uptake and
storage of water in soil and vegetation. An ET cover is
one type of a water-balance cover, illustrated in Figure 3-6.
In limited cases, vegetative covers may be used as an
alternative to traditional covers that employ a resistive barrier (i.e., a multilayered cover with a relatively impermeable component). Vegetative covers may be appropriate
to address contaminated surface soil or sludge, certain
waste disposal units, waste piles, and surface impoundments.
In general, the application of any cover system should
provide the following functions:
• isolate underlying waste from direct human or wildlife
exposure (e.g., prevent burrowing animals from reaching the contaminants);
• Phytoremediation Cover: A cover consisting of soil and
plants to minimize infiltration of water and to aid in the
• minimize the percolation of water into the underlying
waste;
Root
system
Vegetative grasses
and legumes soil = 6”
Geomembrane
Clay
Uncompacted soil
Waste up to 60’
Conventional Cover
Figure 3-6. Illustration of an Evapotranspiration (ET) cover.
36
Vegetative Cover
0
2’
4’
Waste
Waste
8’
Waste
60’
T=0
Trees planted
T=1
Tree roots penetrate waste
Remediation
T= Mature
Soil created
Water balance established
Figure 3-7.
• achieve long-term performance and minimize maintenance needs (e.g., control surface water runoff and
reduce soil erosion); and
Vegetative covers are not appropriate for certain landfill
units, such as municipal solid waste (MSW) landfills, that
generate gas in chronic, large, or uncontrolled amounts.
As reported by Flower et al. (1981), landfill gases can be
toxic to plants and therefore must be considered. To date,
vegetative cover systems have not been shown to prevent
the diffusion of gases from landfills. Gas emissions from
MSW landfills are governed by two sets of regulations.
• prevent the migration or release of significant quantities of gas produced.
The acceptability of vegetative covers as a final cover
for certain waste disposal units, such as landfill cells, is
dependent on applicable regulatory requirements (e.g.,
RCRA). EPA’s minimum technical requirements for landfill
cover systems have evolved within a framework referred
to as the “liquids management strategy.” The two primary
objectives of the strategy are: (1) to minimize leachate formation by keeping liquids out of the landfill (or source area);
and (2) to detect, collect, and remove the leachate that is
generated (EPA, 1987, 1991). A vegetative cover must
demonstrate equivalent performance with generic cover
designs specified in EPA guidance [i.e., Design and Construction of RCRA/CERCLA Final Covers (EPA/625/4-91/
025); Design, Operation, and Closure of Municipal Solid
Waste Landfills (EPA/625/R-94/008); and Technical Guidance For RCRA/CERCLA Final Covers (EPA/OSWER
Draft)].
• 40 CFR §258.23, under RCRA Subtitle D, addresses
the personal and fire/explosion safety aspects of landfill
gas.
• New Source Performance Standards (NSPS) and
Emissions Guidelines (EG) promulgated under the
Clean Air Act (CAA), 40 CFR Part 60 Subparts Cc and
WWW,, regulate emissions of non-methane organic
compounds (NMOCs) as a surrogate to total landfill
gas emissions.
3.8.2
Media
ET and phytoremediation covers are used in the uptake
of infiltrating surface water. A phytoremediation cover can
37
also be used in the treatment of soil, sludge, and sediments.
3.8.3
• Phytoremediation Cover: Contaminants in the waste
materials should not be at phytotoxic levels because
for degradation to occur, the plant roots need to be in
contact with the contaminated waste.
Advantages
A vegetative cover may have the following advantages:
3.8.6
• May reduce maintenance needs and requirements,
such as minimizing surface erosion by establishing a
self-sustainable ecosystem.
For an evapotranspiration cover, the depth of the underlying waste is generally not a factor because the mechanisms (i.e, water evaporation, transpiration, and storage)
occur above the waste.
• The use of vegetative covers is generally considered
cost-effective, as evaluated in the Alternative Landfill
Cover Demonstration (Dwyer, 1997a) for an ET cover.
The effective depth of contaminant degradation for the
phytoremediation cover is the root depth of the plants.
• Vegetation has been shown to be an effective final layer
for hazardous waste site covers (EPA 1983; McAneny
et al. 1985).
3.8.7
3.8.8
• Phytoremediation covers have the potential to enhance
the biodegradation of contaminants in soils, sludges,
and sediments.
Site Considerations
Several factors should be evaluated when considering
the use of a vegetative cover such as soil physical properties, plant community activities, the potential for gas production from the biodegradation of waste, and climatic variables (e.g., precipitation quantity, type, intensity, and seasonality, temperature, humidity).
Disadvantages
A vegetative cover may have any or all of the following
disadvantages:
3.8.8.1
• Proper long-term inspection and maintenance may be
required to ensure appropriate plant cover. Natural succession of plants may lead to a predominance by plant
species other than those originally planted as part of
the cover.
Soil Conditions
Soils most suitable for a vegetative cover should have a
high water storage capacity. The soil should be a high mixture of clays and silts (e.g., fine-grain soils). Soils with rapid
drainage are to be avoided, although a carefully designed
and maintained cover may include a coarser-grained material.
• Surface water may have a tendency to follow macropores
opened by decaying roots and consequently flow downward to underlying waste or groundwater.
3.8.8.2
• For a phytoremediation cover, contaminants may be
taken up by plants intended or used for human, domestic animal, or wild animal consumption, and potential
adverse effects on the food chain could occur.
Ground and Surface Water
Water tables that are relatively high may result in soils
with less available water storage capacity, if evaporation
and transpiration processes are not sufficient. However,
with an appropriate thickness of soil to provide a sufficient
water storage capacity, the water table may not be a factor
in the performance of the cover.
• Most plant based cover designs will be effective only in
a specific climate. Universally applicable designs may
not be possible.
3.8.8.3
• If trees planted as part of a vegetative cover are toppled
by wind, buried waste may be exposed.
Climatic Conditions
Areas with high precipitation rates require more water to
be transpired or stored in the soil. In humid regions (i.e.,
more than 20 inches of annual precipitation), inadequate
evapotranspiration may occur seasonally, and soil layers
will need to be thicker than in arid and semi-arid regions to
provide adequate water storage capacity.
• Most alternative cover designs do not contain and collect landfill gas.
3.8.5
Plants
Poplar trees and grasses have been used commercially
to construct vegetative covers. Ideally, the vegetation selected for the system should be a mixture of native plants
and consist of warm- and cool-season species.
• Vegetation may encourage aerobic microbial activity
in the root zone; such activity could discourage formation of anaerobic landfill gases or degrade them.
3.8.4
Root Depth
Applicable Contaminants/
Concentrations
3.8.9
3.8.5.1 Organics and Inorganics
Current Status
Vegetative covers have been constructed, including numerous testing facilities as described in the Alternative
Covers Assessment Project’s “Phase I Report.” There are
no performance evaluations at present; each installation
must be approved on a site by site basis.
• Evapotranspiration Cover: The concentration of the
contaminants in the underlying material is not a concern, as long as the plants are not in contact with materials having phytotoxic concentrations.
38
this technology. Models used to assess landfill covers
are included in the discussion.
3.8.10 System Costs
In general, vegetative covers are considered cost-effective remedies. Cost estimates indicate notable savings for
an evapotranspiration cover compared to a traditional cover
design (RTDF 1998).
3.8.11
EPA. 1991. Design and Construction of RCRA/CERCLA
Final Covers. (Seminar Publication, EPA 625-4-91-025).
This seminar publication provides regulatory and design
personnel with an overview of design, construction, and
evaluation requirements for cover systems for RCRA/
CERCLA waste management facilities.
Selected References
Dobson, M. C., and A. J. Moffat, 1993. The Potential for
Woodland Establishment on Landfill Sites. HMSO Press.
EPA. 1994. Design, Operation, and Closure of Municipal Solid Waste Landfills. (Seminar Publication, EPA 625R-94-008).
The information presented in this report focuses primarily on the effects of the landfill environment on tree
growth, the typical rooting pattern of trees, the likelihood
of windthrow, and the possible effects of trees on landfill
hydrology.
This seminar publication provides a documented summary of technical information presented at a series of
2-day seminars. The goal of the seminars were to
present state-of-the-art information on the proper design, construction, operation, and closure of Municipal
Solid Waste Landfills.
Flower, F. B., E. F. Gilman, and I.A. Leone. 1981. Landfill Gas, What It Does to Trees and How Its Injurious Effects may be Prevented. J. of Arboriculture. 7(2):February
1981.
EPA. Draft December 1998. Technical Guidance For
RCRA/CERCLA Final Covers.
Methods are suggested for preventing the entry of landfill gases into the root zones of the trees and in accommodating other tree growth problems found to be associated with former refuse dumping areas. These methods include gas venting and blocking, irrigation, planting adaptable species, using small sized specimens in
preference to large, and providing adequate maintenance.
The purpose of this guidance document is to provide
information to facility owners/operators, engineers, and
regulators regarding the regulatory standards, performance monitoring, and maintenance of final cover systems for municipal solid waste and hazardous waste
landfills regulated under RCRA, and sites being
remediated under CERCLA. When released (approximately December 1999), it will be an update to the 1991
EPA document entitled, Design and Construction of
RCRA/CERCLA Final Covers (EPA-625-4-91-025).
Environmental Science and Research Foundation Conference Proceedings. 1997. Landfill Capping in the SemiArid West: Problems, Perspectives, and Solutions. May
21-27, 1997. Grand Teton National Park.
3.9 Riparian Corridors/Buffer Strips
3.9.1 Definition/Mechanism
These conference proceedings address the following:
• Regulatory performance and monitoring requirements
for landfills and caps
Riparian corridors/buffer strips are generally applied along
streams and river banks to control and remediate surface
runoff and groundwater contamination moving into the river.
These systems can also be installed to prevent downgradient
migration of a contaminated groundwater plume and to degrade contaminants in the plume. Mechanisms for
remediation include water uptake, contaminant uptake, and
plant metabolism. Riparian corridors are similar in conception to physical and chemical permeable barriers such as
trenches filled with iron filings, in that they treat groundwater
without extraction containment. Riparian corridors and buffer
strips may incorporate certain aspects of hydraulic control,
phytodegradation, rhizodegradation, phytovolatilization, and
perhaps phytoextraction.
• Perspectives and problems with landfill closure
• What landfill covers do and how they do it
• Different approaches to landfill caps
• Perspective and alternative cap designs
• Economic issues
RTDF. 1998. Summary of the Remediation Technologies
Development Forum Alternative Covers Assessment Program Workshop. February 17-18, 1998, Las Vegas, NV.
http://www.rtdf.org.
3.9.2
Media
Riparian corridors/buffer strips are used in the treatment
of surface water and groundwater.
The meeting minutes of these workshops on alternative
covers include a discussion of the technical and regulatory issues relating to the use of vegetative covers. Regulatory and industry participants present their views on
the use of alternative covers for a variety of geographic
regions and on the research needs required to validate
3.9.3
Advantages
Secondary advantages include the stabilization of stream
banks and prevention of soil erosion. Aquatic and terres-
39
trial habitats are greatly improved by riparian forest corridors.
3.9.4
3.9.8.2
Groundwater must be within the depth of influence of
the roots.
Disadvantages
The use of buffer strips might be limited to easily assimilated and metabolized compounds. Land use constraints
may restrict application.
3.9.5
3.9.8.3
Applicable Contaminants/
Concentrations
3.9.9
3.9.10 System Cost
Cost information is not available.
3.9.11 Selected References
Root Depth
Licht, L. A. 1990. Poplar Tree Buffer Strips Grown in Riparian Zones for Biomass Production and Nonpoint Source
Pollution Control. Ph.D. Thesis, University of Iowa, Iowa City,
IA.
Plants
This thesis describes the use of poplar trees to control
nitrate-nitrogen contamination from agricultural fields.
Methods and results of field work are presented. Poplar
trees successfully established in riparian zones removed
nitrate-nitrogen from soil and groundwater.
Poplars have been used in riparian corridors and buffer
strips.
3.9.8
Site Considerations
Sufficient land must be available for the establishment
of vegetation. Typically a triple row of trees is installed,
using 10 meters at minimum. Larger corridors increase
capacity, and wider areas allow for more diverse ecosystem and habitat creation. Native Midwestern songbirds,
for example, prefer corridors 70 meters and more.
3.9.8.1
Current Status
Buffer strips have been researched and installed commercially with success.
Uptake occurs within the root zone or the depth of influence of the roots.
3.9.7
Climatic Conditions
The amount of precipitation, temperature, and wind may
affect the transpiration rate of the plants.
Nutrient and pesticide contaminants are among the water-soluble organics and inorganics studied the most often
using this technology. The nitrate concentration in groundwater was 150 mg/L at the edge of a field, 8 mg/L below a
poplar buffer strip, and 3 mg/L downgradient at the edge
of a stream (Licht and Schnoor 1993).
3.9.6
Ground and Surface Water
Licht, L. A., and J. L. Schnoor. 1993. Tree Buffers Protect
Shallow Groundwater at Contaminated Sites. EPA Ground
Water Currents, Office of Solid Waste and Emergency Response. EPA/542/N-93/011.
Uptake of nitrates by poplars planted between a
stream and a corn field was studied at an agricultural
field site. The poplars decreased nitrate levels from
150 mg/L in the field to 3 mg/L at the stream. Poplar
trees were used with atrazine and volatile organic
compounds in toxicity studies conducted in laboratory chambers and in the field. Atrazine was mineralized, and deep-rooted poplars slowed migration of
volatile organics.
Soil Conditions
The primary considerations for this technology are the
depth and concentration of contaminants that affect plant
growth. Soil texture and degree of saturation are factors to
be considered for use of this system. Planting technique
can mitigate unfavorable soil conditions.
40
Chapter 4
Phytoremediation System Selection and Design Considerations
This chapter discusses considerations involved in the
selection, design, and implementation of phytoremediation
systems. It presents information that will help a site manager to identify whether phytoremediation may be appropriate for a site and to select a particular phytoremediation
technology, based on the conditions occurring at or applicable to a site. This chapter introduces issues and concepts that should be considered in the design and implementation of a phytoremediation system. Because
phytoremediation is not yet a developed technology, this
discussion is not intended to serve as a design manual.
Rather, it is a foundation on which to develop a
phytoremediation system in consultation with
phytoremediation professionals on a site-specific basis.
It is not meant to encourage or discourage the use of
phytoremediation; rather, it indicates that the use of
phytoremediation should be based on thorough and sound
evaluation.
This discussion does not include a comparison of
phytoremediation to other technologies, since the only intent is to provide information on phytoremediation. It is assumed, however, that such a comparison of effectiveness,
cost, and time frames will be conducted. It is possible that
other technologies might remediate a site more effectively.
A decision-making process for evaluating whether or not
phytoremediation is a viable option is provided by the following outline of the steps for applying phytoremediation:
The main questions when considering phytoremediation
as a remedial alternative are (1) What are my
phytoremediation choices?; (2) Will phytoremediation be
effective and economical in remediating the site?; and (3)
What will it take to implement phytoremediation? This chapter discusses the considerations that need to be evaluated when answering these questions; however, economic
considerations and potential costs of phytoremediation are
discussed in Chapter 2.
• Define Problem
- Conduct site characterization
- Identify the problem: media/contaminant
- Identify regulatory requirements
- Identify remedial objectives
- Establish criteria for defining the success of the
phytoremediation system
• Evaluate site for use of phytoremediation
- Perform phytoremediation-oriented site characterization
- Identify phytoremediation technology that addresses
media/contaminant/goals.
- Review known information about identified phytoremediation technology
- Identify potential plant(s)
The main considerations in the evaluation of
phytoremediation as a possible remedial alternative for a
site are the type of contaminated media, the type and concentration of contaminants, and the potential for effective
vegetation to grow at the site. Recommendations for the
selection of a particular type of phytoremediation technology are provided where appropriate, such as for a particular media and contaminant encountered at a site. Information on selection of appropriate vegetation is provided.
Other site-specific factors that need to be considered to
determine if a potential phytoremediation technology will
work at a site are also discussed. This site-specific evaluation of phytoremediation considerations will lead to a decision regarding the selection of a phytoremediation technology to be used at a particular site. The more positive
responses encountered when going through this list of considerations, the more phytoremediation is likely to work at
the site.
• Conduct preliminary studies and make decisions
- Conduct screening studies
- Perform optimization studies
- Conduct field plot trials
- Revise selection of phytoremediation technology, if
necessary
- Revise selection of plant(s), if necessary
• Evaluate full-scale phytoremediation system
- Design system
- Construct system
- Maintain and operate system
- Evaluate and modify system
- Evaluate performance
This discussion can be considered as a checklist of items
to evaluate that are specific to a particular site, and also
as a reality check for the use of phytoremediation at a site.
41
• Achieve objectives
- Perform quantitative measurement
- Meet criteria for success
• Hydraulic control (plume control)
• Vegetative cover
• Riparian corridors/buffer strips
When planning any remediation system, it is important
to first define the desired remedial objectives: the desired
fate of the contaminant(s) and the desired target
concentration(s). An appropriate remediation technology,
or different technologies as part of a treatment train, can
then be selected based on the characteristics and performance of that technology in meeting the remedial goals.
Remedial objectives are discussed in Chapter 5.
Groundwater, surface water, and wastewater have been
treated using constructed wetlands or similar technologies;
however, a discussion of those technologies is beyond the
scope of this document.
Plants useful for groundwater phytoremediation include
trees (especially Salix family — poplars, willows, cottonwoods), alfalfa, and grasses. The plant transpiration rate
is an important consideration for groundwater
phytoremediation (see Section 4.3).
4.1 Contaminated Media Considerations
Phytoremediation can be used for in situ or ex situ applications. Phytoremediation is generally considered for in
situ use by establishing vegetation in areas of contaminated soil or groundwater. However, soil can be excavated
and placed into a treatment unit where phytoremediation
will be applied. Groundwater or surface water can be
pumped into a treatment unit established for
phytoremediation or it can be sprayed onto vegetation.
4.1.1
The primary considerations for remediation of groundwater contamination are the depth to groundwater and the
depth to the contaminated zone. Groundwater
phytoremediation is essentially limited to unconfined aquifers in which the water table is within the reach of plant
roots and to a zone of contamination in the uppermost
portion of the water table that is accessible to the plant
roots. Plant roots are very unlikely to reach through clean
groundwater to a deeper contaminated zone. The seasonal
fluctuation of the water table will affect the root depth: relatively little fluctuation is desirable to establish a root zone.
If remediation of deeper contaminated water is desired,
careful modeling must be done to determine if the water
table can be lowered by the plants or through pumping, or
if groundwater movement can be induced toward the roots.
Soil, Sediment, and Sludge
The following phytoremediation technologies are used
in the treatment of these media:
• Phytoextraction
• Phytostabilization
• Rhizodegradation
Another consideration for groundwater phytoremediation
is the rate of water movement into the root zone of the
area to be treated. Groundwater remediation will be slow
when the rate of water movement is low. Soil water content will also affect the rate of phytoremediation. Although
root hairs can reach into relatively small pores and plant
roots can extract water held at relatively high matrix suctions (about 15 bars), the low hydraulic conductivity of relatively dry soils will decrease the rate at which dissolved
contaminants are moved toward the plant. Sufficient
groundwater and precipitation must be available to serve
the water requirements of the plants, or irrigation will be
necessary.
• Phytodegradation
• Phytovolatilization (to a lesser degree)
• Vegetative cover
The primary considerations for phytoremediation of soil
are the depth and volume of contamination and soil characteristics that affect plant growth, such as texture and
water content (degree of saturation).
Phytoremediation is most appropriate for large areas of
low to moderately contaminated soil that would be prohibitively expensive to remediate using conventional technologies. The contaminated soil should be within the root zone
depth of the selected plant. Small volumes of contaminated
soil concentrated in just a few areas are likely to be more
efficiently remediated using other technologies.
4.1.2
For groundwater containment, the rate of groundwater
flow should be matched by the rate of water uptake by the
plants to prevent migration past the vegetation. Generally,
the greater the groundwater flow rate, the larger the plants
need to be, and/or the greater the density of the planting.
Groundwater geochemistry also must be conducive to
plant growth. For example, saline waters will be detrimental to the plants unless halophytes (salt-tolerant plants) are
used.
Groundwater
The following phytoremediation technologies are used
in the treatment of groundwater:
Canopy closure is the shading of soils by plant leaves,
and total canopy limits evapotranspiration. Large plants
provide a larger area of shading than small plants. Therefore, the mature size of the plants selected for remediation
should be considered in the design of a treatment plot.
• Phytodegradation
• Phytovolatilization
• Rhizofiltration
42
4.2 Contaminant Considerations
of metals than for one metal alone (Ebbs et al. 1997). The
interaction of the metals in a mixture might need to be investigated, especially in terms of the ability to take up one
or more metals and nutrients.
The applicability of phytoremediation has been researched for some of the most significant and widespread
contaminant classes. Table 4-1 indicates the applications
of phytoremediation and provides the relevant
phytoremediation technology for different types of contaminants.
4.2.3
Most phytoremediation research has focused on individual classes of contaminants and not on mixtures of different types of contaminants. Although there is some evidence that plants can tolerate mixed organic and metal
contamination, it has generally not been investigated if one
type of vegetation can successfully remediate different
classes of contaminants (for example, heavy metals and
chlorinated solvents at the same time). The use of several
types of vegetation, each to remediate a different contaminant, might be required either at the same time or sequentially.
The following sections contain additional discussion on
particular contaminants or classes of contaminants. After
a potential phytoremediation technology is identified
through a review of the information in this chapter, additional, specific information on the technology can be obtained in Chapter 3.
4.2.1
Organic Contaminants
The hydrophobicity of an organic compound (as indicated by the octanol-water partition coefficient, kow) will
affect the uptake and translocation of the compound. In
general, moderately hydrophobic organic compounds (with
log kow between 0.5 and 3.0) are most readily taken up by
and translocated within plants. Hydrophobic (lipophilic)
compounds also can be bound to root surfaces or partition
into roots but not be further translocated within the plant
(Schnoor et al. 1995a; Cunningham et al. 1997).
4.2.2
Waste Mixtures
4.2.4
Contaminant Concentrations
The primary consideration in this area is that the contaminant concentrations cannot be phytotoxic or cause unacceptable impacts on plant health or yield. A literature review or a preliminary laboratory or field plot screening study
will be needed to determine if the given concentrations are
phytotoxic.
Inorganic Contaminants
The contaminant concentrations necessary for successful phytoremediation must be determined in comparison to
the concentrations that could be treated by other, more effective remedial technologies. In general, the highest
concentrations will comprise relatively small hot spots that
Phytoextraction coefficients describe the relative ease
of extraction of different metals; for example, one study
showed that the easiest to most difficult to extract were:
Cr6+, Cd2+, Ni2+, Zn2+, Cu2+, Pb2+, and Cr3+ (Nanda Kumar et
al. 1995). Phytoremediation may be different for mixtures
Table 4-1. Phytoremediation Technologies Applicable to Different Contaminant Types1,2
Technology
Media
Chlorinated
solvents
Metals3
Metalloids
Munitions
Nonmetals
Nutrients
PAHs
PCBs
PCP
Pesticides
Petroleum
hydrocarbons
Radionuclides4
Surfactants
Phytoextraction
Soil
Water
Rhizofiltration
Water
Phytostabilization
Soil
T
F
T
Rhizodegradation
Soil
F
F
F (Se)
F
Phytodegradation
Soil
Water
G
F
F
T
G
G
Phytovolatilization
Soil
Water
T
T
T (Hg)
G
F (Se)
F
T
F5
G
F
T
F
F
F
G
T
G
F
F
F/F
F
F
F
T
T
G
T
1
The applicability of a particular method of phytoremediation to each contaminant type has been judged by the current state or stage of the
application.
This is indicated in the table by the following designations:
T - The application is at the theoretical stage.
G - The application has been researched in the greenhouse or laboratory.
F - The application has been researched using field plots or has been applied in full-scale field systems.
2
All contaminants can be controlled using vegetative covers. The vegetative cover, riparian corridors, buffer strips, and hydraulic control are not
included in the table because they can be considered combinations of the other phytoremediation technologies.
3
Reeves and Brooks 1983; Baker 1995, Salt et al. 1995; Nanda Kumar et al. 1995; Cornish et al. 1995.
4
Salt et al. 1995; Nanda Kumar et al. 1995; Cornish et al. 1995.
5
In constructed wetlands.
43
could be more effectively treated through excavation and
other treatment means. However, costs and remedial time
frames need to be considered as well as concentrations.
(1) Plants that have been shown to be effective or that
show promise for phytoremediation. These plants
have been discussed in this handbook, they can be
found in research publications on phytoremediation,
or they can be enumerated by phytoremediation specialists.
Higher concentrations of organics and nutrients might
be tolerated more readily by plants than by soil microorganisms (Schnoor et al. 1995a). In addition, plants (as measured by seed germination tests) were less sensitive to
heavy metals than were bacteria in toxicity screening (Miller
et al. 1985). Thus, it might be possible that
phytoremediation (except for microbially-based
rhizodegradation) could be effective in cases where
bioremediation fails due to the presence of metals or to
high toxic levels of contaminants. This is speculative, however, because the relative tolerance of plants and microorganisms to high contaminant concentrations might be different under field conditions as compared to laboratory
toxicity screening, due to acclimation of microorganisms
in the field.
4.2.5
(2) Native, crop, forage, and other types of plants that
can grow under regional conditions. A list of these
plants can be obtained from the local agricultural extension agent.
(3) Plants can also be proposed based on those plants
growing at the site, extrapolations from phytoremediation research, inferences drawn from unrelated research, or other site-specific knowledge. The efficacy
of these plants for phytoremediation would need to be
confirmed through laboratory, greenhouse, or field
studies or through screening.
Contaminant Depth and Distribution
in the Soil Profile
Ideally, there would be a plant common to lists (1) and
(2), or there would be evidence that a plant common to
lists (2) and (3) would be effective. These lists of plants
can be narrowed down according to the criteria discussed
in the outline of the steps for selecting a suitable plant (see
Table 4-2). Following the Plant Selection Process outline,
additional information on topics discussed in the plant selection process is provided.
The contaminated soil must be within the root zone of
the plants in order for the vegetation to directly impact the
contamination. The depth to contamination is of less concern in the use of a vegetated cover designed to prevent
infiltration.
4.2.6
Contaminant Characteristics
During the plant selection process, additional information should be gathered regarding candidate plants. Information can be obtained by telephone or the Internet from
local, state, or Federal agencies and offices, or from universities. The Internet has numerous locations with this
information. One very useful source is the Plant Materials
program of the USDA Natural Resources Conservation
Service (http://Plant-Materials.nrcs.usda.gov/).
The contaminant type, pH, physical form of light nonaqueous phase liquid (LNAPL) or dense nonaqueous
phase liquid (DNAPL), mixtures, or oily contamination can
adversely affect the water movement, air movement, or
uptake of nutrients necessary for plant growth. An NAPL
or oily contaminant can significantly decrease plant growth.
Aged compounds in soil can be much less bioavailable.
This may decrease phytotoxicity, but may also decrease
the effectiveness of phytoremediation technologies that rely
on the uptake of the contaminant into the plant. To judge
the effectiveness of an actual phytoremediation design, it
is important that treatability studies use contaminated soil
from the site rather than uncontaminated soil spiked with
the contaminant.
4.3.2
A fibrous root system has numerous fine roots spread
throughout the soil and will provide maximum contact with
the soil due to the high surface area of the roots. Fescue is
an example of a plant with a fibrous root system (Schwab
et al. 1998). A tap root system is dominated by a central
root. Alfalfa is an example of a plant with a tap root system
(Schwab et al. 1998).
4.3 Plant Considerations
It must be remembered that engineers, hydrogeologists,
and other professionals typically involved in site
remediation are not farmers, and that it might be difficult to
have vegetation conform to standard engineering practices
or expectations. Phytoremediation adds an additional level
of complexity to the remediation process because plants
comprise a complex biological system that has its own characteristics.
4.3.1
Root Type
4.3.3
Root Depth
Root depth can vary greatly among different types of
plants. It can also vary significantly for one species depending on local conditions such as depth to water, soil
water content, soil structure, soil density, depth of a hard
pan, soil fertility, cropping pressure, or other conditions.
The bulk of root mass will be found at shallower depths,
with much less root mass at deeper depths. The deeper
roots will also provide a very small proportion of the water
needed by the plant, except in cases of drought.
Phytoremediation Plant Selection
The goal of the plant selection process is to choose a
plant species with appropriate characteristics for growth
under site conditions that meet the objectives of
phytoremediation. There are several starting points for
choosing a plant:
The depth of in situ contamination or of excavated soil
generally should not exceed the root zone depth. Exceptions to this could be made if it is verified that upward movement of dissolved contaminants can be induced toward
44
Table 4-2. Plant Selection Process
1. Identify phytoremediation technology and remedial goals.
2. Gather site information.
• Location (also relative to plant/vegetation/ecosystem zones)
• Temperatures: averages, range
• USDA plant hardiness zone (range of average annual minimum temperature)
• Precipitation: amount, timing
• Length of growing season
• Amount of sun/shade
• Soil texture, salinity, pH, fertility, water content, structure (hardpans, etc.)
• Contaminant type, concentration, form
• Site-specific conditions or considerations
• Identify plants growing in contaminated portion of site. (Optional)
- Do these provide a clue as to what plants to select?
- If not, will these plants compete with the selected plant?
- If the native plants do compete with the selected plant, are they easily removed?
• Identify local plants and crops. (Optional)
- Do these plants provide a clue as to what plants to select?
- Will a selected plant interfere with local plants?
3. Identify important criteria for plant selection.
General:
• Disease resistance
• Heat tolerance
• Cold tolerance
• Insect tolerance
• Drought resistance
• Salt tolerance
• Chemical tolerance
• Stress tolerance
• Legume/nonlegume
• Annual/biennial/perennial
• Cultural requirements: Due to the added stress of a contaminated soil environment, the cultivation and maintenance factors may have to
be
carefully monitored.
- Seed pretreatment before germination (such as for some prairie grasses)
- Planting method (seeds, sod, sprigs, whips, plugs, transplants), timing, density, depth (of seeds, root ball, or whips)
- Mulching, irrigation, soil pH control, fertilization, protection from pests and disease
- Fallen leaves, debris
- Harvesting requirements
- Labor and cost requirements should not be excessive
• Invasive, undesirable, or toxic characteristics
• Plant/seed source
• Establishment rate
• Reproduction method/rate
• Growth rate/biomass production
• Competitive or allelopathic effects
• Value of plant as cash crop
Phytoremediation-related:
• Demonstrated efficacy of plant: The plant can take up and/or degrade contaminants, produce exudates that can stimulate the soil microbes,
or possess enzymes that are known to degrade a contaminant. The potential for the success of phytoremediation can be increased by
screening plants for useful enzymes (Fletcher et al. 1995).
• Phytotoxicity of contaminant: The contaminant should not be phytotoxic at the concentrations found at the site. Contaminant phytotoxicity and
uptake information can be found in the phytoremediation and agricultural literature, or determined through preliminary germination and phytotoxicity screening studies. Chapter 3 provides examples of applicable contaminant concentrations. Databases such as PHYTOTOX or UTAB
have been used to summarize and investigate phytotoxicity and uptake information (Fletcher et al. 1988; Nellessen and Fletcher 1993a,
1993b), although these databases might not be readily accessible.
• Root type and shape: Fibrous root system versus tap root system.
• Root depth: The range of root depths of a given plant must be considered.
• Contaminant depth and distribution: The contaminant depth must be similar to the root depth. The distribution of the contamination at various
soil depths is also important in planning the plant type and planting method. The contaminant concentrations in the seed bed layer of the soil
profile may have a strong effect on the ability to establish vegetation. A surface layer with minimal contamination underlain by greater contaminant concentrations might allow more successful seed germination than if the surface layer is heavily contaminated. Root growth into the
more contaminated layer is then desired, and since it is not guaranteed, must be verified. Make general decisions.
• Deciduous/nondeciduous: Deciduous trees will be dormant for part of the year, resulting in lowered transpiration rates.
• Monoculture vs. mixed species: The use of mixed species of vegetation can lead to more success due to the increased chance that at least
one species will find a niche. However, there could be competition between plants for nutrients and space. A monoculture relies on just one
plant type, possibly requiring more management to ensure its growth against adverse conditions. Despite this, a well-established stand of
one plant that has been shown to be effective could be the most efficient means of phytoremediation.
• Native vs. non-native: Native, nonagricultural plants are desirable for ecosystem restoration. In most applications, plants that are adapted to
local conditions will have more chance of success than nonadapted plants.
• Growing season: Warm season and cool season grasses could be used in combination to address different seasonal conditions, prolonging
a vegetative cover throughout more of the year.
(Continued)
45
Table 4-2.
(continued)
•
Sterile/male/female: The ability to reproduce is necessary for the long-term establishment of vegetation. In cases where the spread of the
plant to surrounding areas is undesirable, however, the plants should be selected to prevent reproduction.
• Plant rotation; planned or natural plant succession: The long-term establishment of vegetation at a site is dependent on the project goals and
future uses of a site. For long-term, no-maintenance vegetation establishment as part of ecosystem restoration, it is likely that there will be a
succession of plants at a site. If so, this succession could be planned when considering the types and timing of vegetation. Plant rotation
could conceivably be important when short-lived vegetation is used that does not reach remedial goals and that should not be replanted in the
same place.
5. Match above criteria with list of available/proposed plants.
• Select all appropriate candidates (eliminate all inappropriate plants).
• Conduct detailed evaluation of remaining candidates against criteria in items 1 to 4 of this table.
• Conduct cost/benefit analysis of top candidates:
- Plant costs
- Plant effectiveness in reaching goal
- Plant value (cash crop)
• Conduct preliminary studies to assess germination, survivability, and biomass. It might not be possible to assess the success of some forms
of phytoremediation (i.e., rhizodegradation) due to the insufficient time for preliminary testing. Because phytoremediation can be a long-term
process, however, spending one or two years in preliminary trials won’t substantially increase the overall remediation time.
- Germination screening studies for phytotoxicity
- Small-scale greenhouse or laboratory chamber studies
- Field plot trials
6. Select plant and implement phytoremediation.
• Monitor and evaluate plant growth and phytoremediation success.
• Reevaluate plant selection on basis of observations: Variability in phytoremediation efficacy in varieties, cultivars, or genotypes of a given
species has been encountered in alfalfa for hydrocarbon rhizodegradation (Wiltse et al. 1998). A screening of cultivars/varieties might be
required.
• Reseed/replant as necessary with same or different plant.
the roots, or if soluble root exudates can be transported
deeper into the soil.
practical purposes, trees are useful for extraction of groundwater less than 30 feet deep. In addition, a contaminated
zone below the water table is not likely to be reached by
roots, as the roots will obtain water from above the water
table.
The root depth ranges provided below represent maximum depths:
• Legumes: Alfalfa roots can go quite deep, down to
about 30 feet, given the proper conditions.
4.3.4
Growth Rate
The growth rate of a plant will directly affect the rate of
remediation. Growth rates can be defined differently for
different forms of phytoremediation. For rhizodegradation,
rhizofiltration, and phytostabilization, for example, it is desirable to have fast growth in terms of root depth, density,
volume, surface area, and lateral extension. For
phytoextraction, a fast growth rate of aboveground plant
mass is desirable.
• Grasses: Some grass fibrous root systems can extend
8 to 10 feet deep (Sloan and Woodward 1996). The
roots of major prairie grasses can extend to about 6 to
10 feet.
• Shrubs: The roots of phreatophytic shrubs can extend
to about 20 feet (Woodward 1996).
• Trees: Phreatophyte roots will tend to extend deeper
than other tree roots. Phreatophytic tree roots can be
as deep as 80 feet. Some examples are mesquite tap
roots which range from 40 to 100 feet and river birch
tap roots which go to 90 to 100 feet (Woodward 1996).
A large root mass and large biomass are desired for an
increased mass of accumulated contaminants, for greater
transpiration of water, greater assimilation and metabolism of contaminants, or for production of a greater amount
of exudates and enzymes. A fast growth rate will minimize
the time required to reach a large biomass.
• Other plants: Indian mustard roots generally are about
6 to 9 inches deep.
Metal hyperaccumulators are able to concentrate a very
high level of some metals; however, their generally low
biomass and slow growth rate means that the total mass
of metals removed will tend to be low. For phytoextraction
of metals, the metals concentration in the biomass and
the amount of biomass produced must both be considered. A plant that extracts a lower concentration of metals,
but that has a much greater biomass than many
hyperaccumulators, is more desirable than the
hyperaccumulator because the total mass of metals removed will be greater.
These maximum depths are not likely to be reached in
most situations, due to typical site conditions such as soil
moisture being available in the surface soils or poorer soil
conditions at greater depths. A review of the literature found
that maximum depths of tree roots were generally 3 to 6
feet, with almost 90% of the roots in the top 2 feet (Dobson
and Moffat 1995).
The effective depth for phytoremediation by most
nonwoody plant species is likely to be only 1 or 2 feet. The
effective depth of tree roots is likely to be relatively shallow, less than 10 or 20 feet. Gatliff (1994) indicates that for
46
• Are the seeds/plants viable or healthy?
Poplars have been widely used in phytoremediation research and applications due to their fast growth rate. They
can grow 9 to 15 feet/year (Gordon 1997).
4.3.5
• The seeds/plants must be high quality: no undesirable
weed seeds, diseases, etc.
Transpiration Rate
4.3.7
The transpiration rate of vegetation will be important for
those phytoremediation technologies that involve contaminant uptake, and for hydraulic control. The transpiration
rate depends on factors such as species, age, mass, size,
leaf surface area, canopy cover, growth stage, and climatic
factors, and will vary seasonally. Thus, well-defined numbers for a given type of vegetation cannot be assumed.
Allelopathy
Allelopathy refers to the inhibition of growth of one plant
species due to the presence of chemicals produced by a
different plant species. Allelopathic effects could be investigated when considering co-establishment of several species of vegetation to ensure that one species won’t hinder
the growth of another. Allelopathic effects could also be
due to plant residue that is incorporated into the soil in an
attempt to increase the fertility of the soil. For example,
root, stem, and leaf residues from canola inhibited the
growth of corn, wheat, and barley (Wanniarachchi and
Voroney 1997).
Estimates for certain cases, however, provide a rough
guide to the order of magnitude that might be expected.
For Populus species, approximately 26 gpd for a 5-yearold tree was estimated (Gordon 1997). It was reported that
5000 gpd was transpired by a single willow tree, which is
comparable to the transpiration rate of 0.6 acre of alfalfa
(Gatliff 1994). Individual cottonwood trees were estimated
to transpire between 50 and 350 gpd, based on analysis
of drawdown near the trees (Gatliff 1994).
These transpiration rates, given in terms of gpd for individual trees, should be viewed with caution because the
transpiration rate varies with tree size and other factors,
as mentioned above. A more appropriate measure would
be to look at the total water usage in a given area of vegetation.
The phenomenon of allelopathy might also provide clues
as to the usefulness of a particular plant species. The allelopathic chemicals produced by the plant could be investigated to determine if they are suitable chemical substrates for microbial cometabolism of soil contaminants.
Allelopathy also indicates that compounds exuded by plant
roots influence the surrounding soil. The distance that this
influence extends could be estimated by examining the
spacing between such allelopathic plants and their neighbors. This would provide a clue as to how far soil
phytoremediation could reach.
4.3.6
4.3.8
Seed/Plant Source
Forensic Phytoremediation
Areas with contaminated soils or groundwater can become revegetated through the establishment of naturallyoccurring plants. Forensic phytoremediation refers to the
investigation of naturally-revegetated contaminated areas
to determine which plants have become established and
why, and to determine the impact of these plants on the
contamination. This investigation can identify plants that
are capable of surviving in contaminated areas, some of
which might also be capable of contributing to the degradation of the contaminants.
Important considerations for the source of the plant include:
• Are the plants/seeds local or from a comparable climate? It is important to have the seed or plant supplier verify where the seeds were produced since a
supplier may sell seeds that have been collected from
a wide variety of geographic locations. It is generally
best to use seeds or plants (and varieties) that are
local or from the region of the site so that the plants
are adapted to the particular climatic conditions. The
seed supplier or local agricultural extension agents can
provide information regarding local seeds and plants.
A comment during a phytoremediation presentation at
the Fourth International In Situ and On-Site
Bioremediation Symposium (April 28 - May 1, 1997,
New Orleans, LA, sponsored by Battelle Memorial Institute) indicated that poplars were purchased for a
project; however, the source was in a different climate,
and all the trees died.
Because the vegetation has often been present at the
site for a relatively long period compared to the time interval for planned phytoremediation field studies, a researcher
has the additional benefit of not having to wait many more
years to investigate the effects of the revegetation (which
are evident now at the naturally-revegetated site). Natural
revegetation of a site is essentially a form of intrinsic
bioremediation. Phytoremediation intrinsic bioremediation
and forensic phytoremediation approaches have been intensively investigated at a petroleum refinery sludge impoundment that was naturally revegetated (Fletcher et al.
1997; Wong 1996).
• Can the source supply the quantity needed when they
are needed (whether in season or out of season)? Is
the supplier reliable?
4.3.9
Plants Used in Phytoremediation
A compilation of plants used in phytoremediation research or application is given in Appendix D. This Appendix includes a table giving the common name followed by
the scientific name, and a table with the scientific name
followed by the common name.
• Are there any transport/import/quarantine restrictions
or considerations?
• Can the supplier provide information on the cultivation
of the plants?
47
The following are examples of commonly-investigated
or used plants:
of contaminant are the primary considerations. However,
any former incidental use of chemicals could affect
phytoremediation, such as herbicide use at the site to suppress vegetation during site activities. The former or existing vegetation at the site can negatively influence the establishment of vegetation. Examples include allelopathic
plants, well-established undesired vegetation, and soil
pathogens in former vegetation. The former activities and
vegetation could be investigated to determine if any of these
factors could increase the difficulty of establishing the remedial vegetation.
• Trees:
- Poplars (hybrids)/cottonwoods
- Willows
• Grasses:
- Prairie grasses
- Fescue
• Legumes:
- Alfalfa
4.4.1.2
Current Site Activities
The site must have sufficient open space, and no physical structures or site activities could interfere with the vegetation. In addition, site facilities or debris may have to be
removed. For ex-situ soil treatment, the volume of soil to
be remediated is divided by the depth of the root zone to
find the land area required for phytoremediation. The required land area must remain undisturbed by site activities, uses, or traffic.
• Metal-accumulators:
- Hyperaccumulators
Thlaspi caerulescens
Brassica juncea
- Accumulators
Sunflower
• Aquatic plants:
- Parrot feather
- Phragmites reeds
- Cattails
The following general plant characteristics are optimum
for different forms of phytoremediation:
The impact of tree roots on foundations and subsurface
utility lines must be considered as well as the impact of
tree branches on overhead utility lines. Potentially impacted
utilities may have to be removed or relocated.
Phytoremediation using trees would have to be curtailed if
such removal or relocation cannot be done.
• Rhizofiltration and phytostabilization:
- Able to remove metals.
- No translocation of metals from the roots to the
shoots.
- Rapidly growing roots.
Fencing might have to be installed around the
phytoremediation system to keep out animal pests that
might damage the vegetation. Additionally, the site must
have access to a water supply (groundwater, surface water, or municipal) if irrigation is required.
• Phytoextraction:
- Tolerates, translocates, and accumulates high concentrations of heavy metals in the shoots and leaves.
- Rapid growth rate and high biomass production.
- Is not favored for consumption by animals (this decreases risk to the ecosystem).
4.4.1.3
4.3.10 Optimum Plant
Future activities planned for the site can impact the selection of a phytoremediation technology. Portions of the
site might need to remain undisturbed to allow long-term
plant growth. If use of the site is required in the near future, the establishment of trees or the use of slow-growing
metal accumulators is not desirable. Fast-growing trees
such as hybrid poplars might be grown for a short period,
however, and then harvested if the remediation is predicted
to be relatively short-term.
• Rhizodegradation:
- Possesses appropriate enzymes and should not take
up the contaminant.
- Appropriate root growth (depth and or extent).
Future use of the site might be for industrial, residential,
or recreational purposes. Different remedial criteria could
apply to these different uses. Institutional controls might
be necessary, depending on the proposed use of the site.
• Phytodegradation:
- Able to take up the contaminant.
- Degradation products are not toxic.
Agricultural uses of the site such as for grazing or for
crops will entail more concern regarding accumulation of
toxic compounds within the plants. Ecosystem or habitat
restoration uses will also raise concerns about possible
effects on the food chain.
• Phytovolatilization:
- Able to take up the contaminant.
4.4 Site Considerations
4.4.1 Site Activities
4.4.1.1
Proposed Site Activities
4.4.2
Former Site Activities
Climatic Considerations
Climatic factors cannot be predicted with certainty, and
their effects cannot always be controlled. As a complex
Former site activities will affect the selection of plants
for phytoremediation. The location, extent, degree, and age
48
• Aesthetics: The presence of weeds and plant debris
can affect the perception and acceptance of a
phytoremediation site.
biological system, a phytoremediation system can be severely impacted by extreme weather events; thus, this possibility must be considered during the planning of remedial
activities.
• Inappropriate plant introduction: The introduction and
spreading of a potentially undesirable plant (noxious or
invasive weeds) that will take over local vegetation must
be avoided. Plants should not have an adverse effect
on the local ecosystem. The vulnerability of the surrounding area to the selected vegetation and the
vegetation’s impact must be examined.
Precipitation. The amount and timing of rainfall and snowmelt will determine the time of soil preparation, time of planting, and need for irrigation.
Air temperature. The mean, extremes, and fluctuations
in air temperature will affect the ability of plants to grow.
Sunlight. The amount of sunlight affects plant growth,
air temperature, and evapotranspiration.
• Pollen and allergies: If plants are used that would contribute an unacceptable amount to local allergen loadings, the plants must be harvested before release of
the allergen or treated to decrease the impact.
Shade. The amount of shading from nearby buildings or
from mixed vegetation can affect the ability of plants to
grow.
• Effect on nearby crops and vegetation:
- Pesticide drift: If pesticides are used during preparation or maintenance of the system, the impact of
any spraying must be carefully monitored, and negative impacts on nearby crop or residential areas pre
vented.
- Interbreeding: The impact of the selected plants on the
surrounding vegetation must be examined to ensure that
hybridization does not occur in a nearby crop.
Length of growing season. Phytoremediation processes
are more likely to be active during the growing season.
The length of the growing season must be considered in
predicting overall remedial timeframes.
Wind. The amount of wind affects evaporation, causes
damage to plants, and disperses volatiles and debris. Windbreaks may need to be installed.
- Airborne plant diseases could impact nearby vegetation.
Location. Regional and local weather patterns will affect
the factors described above.
• Attraction of pests: The plants might attract unwelcome
animals that become pests, such as birds (noise and
droppings), poisonous snakes (danger to humans), rats
(disease-carriers or food-destroyers), or insects (disease-carrying vectors).
4.4.3 Water Considerations
Surface water drainage and runoff will affect how soon
the soil can be worked, the soil temperature, and the stability of the soil, seeds, and plants. Subsurface water drainage will affect soil water content and soil temperature. Artificial drainage might need to be provided to encourage
remedial success. Poor growth and shallow roots in
buffalograss (Buchloe dactyloides) and warm season prairie grasses resulted from water-logging during a field test
of phytoremediation in soils contaminated with relatively
low levels of PAHs and PCP (Qiu et al. 1997).
• Safety issues: blocked vision/sightlines, tree limbs, concealment, fire hazard due to accumulated plant matter.
• Plant toxicity to people, birds, mammals (such as foraging animals), other plants (through allelopathy), or
beneficial insects such as honeybees. The inherent
toxicity of useful plants, such as that of Datura innoxia
(thornapple), must be considered in any risk analysis.
Potential bioconcentration of toxic contaminants in
plants is a concern, and the fate of the plant must be
controlled to prevent chemical or toxin ingestion by
animals or humans.
Irrigation is likely to be necessary during phytoremediation.
The source, availability, volume, cost, quality, and timing of
the irrigation water need to be considered.
4.4.4
Potential Adverse Effects/
Neighborhood Concerns
• Root damage to foundations, underground utilities, or
other structures.
Phytoremediation could potentially have adverse impacts
on the site or surroundings. The list of potential adverse
impacts listed below is not meant to discourage the potential use of phytoremediation, but rather to make the reader
aware of potential pitfalls. Possible adverse impacts or disadvantages of phytoremediation include:
• Impact on contaminant transport: The interactions of
the plants and all contaminants at the site could be
studied. Fertilizer application to optimize plant growth
may result in an increase in the mobility of some metals in the soil because many common nitrogen-containing fertilizers lower the pH of soil. This might result
in leaching of metals to groundwater.
• Dust from tilling operations.
• Odor: Soil preparation that generates odors from volatile contaminants might be required during
phytoremediation, but not with other remedial technologies. The selected plant might be odorous at certain
stages of growth or decay.
Phytoremediation might positively or negatively impact
other remediation activities. A potential positive impact is
that vegetation could be a visual, odor, dust, and noise
49
barrier to block other site activities from the surrounding
areas. Potential negative impacts include the need to have
healthy vegetation, which requires that the plants not be
significantly disturbed. Thus, vehicles or equipment should
not be used or stored on the vegetated areas, which might
limit activities that could otherwise occur at the site.
4.4.5
of tilling might need to be done to prepare the seed
bed. A good seed bed will increase the chances for
the establishment of a healthy stand of vegetation. Dust
will need to be suppressed during soil preparation work
or tilling.
• Planting considerations include the density, timing, and
method of applying seeds or plants.
Agronomic Considerations for
System Installation and
Maintenance
• pH maintenance
All of the factors that must be considered in successful
agriculture also must be considered during
phytoremediation. These factors will be more critical or more
difficult to control due to the additional stress placed on
the system by the contamination.
4.4.5.1
• Mulching
• Fertilization: Fertilizers or organic matter amendments
might be necessary. The effect of fertilizers on soil pH
(for phytoremediation of metals) and on soil microbes
(for rhizodegradation) could be assessed.
Pre-Plant Selection
• Irrigation equipment and scheduling
Pre-plant selection includes the following:
• Control of plant pests/desirable animals/undesirable
pests
- Birds: netting
- Grazers, vermin: fences, trapping
- Insects: pesticides
- Plant competitors: herbicides
- Diseases: pesticides, nutrients, pH, drainage
• Soil must have a pH range that will allow plant growth.
The soil pH might need to be modified and controlled
through liming to increase pH or through acidification
to lower pH.
• Soil fertility and nutrient content
• Soil structure
• Aesthetics/debris cleanup (wind damage, fallen leaves,
etc.)
• Soil tilth
• Odor control: from plant, from soil prep, from contaminant
• Soil salinity
• Soil water content
• Biomass disposal
- Harvesting: determine method
- Plant debris (uncontaminated): occasional, periodic
- Plant debris (contaminated, biomass with metals)
• Air-filled porosity: Affects aeration.
• Soil texture: Affects moisture content and drainage.
• Soil temperature: Affects germination of seeds.
• Effect of contaminant on nutrient or toxin availability
(some metals, or modification to enhance metal solubility or chelation, may make nutrients unavailable or
enhance adverse impacts of toxins).
• Soil depth: The depth to bedrock, a hardpan, or infertile soil (along with soil water contents and soil nutrients) can control the maximum depth of roots.
4.4.6
• Irrigation requirements
Due to the growth of vegetation, the mass of plant material will increase with time. Depending on the type of
phytoremediation, the biomass that must be removed from
the active system will vary. Relatively permanent long-term
systems that rely on the establishment of mature vegetation (e.g., poplar trees or grass for rhizodegradation) will
not require periodic planned removal of the biomass. In all
phytoremediation systems, however, some biomass such
as dead or diseased plants, fallen leaves, fallen limbs, or
pruned material might have to be removed occasionally to
maintain good operation of the system. These uncontaminated plant materials will need to be harvested, stored,
and disposed of as necessary. It will be important to confirm that the plant material does not contain any hazardous substances. After this confirmation, the material could
be composted or worked into the soil on site. If that is not
possible, off-site disposal will be required.
• Control of plant pests: birds, grazers, insects
4.4.5.2
Disposal Considerations
Post-Plant Selection
Post-plant selection includes the following:
• Soil preparation: This preparation can include screening out debris or rocks, and (if desired) mixing and
diluting of the contaminated soil. Bulking agents and
organic matter amendments might need to be added
to improve the fertility or moisture-holding capacity of
the soil. Adding metal-chelating agents (such as
EDTA), maintaining a moderately acid pH, and adding
reducing organic acids to alter the redox status of the
soil can all increase the bioavailability of metals.
• Seed bed preparation: Preparation of the soil will likely
be required before seeding or planting. Various types
50
The operation of some phytoremediation systems, such
as with phytoextraction and rhizofiltration, does depend
on the periodic removal of biomass. In these cases, proper
harvesting, storage, and disposal of contaminated biomass (e.g., containing heavy metals or radionuclides) will be
necessary to prevent potential risk pathways such as introduction to the food chain. An appropriate disposal facility must be identified, and it is likely that costs will be greater
than with uncontaminated biomass. Regulatory requirements for the handling and disposal of this material will
have to be followed.
experience is gained in researching and applying
phytoremediation, the most relevant disciplines will likely
be identified and more specific information can be provided as to how the following professionals can assist in
phytoremediation.
Agricultural extension agents or state university agricultural departments can provide invaluable information on
the particular plants that grow in the local region, the cultural practices for these plants, and the local soils. Most
information is likely to be about commodity crops grown in
the region and the weeds that affect these crops.
If the selected phytoremediation technology results in
uncontaminated biomass, it might be possible to harvest
the vegetation as a cash crop to offset some of the remedial costs. Examples include the harvest of grasses or alfalfa for animal feed, or lumber from poplars. It must be
verified, however, that the plant materials do not contain
hazardous substances.
Agricultural engineers are more likely to have experience
with soil properties, drainage, tilling equipment, and irrigation than other engineers.
Agronomists can provide assistance in working with soil
and crops.
Botanists can provide critical information on the identification, growth, properties, and behavior of plants.
4.5 Treatment Trains
Phytoremediation could be part of a treatment train at a
site. Pretreatment of soil or water might be necessary before application of phytoremediation, such as with the adjustment of inflow water chemistry into an engineered
rhizofiltration system. Phytoremediation might also be used
as a finishing step to decrease contaminant concentrations below what is achieved by a different initial remedial
technology, such as land treatment biodegradation. The
disposal or treatment of plant matter that contains the contaminants will be the final step in a treatment train. In general, however, research has focused on phytoremediation
as a stand-alone technology, with little or no integration of
phytoremediation with other remedial technologies.
Ecologists will be crucial when hazardous waste site
remediation is also part of a longer-term ecosystem restoration project.
Environmental/civil engineers have significant experience
using many technologies to characterize and remediate
hazardous waste sites.
Food scientists, vegetable crop specialists, and pomologists can provide information on contaminants in food crops
and fruits; this information can provide clues as to which
plants are useful in the uptake of contaminants.
Foresters can provide tree propagation and culture information.
Phytoremediation could be a partial solution at a site.
For example, excavation of highly-contaminated soil and
treatment by other remedial technologies could be followed
by a phytoremediation technology. A backup remedial technology might also be necessary for times when the
phytoremediation system is not working effectively, such
as during winter when plant growth has stopped or when
the vegetation is damaged by pests or weather.
Hydrogeologists can evaluate the contaminated media
in the system and can evaluate the interactions with surface water, groundwater, and soil water.
Land reclamation specialists have knowledge of the
plants and techniques used to restore degraded land; some
of the contaminants at hazardous waste sites, however,
have not been encountered in most cases of land reclamation.
4.6 Additional Information Sources
Successful phytoremediation requires a multidisciplinary
approach. This approach will call for the knowledge, input,
and/or participation of a wide range of professionals and
practitioners. Many of these fields have conducted research
on topics relevant to phytoremediation before the applied
concept of phytoremediation was developed. In addition,
valuable information on potentially useful local plants or
cultural practices can be obtained from less-commonlyused resources such as farmers, agricultural extension
services, and even local garden clubs and nurseries.
Landscape architects can advise on the selection and
placement of plants.
Nurseries and seed companies can provide advice on
the selection and care of seeds and plants under local
conditions.
Soil scientists are specialists in understanding soil properties.
Soil microbiologists will be particularly useful for work
involving rhizodegradation, and as an aid in explaining how
soil microorganisms will interact with plant exudates, contaminants, and any amendments to the soil.
Relevant disciplines, resources, and information sources
are described below. Since a typical phytoremediation system is not likely to be a research project, all of these information sources do not need to be fully utilized. As more
51
Chapter 5
Remedial Objectives, Treatability, and Evaluation
5.1 Remedial Objectives
tat, destruction of the contaminant is preferred over contaminant containment. Processes that transfer the contaminant to another location or phase are also less desirable.
The remedial objectives that will be appropriate for a particular site are determined by site-specific conditions and
the requirements of the Federal or state program under which
the cleanup action will be conducted. Such cleanup programs include the RCRA Corrective Action and Underground
Storage Tank Remediation programs, which are Federal programs typically implemented by the states; the Federal
Superfund program; and state cleanup programs. These
cleanup programs generally require that remedial measures
be taken to:
5.1.1
Cleanup Levels
The target concentration for each contaminant may be
driven by environmental regulations such as RCRA,
CERCLA, the Clean Water Act, or state-specific cleanup
requirements. For example, surface water discharges, if
any, from the site may be required to meet National Pollutant Discharge Elimination System (NPDES) limitations. If
soil or water is removed from the site for treatment or disposal, RCRA standards are applicable.
• Prevent contaminants from reaching human or environmental receptors above acceptable risk levels (prevent
exposure);
A specified contaminant concentration is often a goal in
soil or groundwater remediation. Because phytoremediation
is an emerging technology, there is still uncertainty regarding the contaminant concentrations that are achievable by
the various types of phytoremediation. The information compiled in Chapter 3 provides a rough guide as to the contaminant concentrations that have been achieved in research studies.
• Control further migration of contaminants from source
materials to groundwater or surface water (source control); and
• Control further migration of contaminated groundwater (plume control).
Some programs have additional requirements. For example, the Superfund program generally requires that treatment remedies be used for source materials that are determined to be “principle threat” wastes, and generally allows containment remedies for source materials determined
to be “low level threat” wastes. Also, the Superfund and
RCRA Corrective Action programs generally require that
contaminated groundwater be restored to cleanup levels
appropriate for current or future beneficial uses (e.g., drinking water). Remedies that involve treatment of source
materials and restoration of groundwater will also set
cleanup levels to be attained by the remedy.
5.1.2
Fate of the Contaminant
Destruction of each contaminant is the preferred remedial objective. However, depending on the phytoremediation
technology selected, contaminants may be contained and
left in place, or extracted or taken up by the plant into the
plant tissue and then left in place, removed, or volatilized.
Table 5-1 summarizes the methods of contaminant control
for each phytoremediation technology.
Table 5-1. Summary of Phytoremediation Technologies and Method
of Contaminant Control
Method
Two factors must be considered to determine the remedial objectives for phytoremediation projects: 1) the target
concentration for each contaminant in each type of media
(soil, water, etc.) and 2) the desired fate of each contaminant, i.e., containment, uptake and removal, destruction,
or a combination of these options.
Phytoextraction
(concentration)
Rhizofiltration
Phytostabilization
Rhizodegradation
Phytodegradation
Phytovolatilization
Plume control
Vegetative cover
Riparian corridors
Ecosystem restoration could be a primary or secondary
objective in combination with soil or groundwater
remediation. Although remediation might be a secondary
goal as opposed to reestablishment of vegetation and habi-
a
b
52
Destruction
Extraction/Uptake
Containment
√
√
√
√a
√
Phytoremediation cover.
Evapotranspiration cover .
√
√
√
√
√b
√
5.2 Treatability Studies
degradation or uptake. Successful enhancements to
phytoremediation have been noted such as the inoculation of wheat seeds with TCE-degrading bacteria (Yee et
al. 1998).
Phytoremediation techniques are almost by definition innovative. Most have not been applied very often. In spite
of the body of information concerning applications of
phytoremediation to contaminated soils, groundwater, and
surface waters, there is still a need to determine a priori if
the specific plant(s) and treatment procedures indicated
for cleanup will work to remediate the contaminant(s) in
the soil or water at a specific site. Many factors will influence the success of phytoremediation at a given site, including contaminant concentration, availability of nutrients,
daily maximum and minimum temperature, rainfall or possibility of irrigation, grade on site, aesthetic considerations,
daily illumination level, relative humidity, wind patterns, and/
or the presence of growth-suppressing contaminants. The
desired level of cleanup and the desired rate of decontamination also need to be considered. All of these factors
need to be evaluated prior to a substantial expenditure of
time and money on a large-scale phytoremediation effort.
5.2.1
5.2.2
Other Considerations
Treatability studies could also provide information relating to disposal of contaminated biomass. Such disposal is
a major consideration in the cleanup of metal-containing
soils. Depending on regulations and plant concentrations
of metals, plants may need to be landfilled, or the metals
reclaimed through smelting, pyrolysis of biomass, or extraction. In a discussion of the reclamation of metal-contaminated plant tissue by smelters, Dibakar (1997) stated
that plant tissue with a dry-weight concentration of over
one percent metal was amenable to reclamation.
Insecticides or herbicides might be used at the field treatment site to preserve the plant species selected for
phytoremediation or to prevent the overgrowth of hardy
indigenous species. The site survey should consider the
prevalence of insect pests and invasive native species. Subsequent laboratory trials may need to evaluate pesticide
usage to ensure that it does not interfere with
phytoremediation.
Optimization Studies
The phytoremediation system should be optimized prior
to the actual field application. There may be a need to
modify the soil or water pH or to add soil amendments
such as chelating agents (to make metals more
bioavailable), nutrients (to increase rate of plant growth),
and/or organic matter (to facilitate the growth of the desired plant species or to improve soil microbial viability).
Organic amendments used in phytoremediation studies
have included leaf mulch, ground corn stalks, peanut shells,
cotton gin debris, and ground bark. Caution should be exercised in the use of plant-derived amendments because
some plant materials have been shown to possess phytotoxic properties. Canola leaf and root residues have been
shown to suppress the growth of corn, barley, and wheat
(Wanniarachchi and Voroney 1997). Such naturally occurring phytotoxins probably have an evolutionary advantage
by suppressing competition for nutrients. Prior to full-scale
implementation, candidate amendments should be tested
in small-scale studies for their ability to suppress the growth
of the desired phytoremediating species.
Treatability studies often use radiolabeled contaminant
preparations to assess the toxicity of plant-generated metabolites of the contaminant(s) of interest or to assess the
possibility of volatilization or solubilization of toxic contaminants. This use of radiolabels allows for a much greater
sensitivity in the analysis for contaminant or metabolites
(much lower detection limit), thereby facilitating the tracking of metabolic transformation of the contaminant in the
phytoremediation system. Studies using radiolabeled contaminants are usually performed in greenhouses or growth
chambers, although limited studies have been done in the
field using nonvolatile radiolabeled contaminants and encasing the plant roots in an impermeable barrel-like container to prevent migration of radiolabels into the surrounding soil or into the groundwater. If volatility of contaminant
or metabolites is a concern, then studies should be performed in a greenhouse. A gas-tight barrier can be installed
between the soil surface and the air so that evaporation
from the soil can be differentiated from plant uptake and
subsequent volatilization. Several studies are underway
using Populus species in an attempt to discern the
mechanism(s) by which poplars remove TCE from contaminated groundwater.
Dibakar (1997) recommends groundwater monitoring if
amendments might mobilize contaminants. An interesting
example of a pilot study using amendments is the work by
Blaylock et al. (1997) concerning the use of several chelates at multiple concentrations. In this study, the
phytoremediation potential of Indian mustard (Brassica
juncea) was tested for removing metals from soils. This
study and other studies are discussed in section 5.2.5.
Amendments have also been used to adsorb contaminants so that they could later be available to plants or degraded by soil microbes. Cunningham et al. (1995b) assessed the stabilization of lead in soil by adding an alkalizing agent, phosphates, mineral oxides, organic matter, or
biosolids.
As a special case, phytoremediation studies that deal
with contaminant removal from aqueous media (groundwater, waste water, wetlands) might use a radiolabeled
contaminant to address the comparative rates of transpiration, bioconcentration, and/or degradation. Pilot studies
have been performed with radiolabeled contaminants to
evaluate phytoremediation treatment of groundwater to
remove persistent herbicides (Burken and Schnoor 1997)
and metals (Salt et al. 1997).
Phytoremediation might also be enhanced by the addition of microbial innocula that would increase rhizosphere
Treatability studies often also use hydroponic systems
in initial, proof-of-concept trials. Although hydroponic sys-
53
successfully grown at a specific field site, then (as is the
case for chemically similar compounds) similar strains/cultivars, species, or genera should be assessed in a pilot
study.
tems should not be used to infer the rate of uptake or degradation for soil systems, such studies can determine if a
particular plant can be used with a particular contaminant.
Hydroponic systems are often used for screening several
options (e.g., several concentrations of a contaminant),
since these systems are inexpensive and allow rapid growth
of plant tissue.
Soil samples should be taken in conjunction with plant
samples from the site to assess the concentration of contaminants in the immediate soils around the plants at the
site; soil contaminants have been shown to be degraded
by the microbial population found around the roots of plants
growing in contaminated soils (Anderson and Walton 1995).
These soil samples might also serve as sources of innocula
for microbial seeding of soils, seeds, or roots during subsequent remediation studies.
The issue of available time should be considered in the
design of treatability studies. The candidate plant species
must have sufficient time to develop roots and biomass
(and possibly metabolic enzymes) to perform the
phytoremediation. The ability of a plant to degrade or to
take up a contaminant varies with the age and metabolic
status of the plant (water content, diurnal cycles, temperature). Factors to consider include growth rate, period of
dormancy (deciduous plants), and any other known factors, such as the development of metabolizing enzymes,
that change with the age of the plant. Table 5-2 shows
experimental factors to consider in conducting treatability
studies.
The greenhouse or laboratory trial should use soil or
water from the site, if possible. This allows for assessment
of soil toxicity and assessment of the possibility of migration of the contaminant in the soil column (leachability).
Soil toxicity should be assessed using Standard Practice
E 1598-94 from the American Society for Testing and Materials (ASTM). The soil-dwelling microflora that aid in
phytoremediation are affected by the type and level of contamination in the soil on site. These microflora are estimated to take 2 to 16 weeks to recover from pesticide
treatment, more than 10 years to recover from toxic insult
due to oil spills on soil, and 50 to 100 years to recover from
metal contamination (Shimp et al. 1993). If soil from the
site cannot be secured, then a soil of the same USDA type
should be used and adjusted for pH at the site. Commercial suppliers of standard USDA soil types should be contacted as needed. In most studies, soils are screened
through a coarse mesh to remove rocks and large biomasses; however, this process may perturb the soil sample
(Nwosu 1991).
Table 5-2. Experimental Factors for Testing in Treatability Studies
Essential
Optional
Contaminant reduction
Different soil types on site
Phytotoxicity of contaminant(s)
levels
at site
Different contaminant
Growth of plant species on site
(soil type, nutrients, temperature, rainfall,
illumination)
Allelopathy
Rate of cleanup
Aesthetic considerations
Level of cleanup
Soil amendments
Microbial innocula
Pesticide usage
Disposal options for plant
materials (metals,
radioisotopes)
5.2.3
on site
The trial should duplicate the illumination and moisture
conditions at the site as closely as possible since these
factors often have a significant influence on the rate of
remediation. Temperature and relative humidity have also
been shown to affect the rate of uptake of contaminants
(Dibakar 1997). If the site has several soil types, samples
of each soil type should be collected to assess growth in
each type. If uncontaminated areas are within the site, then
soils should be collected from these areas also for use as
experimental controls and for use in assessing the maximum tolerable level for a plant species to a given contaminant through addition of the contaminant of interest to the
uncontaminated soil. Table 5-3 summarizes factors to consider in designing a phytoremediation trial.
Experimental Design for Plant
Selection
If a contaminant has not been studied for
phytoremediation, but a chemically similar contaminant was
successfully treated, a trial might be conducted to determine if the species used to treat the chemically similar
contaminant would work. If initial trials with one species
are unsuccessful, then different cultivars/strains, or related
species in the same genus, or related genera in the same
plant family might be assessed.
5.2.4
Experimental Design
Most published phytoremediation pilot studies have utilized a block design, with a first-level assessment of all
possible combinations of several key factors (e.g., several
plant species/several pH levels/several chelators). A second series of tests could then evaluate the best combination of first-level factors (e.g., the best combination of plant/
pH/chelator) for another set of factors (e.g., plant-tolerated level of contaminant and rate of cleanup). Such an
A survey of the site vegetation should be undertaken to
determine what species of plants are able to grow on that
site. Plants from the site can be assessed for uptake of the
contaminant, bioconcentration, and/or biodegradation.
The species chosen for a field study must be suited to
the soil, terrain, and climate at the site. If a plant species
previously used elsewhere for phytoremediation cannot be
54
to enhance uptake of the metals by Indian mustard, Brassica juncea (Blaylock et al. 1997).
Table 5-3. Information Needed for a Pilot Treatability Study
•
•
•
•
•
•
•
•
•
•
•
•
Identification of contaminant(s)
Level (concentration) of contaminant(s)
pH
Average monthly temperature, plus expected maximum
and minimum temperatures
Soil nutrient levels (P, K, N) and organic matter levels
Average monthly rainfall
Grade on site
Aesthetic considerations (proximity to commercial or
residential properties or recreational areas)
Daily illumination
Average relative humidity
Wind patterns (prevailing direction and velocity)
Presence of growth-suppressing contaminants
Hybrid poplar cuttings were rooted in aqueous medium
and planted in 1-liter bioreactors filled with uncontaminated
soil or sand. Each bioreactor system was dosed with 14Clabeled atrazine. After controlled incubation, uptake and
degradation were measured and metabolites were identified, where possible. 14C was rinsed from some sand-containing bioreactors, and it was demonstrated that degradation of atrazine could be accomplished in plant tissue. A
mathematical model was developed to describe atrazine
uptake, distribution, and metabolism (Burken and Schnoor
1997).
In a review paper, Cunningham et al. (1995a) discuss
soil amendments and their use in phytoremediation.
experimental design allows for the efficient use of time and
funds, with assessment of multiple factors concurrently.
Information can be collected on interactions between factors affecting phytoremediation, and an optimal field
phytoremediation plan can be developed. The analysis of
data from such studies can be complex. Analysis of variance methods (ANOVA) can be used if certain conditions
are met. Standard statistics texts present a discussion of
ANOVA procedures. The best approach, however, is to
consult a statistician experienced in experimental design.
5.2.5
A greenhouse study evaluated three conditions (nutrient-amended soil/ryegrass, nutrient amended soil/no
plants, and unamended soil/no plants) for remediation of
soils contaminated with pentachlorophenol or a mixture of
polycyclic aromatic hydrocarbons. Replicate soil columns,
subjected to the three treatments, were analyzed over time
for contaminant concentration (Ferro et al. 1997).
In a small-scale field trial, a series of self-contained plots
was established with or without poplar trees and with/without trichloroethylene (TCE), which was added to the water
supply during the growing season. The poplar/TCE and
the poplar/no TCE plots were replicated, while the other
conditions were not. Removal of TCE from the water and
metabolite formation were measured (Newman et al.
1997c).
Completed Pilot-Scale Studies
Several pilot studies of phytoremediation options have
been published. The following paragraphs include brief
notes on the methods used, the experimental design, and
other information relevant to the design of a pilot study.
Rhizosphere soils (soils around the plant roots) were
collected from several plant species growing in an area
with prior herbicide application. These soils were dosed
with 14C-labeled metolachlor and incubated in biometer
flasks under controlled conditions. Evolution of 14CO2 was
measured as an indication of herbicide degradation (Anderson and Coats 1995).
Axenic tumor plant cell cultures were utilized to demonstrate metabolism of TCE in plant tissue, as contrasted
with metabolism by rhizosphere microbes (Newman et al.
1997a).
Soil was collected at a contaminated site, and the germination of cucumber and wheat seeds was assessed in containers incubated on site. Seed germination was evaluated
against matched controls composed of the same seeds
planted in clean sand and incubated on site (Nwosu et al.
1991).
A greenhouse study evaluated the efficacy of the
remediation of PAH-contaminated soil by eight species of
prairie grasses. Reaction units were PVC pipe, capped at
one end. The eight reaction units were dosed with a mixture of four PAHs; of these units, four were seeded and
four were not seeded. Additional controls were four undosed
units (two seeded and two unseeded). The concentration
of the PAHs was measured in the soils and leachates (Aprill
and Sims 1990).
A static renewal bioassay was developed in which plants,
grown several weeks in uncontaminated soil in a greenhouse, were then exposed to several different concentrations of a solution of nutrients and the contaminant of interest. Growth was evaluated through measurement of dry
weight, visual observation, and chlorophyll assay. This process was used to establish tolerance levels for a contaminant-plant system (Powell et al. 1996).
Soil additives (KH2PO4, limestone, gypsum, sulfur, various iron compounds, and various organic carbon sources)
were used in soils taken from three industrial sites to immobilize lead for subsequent phytoremediation. A test for
immobilization/leaching potential was described (Berti and
Cunningham 1997).
After a preliminary seed germination study, tall fescue
grass was chosen as the best grass species to use in this
study. Triplicate microcosms of vegetated and unvegetated
soils were dosed with benzo(a)pyrene or hexachlorobiphenyl,
both of which were 14C labeled. Offgassing of radiolabeled
metabolite was monitored. Degradation of the contaminants
Studies in growth chambers used a USDA standard soil
amended with lime and fertilizer and dosed with metal salt
solutions (Cd, Cu, Pb, or Zn). Five chelating agents were
evaluated at four concentrations to determine their ability
55
in soil and binding to soil were also measured. A complete
randomized block design was used and data were analyzed by two-way ANOVA (Qiu 1995).
moisture tension, which then could be related to water content through site-specific calibration. To evaluate processes
designed to impact water movement, the transpiration rate
should be determined.
Four species of aquatic plants grown in glass tanks under controlled conditions were evaluated to determine their
ability to accumulate either HgCl2 or CH3HgCl. Tests were
run in duplicate with two water sources and three sediment types. Uptake of Hg and plant growth were measured (Ribeyre and Boudou 1994).
Because phytoremediation is an emerging technology,
standard performance criteria for phytoremediation systems have not yet been completely developed. Data are
being gathered and assessed to develop performance
measures that can be used to assess the function of an
individual system.
Indian mustard seedlings were grown in aerated water
under controlled conditions and dosed with 210Pb, 85Sr, 109Cd,
63
Ni, 51Cr, or 134Cs. Bioaccumulation of metal-associated
radiolabel was measured. Effects of competing ions (Ca++,
Mg++, K+, SO4= and NO3-) were also assessed (Salt et al.
1997).
Long-term monitoring may be needed for
phytoremediation systems that require long time periods
to demonstrate their effectiveness. Monitoring may be continued after short-term cleanup goals have been met in
order to determine the impact of the phytoremediation system on the ecosystem.
Ten plant species were evaluated for remediation of TNT
and other explosives in groundwater. By-product formation, plant density, and rhizosphere interactions were evaluated. Time-course studies were performed and kinetics
were described (Saunders 1996).
5.3.2
Monitoring Plan
A monitoring plan for the phytoremediation system should
be prepared to collect data to:
• Optimize operation of the phytoremediation system.
Phytoremediation of diesel-fuel-contaminated soils was
studied in this small-scale field study. Four treatments were
used: clover, fescue, bermuda grass, and no plants. Six
replicates were made of each treatment, and each replicate plot contained four sampling sites. Decreases in total
petroleum hydrocarbons and plant growth were measured
over a 1-year period (Schwab and Banks 1995).
• Monitor potential adverse impacts to the ecosystem.
• Measure progress toward the remedial objectives, i.e.,
destruction, extraction, or containment of contaminants.
The monitoring plan should contain the following elements:
Shimp et al. (1993) published a review paper containing
an overview of factors affecting the phytoremediation of
soils and groundwater containing organic contaminants.
The paper’s emphasis is on rhizosphere mechanisms along
with a list of plants/contaminants that have been studied.
• Constituents, parameters, or items to be monitored
• Frequency and duration of monitoring
5.3 Monitoring for Performance Evaluation
• Monitoring/sampling methods
The phytoremediation system must be monitored and periodically evaluated to measure progress toward the remedial objective.
• Analytical methods
5.3.1
• Quality assurance/quality control (QA/QC) requirements.
• Monitoring locations
Performance Evaluation
To evaluate the performance of soil remediation, contaminant and degradation product concentrations in the soil must
be measured. In rhizodegradation, the microbial populations
could be counted and/or identified to confirm biodegradation. In collecting and analyzing the soil, samples should
be collected from the root zone because the proximity and
influence of the root zone, as well as the density of the
roots, may affect how much rhizodegradation or
phytostabilization is measured.
Table 5-4 lists common parameters monitored in a
phytoremediation system. This list is not all-inclusive and
is dependent upon the individual phytoremediation system.
Modeling may be necessary to optimize the
phytoremediation system or to predict behavior. Modeling
may be especially relevant to evapotranspirative covers
where the water balance is critical to the success of the
system. Plant uptake models may be used to predict the
rate at which a contaminant will be degraded within a plant.
To evaluate the performance of groundwater remediation,
the contaminant and degradation product concentrations
should be measured. The depth, flow rate, and volume of
groundwater should be monitored to evaluate the success
of hydraulic control. Periodic water content measurements
should be made, or tensiometers used to measure soil
Monitoring of the ecosystem for potential adverse effects
may be necessary for some phytoremediation systems. If
the system uses phytovolatilization, air sampling might be
necessary to address concerns about contaminants or deg-
56
Table 5-4. Summary of Monitoring Parameters
Monitoring Parameter
Reason for Monitoring
Climatic data
•Temperature
•Precipitation
•Relative humidity
•Solar radiation
•Wind speed and direction
•Maintenance requirements (irrigation)
•Determine water balance and evapotranspiration rates
Plants
•Visual characteristics (viability, signs of stress, damage from insects or
animals, growth, leaf mass, etc.)
•Tissue composition (roots, shoots, stems, leaves, etc.)
•Transpiration gases
•Transpiration rate
•Root density
Soil
•Geochemical parameters (pH, nutrient concentrations, water content,
oxygen content, etc.)
•Microbial populations
•Contaminant and breakdown product levels
•Maintenance (plant replacement, fertilizer, pesticide application, etc.)
•Quantify contaminants and byproducts
•Quantify and/or predict system operation
•Optimize vegetative, root, or microbial growth
•Determine water balance and evapotranspiration rates
•Quantify contaminants and byproducts
•Quantify and/or predict system operation
Groundwater
•Aquifer information (direction and rate of flow, depth to groundwater,
specific yield, etc.)
•Contaminant and breakdown product levels
•Quantify contaminants and byproducts
•Quantify and/or predict system operation
radation products that may be released to the environment. If
soil additives are used to enhance the bioavailability of metals in soil, monitoring may be required to ensure the metals
are not migrating to groundwater. Extraction of contaminants
by plants with uptake to edible portions of the plant such as
leaves and seeds may require monitoring of the food chain for
bioaccumulation of the contaminant.
The monitoring plan should include QA/QC procedures
for sample collection, analysis, and data interpretation.
Since remedial site and analytical personnel may not be
experienced in sampling, preservation, or analytical methods for plant matter, properly developed and validated methods must be used to ensure conclusions are valid.
57
Chapter 6
Case Studies
feet were used for the disposal of chemical warfare agents,
munitions, and industrial chemicals from the 1940s to the
1980s. Disposal methods included open burning of waste
material such as high explosives, nerve agents, mustard
agents, and smoke-producing materials. Wood and fuel were
used to feed the fire. Decontaminating agents used in the
operations were solvent-based. During this burning process,
large volumes of various chlorinated solvents were discharged. As a result, a plume of chlorinated solvents formed
in the aquifer below the burning pits. The predominant solvents in the groundwater are 1,1,2,2-tetrachloroethane (1122TCA) and trichloroethylene (TCE), with maximum concentrations in the groundwater of 170 ppm and 61 ppm, respectively. Total volatile organic compound (VOC) concentrations
in the groundwater range up to 260 ppm.
The six case studies presented in this chapter illustrate
specific field applications of phytoremediation. Site descriptions, design considerations, monitoring recommendations,
status, and cost of various phytoremediation processes are
presented. The completeness of the information provided
varies based on the status of the project (i.e., complete
costs or degree of contaminant removal may not be fully
defined because the project is ongoing).
6.1 Edgewood Area J-Field Toxic Pits Site
Aberdeen Proving Grounds
Edgewood, Maryland
Site name:
Location:
Media:
Primary
contaminants
and maximum
concentration:
Type of plant:
Edgewood Area J-Field Toxic Pits Site
Aberdeen Proving Grounds,
Edgewood, Maryland
Groundwater (8 ft bgs)
1,1,2,2-tetrachloroethane (1122-TCA),
170 ppm
Trichloroethylene (TCE), 61 ppm
6.1.2
Several technologies were considered for cleaning the soil
and groundwater at the site. Soil washing, vapor extraction,
and capping were considered for cleaning up soils, while
pump-and-treat and air sparging were considered for
remediating the groundwater. These technologies were eliminated from consideration for a number of reasons. Technologies that involved a rigid installation design were eliminated because of the potential for unexploded bombs buried on site. Pumping and treating the water would be difficult because of the high concentrations of contaminants
and strict discharge regulations. Thus, the pump-and-treat
system would need to remove high concentrations of contaminants from large volumes of groundwater, and then discharge the groundwater after it had been treated. Soil excavation was eliminated from consideration due to the presence of unexploded ordnance (UXO) and its high cost. After
eliminating the other possibilities, project managers decided
the J-Field site was a candidate for a pilot-scale
phytoremediation system.
Populus tricocarpa x deltoides
(Hybrid poplar)
Area of planting: 1 acre
Date of planting: March/April 1996
6.1.1
Design, Goals, and Monitoring
Approaches
Site Description
The Aberdeen Proving Grounds (APG) in Maryland began serving as a U.S. Army weapons testing facility in 1918.
Military weapons testing and past disposal activities over
the years have caused extensive pollution throughout the
soil and groundwater of the Proving Grounds. As a result,
the entire Edgewood area of Aberdeen appears on the Superfund National Priority List (NPL). The Department of
Defense (DOD) and the U.S. Environmental Protection
Agency (EPA) are jointly funding field-scale applications of
innovative treatment technologies around the facility. At the
J-Field Site in the Edgewood Area, EPA’s Environmental
Response Team (ERT) coordinated the planting of hybrid
poplars over a shallow plume of chlorinated solvents in an
effort to hydraulically contain the contaminants and treat
the groundwater.
Based on site conditions and the possible presence of
UXO at the J-Field Toxic Pits Site of APG, phytoremediation
was deemed a viable remedial alternative to hydraulically
contain the contaminants and treat the groundwater. Applied Natural Sciences, Inc., was subcontracted to design and
install the phytoremediation system. The phytoremediation strategy employed at the J-Field site began in September 1995
with a phytotoxicity assessment of on-site pollutants to
J-Field is located at the tip of Gunpowder Neck, in the
Edgewood Area of APG. Two pits measuring 10 x 15 x 200
58
Table 6-1. Monitoring Approaches at the J-Field Site.
determine any nutrient deficiencies that would hinder tree
growth. Four planting areas were designated at the J-Field
site, totaling approximately 1-acre. Holes were augered to a
depth of 8 feet to allow homogenization of soil layers. Soil
samples were collected and analyzed for VOCs, metals,
and chloride. The design was based on the location of the
toxic pits, various wells which would be utilized in monitoring the system, and the flow of contaminated groundwater.
In March and April 1996, 184 bare-root hybrid poplars (P.
trichocarpa x deltoides [HP-510]) were purchased from a
tree farm in Pennsylvania and planted 2 to 6 feet below
ground surface (bgs) in the areas of highest pollutant concentration around the leading edge of the plume. These trees
were planted in an attempt to intercept groundwater, thus
preventing further contamination of the nearby marsh. The
phytoremediation planting area covers approximately 1 acre
southeast of the toxic pits, and is surrounded by wooded
areas and scattered thickets. Groundwater flows from the
toxic pit area to the south and southeast. Perched groundwater in the planting area varies throughout the year from 2
to 8 feet below ground surface (bgs). To promote growth
down to the saturated zone, each tree was planted with a
plastic pipe around its upper roots. A long piece of rubber
tubing was also added from the surface to the deeper roots
in order to provide oxygen. A drainage system was installed
in May 1996 to remove rainwater and thus encourage the
plants’ roots to seek groundwater. A sweetgum tree growing
on site prior to installation of the phytoremediation system
was left standing. It will be monitored along with the poplars.
Type of Analysis or Observation
Used
Parameters Tested or Methods
Plant growth measurements and
visual observations
Diameter, height, health, pruning,
replacement
Groundwater and vadose zone
sampling and analysis
14 wells and 4 lysimeters to
sample for VOCs, metals, and
nutrients
Soil sampling and analysis
Biodegradation activity, VOCs,
metals
Tissue sampling and analysis
Degradation products, VOCs
Plant sap flow measurements
Correlate sap flow data to
meteorological data
Transpirational gas sampling
and analysis
Explore various methods
Source: Tobia and Compton (1997)
pared with the surrounding tree tissue and transpirational
gas data to determine the degree of success of the study.
6.1.3
Results and Status
Plant tissue samples were taken from certain trees and
analyzed for VOCs and metals. Results have shown parent
compounds and degradation products increasing in concentration through mid-growing season and waning in the fall.
Weather parameters were measured by an on-site meteorological station, correlated with tree data, and were utilized to estimate seasonal, daily, and yearly water uptake.
These parameters included precipitation, incident solar radiation, temperature, humidity, and wind speed. All of these
factors play a role in transpiration rates and sap flow.
Since the Aberdeen project involves a new treatment strategy, extensive monitoring is taking place to determine the
fates of the pollutants, the transpiration rates of the trees,
and the best methods for monitoring phytoremediation sites.
Groundwater contaminant levels, water levels, tree growth,
tree transpiration rates, tree transpirational gas and condensate water contaminant levels, soil community, and tree
tissue contaminant levels were monitored over the second
year growing season to determine the effectiveness of this
emerging technology. The monitoring approaches are summarized in Table 6-1. The sampling design of the site involves collecting soils, transpiration gases, and tree tissues
from the roots, shoots, stems, and leaves. Results will help
determine the concentrations of contaminants and their metabolites along each step of the translocation pathway.
Tree sap flow rates are being monitored in order to determine the pumping rates of the trees. A noninvasive technique was used to measure sap flow on certain trees during
the various sampling seasons. The Dynamax Flow32™ Sap
Flow System was used to measure the water flux of the
trees in grams of water/hour/tree by utilizing the heat balance method.
Transpiration gas sampling was performed by placing a
100-L TedlarTM bag over a section of branch on each of the
selected trees. Air was drawn from the sealed branch by
using a carbon Tenax tube, summa canister, Sciex Trace
Atmospheric Gas Analyzer, and on-site Viking Gas Chromatograph/Mass Spectrometer (GC/MS). The most reliable
results were obtained by collection into the summa canister, then analysis by laboratory GC/MS. The results show
similar patterns to those found in the leaf tissue. The parent
compounds of 1122-TCA and TCE were detected at increasing levels through the mid-growing season (maximum 2,000
ppb), with subsequent decreasing concentrations in the fall.
Condensate water was collected from the bags and analyzed for VOCs. There was a strong correlation (0.92) between condensate water and transpiration gas, with a maxi-
Nine wells were located in the surficial aquifer near the
study area at the time of tree planting. To determine the
effects of the phytoremediation study on groundwater, an
additional five wells and four lysimeters were installed in
November 1996. Monitoring wells were screened from 4 to
14 feet bgs. Two sets of two lysimeters were installed near
the new monitoring wells. The lysimeters were placed in
pairs and set at depths of 4 and 8 feet bgs. These depths
allow for coverage of the capillary zone during seasonal
highs and lows. Groundwater and lysimeters were monitored
on a quarterly basis for VOCs, metals, and chloride. The
data obtained from the lysimeters are currently being com-
59
6.1.5
mum concentration of 640 ppb of 1122-TCA in the condensate water.
Phytoremediation using trees to clean up groundwater
contaminated with volatile organic compounds may be an
ideal choice for this site and others due to the low cost,
low maintenance, and low impact associated with the technology. Much more work needs to be performed to further
confirm: (1) the correlation between transpiration gas and
condensate water; (2) soil community contaminant degradation rate; (3) soil flux rate of VOCs; (4) contaminant exposure to the root zone versus sap and condensate water;
(5) leaf litter exposure pathway; and (6) microwells to determine the zone of contamination.
Soil samples were collected from the rhizosphere of the
selected trees. These samples were analyzed for VOCs,
chloride, and metals, and utilized for soil community comparisons. There were noteworthy changes to the nematode
functional group populations. The nematode community appears to have increased, both in diversity and concentration, from previous samples taken from the area before the
trees were planted. Studies are planned to assess chlorinated solvent degradation by soil microbes.
Growth measurements, visual observations, and maintenance were performed on all trees during planting and at the
end of each growing season to monitor tree growth and
health. Tree diameter and height were measured and tree
health observed, including monitoring of insect damage,
chlorosis, and wilting. Approximately 10% of the trees have
been killed by frost, deer rub during rutting season, and
insect predation.
The environment benefits from the presence of trees regardless of whether or not the technology is effective in
removing contamination. These environmental benefits include habitat for wildlife, protection of the soil against wind
and water erosion, reduction of rainwater infiltration and flushing, an increase in organic matter, and an increase in soil
aeration and microbial activity.
Sap flow rate data indicate that on a daily scale, maximum flow occurs in the morning hours. In addition, increasing amounts of solar radiation seem to increase sap flow
rates, as would be expected in a tree. Groundwater monitoring data from May 1997 indicate that the trees are pumping
large amounts of groundwater. Data indicate that there is
roughly a 2-foot depression in the water table beneath the
trees in comparison to data from April 1996. Tree tissue
samples indicate the presence of trichloroacetic acid
(TCAA), a breakdown product of TCE. These data correlate
with the results from University of Washington greenhouse
scale studies that also found TCAA in plant tissues in both
axenic poplars cell cultures and hybrid poplar tissues. Site
managers at Aberdeen are also finding that chlorinated solvents (TCE and 1,1,2,2-tetrachloroethane) are being
evapotranspirated by the trees. To date, no mass balance
studies have been performed to quantitatively determine
the different fates of chlorinated solvents in this treatment
system. Future monitoring of the site will hopefully answer
some of the questions about solvent fate. To accomplish
this, additional types of monitoring will be employed, such
as on-site infrared spectrometry and on-site GC/MS.
6.1.6
Contacts
Harry Compton
US EPA Environmental Response Team
Edison, NJ 00837
(732) 321-6751
Steve Hirsh
US EPA Region 3
Philadelphia, PA 19103
(215) 814-3352
6.2 Carswell Site
Fort Worth, Texas
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
Based on tree containment measurements, the results of
the second growing season show that the trees are removing contaminants from the groundwater and transpiring parent compounds and their degradation products. The groundwater table has been lowered by tenths of feet in the planting area at the end of the growing season, indicating possible groundwater withdrawal by the trees for containment
of the contaminated groundwater in future growing seasons.
The trees are utilizing the groundwater at rates of 2 to 10
gpd/tree.
6.1.4
Conclusions
Area of planting:
Date of planting:
6.2.1
Former Carswell Air Force Plant
Fort Worth, Texas
Groundwater (12 ft bgs)
Trichloroethylene (TCE),
<1,000 ppb
Populus deltoides
(Eastern Cottonwood)
1 acre
April 1996
Site Description
The efficacy and cost of phytoremediation with respect
to the cleanup of shallow trichloroethylene (TCE) contaminated groundwater are being evaluated at the field scale in
a multiagency demonstration project in Fort Worth, Texas.
This U.S. Air Force project, which is being conducted as
part of the Department of Defense’s (DOD) Environmental
Security Technology Certification Program (ESTCP), as well
as the U.S. Environmental Protection Agency’s (EPA) Superfund Innovative Technology Evaluation (SITE) Program,
Costs
Tree installation cost is about $80/tree, or approximately
$15,000 for the installation of 184 trees. The costs of monitoring are highly varied due to the numerous monitoring techniques employed at the site.
60
entails the planting and cultivation of Eastern Cottonwood
(Populus deltoides) trees above a dissolved TCE (<1,000
ppb maximum concentration) plume in a shallow aerobic
aquifer to investigate the ability of these trees to control
and degrade the plume. The plume is located near Air Force
Plant 4 at the Naval Air Station Ft. Worth, also known as
the Carswell Air Force Base. Data are being collected to
determine the ability of trees planted as short-rotation woody
crops to perform as a natural pump-and-treat system.
6.2.2
Two sizes of trees were planted: whips and 5-gallon buckets. The whips were approximately 3/4 inch in diameter and
were about 18 inches long at planting. The whips were planted
so that about 2 inches remained above ground and the rest
of the tree was below ground to take root. The 5-gallon bucket
trees were about 1 inch in diameter and 7 feet tall when
planted. The 5-gallon bucket trees were estimated to have
about twice as much leaf mass as the whips when planted,
and thus they were expected to have higher evapotranspiration rates.
Design, Goals, and Monitoring
Approaches
The layout for the project (see Figure 6-1) involved planting a separate plot of trees for the whips and the 5-gallon
buckets, with both plots perpendicular to the contaminant
plume. The plume is moving to the southeast, so the plots
were laid out on a northeast axis. The whips section was
planted to the northwest of the 5-gallon buckets, so that the
plume would first travel through the root zone of the whips
and then through the root zone of the 5-gallon buckets. A
control area with monitoring wells was placed to the northwest of the whips, and another in between the whips and
the 5-gallon buckets, along with monitoring wells throughout the treatment site. These control areas enable data to
be collected on the amount of contaminant that enters each
of the treatment areas (whips and 5-gallon buckets), so that
The U.S. Air Force (USAF) Acquisition and Environmental Management Restoration Division and the EPA National
Risk Management Research Laboratory (NRMRL) carried
out the design and implementation of the phytoremediation
strategy at Carswell Site. In April 1996, the USAF planted
660 cottonwoods in an effort to contain and remediate a
plume of dissolved TCE located in a shallow alluvial aquifer
(<12 feet below ground surface). The species P. deltoides
was chosen over a hybridized species of poplar because it
is indigenous to the region. Therefore it has proven its ability to withstand the Texas climate, local pathogens, and
other localized variables that may affect tree growth and
health.
Experimental Design
Legend
Monitoring Well
Monitoring Well
with Data Logger
Nested Wells
Control
Control 2
Whips
Ro
ar
ing
Sp
rin
gs
Ro
ad
1 - 1.5 Inch
Caliper Trees
Cart Path
Farmers Branch Creek
N
0
Mature Tree
Figure 6-1. Experimental Design
61
50
100
200 Feet
a comparison of the performance of each type of tree can
be made.
in the presence of roots from the cottonwood trees. The
products of degradation are anaerobic in the rhizosphere
and aerobic (haloacetic acids and carbon dioxide) in the
canopy. Increased amounts of vinyl chloride and a trace of
TCE as well as iron- and sulfur-reducing conditions in the
rhizosphere were detected at the end of these experiments
(Harvey 1998). The disappearance of PCE in the presence
of roots from a willow tree near the site was even more
remarkable (Wolfe 1997). These experiments indicate that
cottonwoods and willows produce enzymes that can degrade PCE and TCE. Researchers trying to determine how
the trees change the geochemistry of an aerobic aquifer
contaminated with TCE and its breakdown product found
that labile organic matter from the cottonwoods and several
other species of trees is promoting reducing conditions conducive to the degradation of TCE (Harvey 1998).
One unique aspect of Carswell Site is the 19-year-old mature cottonwood growing on the site. This 70-foot-tall tree is
located just southeast of the planting area on the other side
of a cart path. Groundwater monitoring wells were installed
around this tree, and it has been sampled in a similar manner to the planted cottonwoods to see how well a mature
tree functions in this phytoremediation system.
6.2.3
Results and Status
Seventeen months after the trees were planted (summer
1997), several trenches were dug adjacent to selected trees,
and it was determined that the tree roots had reached the
water table (Hendrick 1997). Although the trees are pumping water from the contaminated aquifer, they have not yet
begun to hydraulically control the plume. Combined results
of a transpiration model and a groundwater flow model will
be used to determine when hydraulic control of the plume
might occur. Transpiration measurements indicate that the
largest planted trees pumped approximately 3.75 gpd during summer 1997; the mature 19-year-old cottonwood tree
near the planted trees was determined to pump approximately 350 gpd (Vose 1997).
Groundwater samples had been collected from the 29
monitoring wells and analyzed on three occasions as of
August 1997. Concentrations of TCE, cis-DCE, trans-DCE,
and vinyl chloride were determined from these samples.
They ranged from 2 to 930 ppb TCE in the groundwater, with
most samples falling in the 500- to 600-ppb range (see Figure 6-2). Average concentrations of the contaminants on
the three sampling dates are provided in Table 6-2, with the
exception of vinyl chloride. Vinyl chloride was only detectable in a handful of samples and generally at low levels;
thus, an average concentration was not determined.
Some analytical work has been done on the tree tissues
at the site, but this type of information is still in the early
stages of collection. Data from November 1996 indicated
TCE in the whips that were planted over an area where the
groundwater was the shallowest. This indicates that the
young trees were capable of evapotranspirating TCE after
just one growing season. Qualitatively, both types of trees
were capable of evapotranspiring TCE, and the 5-gallon trees
are evapotranspiring more water than the whips. This was
to be expected because of the greater total surface area of
the leaves of the 5-gallon trees. In addition, the transpiration rates were generally higher in June than May, which is
likely due to a combination of warmer weather and more
fully developed leaves. There also appeared to be a midday
decline in transpiration during June, indicating that the plants
were experiencing water stress during the hottest part of
the day in the summer months. Thus, the water demand for
the tree exceeded the supply during that time. There was
also a notable difference in transpiration rates between days
in June, with cloudier days resulting in lower transpiration
rates. In addition to evapotranspiration information, some
tree growth data have also been collected. In 16 months the
whips grew about 20 feet, and the 5-gallon bucket trees
have grown faster than the whips. Now that the trees have
been on site for over an entire growing season, site managers at Carswell Site have increased monitoring at the site to
include a whole suite of water, soil, air, and tree tissue sample
analysis. Some of the more unique data they are collecting
(in relation to the other case study sites) are analyses of
microbial populations and assays of TCE-degrading enzymes in the trees.
TCE concentrations in groundwater samples collected beneath the 19-year-old cottonwood tree during summer 1997
were about 80% less than concentrations in groundwater
beneath the planted trees, and cis-1,2 DCE (byproduct of
TCE degradation) concentrations were about 100% greater.
These data, along with additional geochemistry data from
the site, are consistent with microbial degradation of TCE
beneath the mature tree (Lee 1997). Microbes with the ability to readily degrade TCE require an environment that is
low in dissolved oxygen and high in an appropriate source
of organic carbon. These conditions, which are often lacking at sites contaminated with TCE, exist in the aquifer under the mature tree and are likely due to the introduction of
organic matter from tree-root activity. Once the planted cottonwood trees have established more mature root systems,
an environment could develop in the aquifer beneath the
trees that would promote biodegradation and result in an
additional mechanism for attenuation of TCE. The effect of
other mature trees such as willows, oaks, junipers, mesquite, ashes, and sycamores on the geochemistry of the
groundwater in the winter and spring is also being explored.
6.2.4
Costs
Some rough estimates of cost for the Carswell Site have
been provided by site managers. These estimates can be
found in Table 6-3. Since this site involves an innovative
treatment technology, these costs are substantially inflated
due to the heavy monitoring taking place at the site. Also,
long-term projected costs and/or total project costs are not
available because the time involved in remediating the site
is uncertain. In addition to the costs in the table, $200,000
Laboratory experiments conducted on root samples from
the site show the disappearance of perchloroethylene (PCE)
62
TCE Concentrations (PPB) from Site Characterization Work
278
438
Ro
830
ar
490
ing
Sp
rin
900
gs
230
Ro
ad
Cart Path
960
890
Farmers Branch Creek
920
620
920
970
890
N
0
50
100
200 Feet
Figure 6-2. TCE Concentrations
More extensive cost and performance data from the demonstration are being compiled to assist others in selecting
phytoremediation as a treatment technology. The subsurface fine biomass study will also define the volume of soil
exploited by the trees at any given point in time. A typical
poplar plantation grown as a short rotation woody crop can
produce up to 50,000 to 75,000 miles of fine roots per acre.
Also, a groundwater flow and transport model of the site is
planned to help determine the relative importance of various attenuation processes in the aquifer to guide data collection at future sites. The model will also be used to help
predict the fate of TCE at the demonstration site in an effort
to gain regulatory acceptance of this remedial action.
Table 6-2. Average Concentrations of TCE, cis-DCE, and trans-DCE
at Carswell Site.
Contaminant
Average Concentration (ppb)
December 1996
May 1997
TCE
cis -DCE
trans-DCE
610
130
4
July 1997
570
140
2
550
170
4
Table 6-3. Estimated Cost of Phytoremediation at the Carswell Site.
Activity
Estimated Cost
Wholesale cost of trees (does not
tree
include delivery or installation costs)
$8/tree for 5-gallon bucket
$0.20/tree for whips
6.2.5
29 wells (including surveying, drilling
and testing)
$200,000
Subsurface fine biomass study (the
vertical and lateral extent of tree
roots less than 2 mm in diameter)
$60,000
There are over 900 Air Force sites with TCE contamination within 20 feet of land surface that could be reviewed for
potential application of phytoremediation by use of poplar
trees (Giamonna 1997). Costs may be 10 to 20% of those
for mechanical treatments. Scale-up costs for large scale
applications of phytoremediation can be minimized by exploiting the body of data developed for the Department of
Energy on the planting and cultivation of poplar trees for the
purpose of biomass production.
will be spent for extensive site monitoring that would not
normally be associated with a phytoremediation system;
thus, this amount was not included in the cost estimates.
63
Conclusions
6.2.6
Contacts
aquifer tests revealed a high yield aquifer, which would
require severe over pumping to create any substantial cone
of influence around the pumping wells. Contaminants trapped
in the silty-clay lens beneath the site would be difficult to
extract in this manner because the transfer rate of contaminants into the groundwater is slow. As a result, large volumes of groundwater would need to be pumped to the surface for treatment, and this water would contain low concentrations of contaminant. Also, neighbors of the property
would be disturbed by the noise created by a pump-andtreat system.
Greg Harvey
US Air Force ASC/EMR
WP AFB, OH 45433
(937) 255-7716, Ext. 302
Steven Rock
US EPA National Risk Management Research Laboratory
Cincinnati, OH 45268
(513) 569-7149
6.3 Edward Sears Properties Site
New Gretna, New Jersey
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
Area of planting:
Date of planting:
6.3.1
Based on these results, a pump-and-treat option would
be expensive and inefficient for the Edward Sears site. Site
managers then moved to consider a phytoremediation option. This site was judged as a potential candidate for a
phytoremediation system due to the nature of the soils and
groundwater. There is a highly permeable sand layer about
4 to 5 feet bgs, but below that exists a much-less-permeable layer of sand, silt, and clay from 5 to 18 feet bgs. This
silt, sand, and clay layer acts as a semiconfining unit for
water and contaminants percolating down toward an unconfined aquifer from 18 to 80 feet bgs. This unconfined aquifer
is composed primarily of sand and is highly permeable. The
top of the aquifer is about 9 feet bgs, which lies in the lesspermeable sand, silt, and clay layer. Most of the contamination is confined from 5 to 18 feet bgs; thus, site managers decided to plant hybrid poplars in order to prevent further migration of the contaminants and ultimately remove
contaminants from the groundwater.
Edward Sears Properties Site
New Gretna, New Jersey
Groundwater (9 ft bgs)
Trichloroethylene (TCE),
<400 ppb
Populus charkowiieensis x incrassata
(Hybrid Poplar)
1/3 acre
December 1996
Site Description
From the mid-1960’s to the early 1990’s, Edward Sears
repackaged and sold expired paints, adhesives, paint
thinners, and various military surplus materials out of his
backyard in New Gretna, NJ (see Figure 6-3). As a result,
toxic materials were stored in leaking drums and containers
on his property for many years. The soil and groundwater
were contaminated with numerous hazardous wastes, including methylene chloride, tetrachloroethylene, trichloroethylene, trimethylbenzene, and xylene. After his death, no
one could be found responsible for the site or its cleanup;
thus, On-Scene Coordinators (OSC) from EPA’s Region 2
Removal Action Branch were called in to remove the leaking drums of hazardous materials, including off-specification paints and solvents. Soil sampling indicated that two
areas, 35 x 40 feet and 15 x 20 feet, were very heavily
contaminated with solvents. These soils were removed to 8
feet below ground surface (bgs) (just above the water table).
Further excavation could not be achieved without pumping
and treating large volumes of groundwater. The excavated
areas were backfilled with clean sand and the OSC activated the EPA’s Environmental Response Team (ERT) of
Edison, NJ to determine the extent of groundwater and deep
soil contamination.
Samples were taken from temporary well points throughout the site. Data from these sampling efforts indicated
trichloroethylene (TCE) concentrations in the groundwater
ranged from 0 to 390 ppb. Most of the TCE was concentrated in a small area on the site. Seven monitoring wells
were installed based on the information obtained from the
temporary well points. Monitoring Well 1 was installed in the
area of highest TCE contamination. Little or undetectable
TCE was found in the groundwater samples from the other
six wells.
6.3.2
Design, Goals, and Monitoring
Approaches
Under the Response Engineering and Analytical Contract
(REAC), a pilot phytoremediation test was conducted at the
Sears site to determine whether hybrid poplar trees can be
used to reduce soil and groundwater VOC contamination
levels in the planted area and to prevent further offsite migration of contaminated groundwater. In October and November 1996, the site was cleared of debris and a 4-inch
clay layer was placed approximately 1 foot bgs to prevent
penetration of rainwater into the upper root zone, thus promoting root growth into the underlying aquifer. This was followed by the replacement and grading of the native surface
soil.
Using innovative hydraulic-push groundwater sampling
techniques, the ERT investigation revealed localized, highly
contaminated groundwater. Based on this information, a limited number of monitoring wells (see Figure 6-4) were installed to determine vertical contaminant migration and to
conduct aquifer tests necessary to evaluate pump-and-treat
options. A pilot test for a pump-and-treat system with air
stripping and activated carbon was then conducted. The
Thomas Consultants, Inc. of Cincinnati, OH were subcontracted to lay out the phytoremediation design. In December 1996, 118 hybrid poplar saplings ( Populus
64
’s C
Job
Fords Residence Occupied
re
ek
(
tid
al
r
ta
bu
tri
y)
Drum Storage
Area
Unoccupied
Trailer
Delaportes
Property - Occupied
Residence - Occupied
Edward Sears
Residence - Occupied
Route 9 North
Unoccupied
Gas Station
Legend
Tree Line
Wetland
Roads
Fence
Residence
Site Map
Edward Sears Property Site
New Gretna, NJ
March 1997
Graphic Scale
110
0
55
110
120
U.S. EPA Environmental Response Team Center
Figure 6-3. Site Map
charkowiieensis x incrassata, NE 308) were planted by
ERT, REAC, and Thomas Consultants personnel in a 1/3acre plot. The trees were planted 10 feet apart on the axis
running from north to south and 12.5 feet apart on the eastwest axis.
and the remaining 5 feet to surface was filled with clays
removed during the boring process.
About 90 poplars still remained after the deep rooting
was completed. These extra trees were planted along the
boundary to the north, west, and east sides of the site.
These trees were only planted to a depth of 3 feet, or shallow rooted. The shallow-rooted trees were added to prevent rainwater infiltration from off site and to replace any
loss of deep-rooted trees. These trees were planted very
close together (about 3 feet apart) under the assumption
that natural thinning would take place over subsequent
growing seasons. A surface water control system was then
installed by planting grasses over the entire site. These
grasses came from commercially available seeds purchased from a lawn and garden store.
A process called deep rooting was used to plant the trees.
In deep rooting, the roughly 12-foot trees were buried 9 feet
so that only about 2 to 3 feet remained on the surface. Deep
rooting the trees involved drilling 12-inch-diameter holes to
a depth of 13 feet. These holes were then back filled to 5
feet below ground surface with amendments such as peat
moss, sand, limestone, and phosphate fertilizer. This backfill was installed to provide nutrients to the roots as they
penetrated down through the soils. Waxed cardboard cylinders 12 inches in diameter and 4 feet long were installed
above the backfill to promote root growth down into the
groundwater. These barriers settled about 1 foot into the
planting holes; therefore, 5-gallon buckets with the bottoms
cut out were placed on top of the cylinders to create a 5foot bgs root barrier. The trees were placed in the cylinders
ERT is conducting an ongoing maintenance and monitoring program at Edward Sears. Monitoring of the site includes periodic sampling of groundwater, soils, soil gas,
plant tissue, and evapotranspiration gas. Continued growth
65
Legend
Clean Backfill
Deep Planted Tree
Shallow Planted Tree
Soil oring
Monitor Well
Property Boundary
Tree
Geoprobe Sampling
Locations
DCW Contamination
TCE Contamination
PCE Contamination
Delaportes
Property - Occupied
Sears
Well
Unoccupied
Gas Station
Route 9
Edward Sears
Residence Unoccupied
Graphic Scale
35
0 17.5
35
70
U.S. Environmental Response Team Center
Response Engineering and Analytical Contract
Sampling Grid with EOC
Outlines-9/96 Data
Edward Sears Property
New Gretna, NJ
October 1997
Figure 6-4. Sampling Grid
piration gas was sampled by placing TedlarTM bags over
entire trees. Data from these air samples suggest that the
trees are evapotranspirating some of the VOC’s. However,
the VOC concentration in the TedlarTM bags matches the
background concentrations of VOCs in control samples.
This could be due to VOCs volatilizing from the soils, or it
could be due to evapotranspirated VOCs that may have
gotten into the control samples. Future sampling designs
will attempt to determine accurate background VOC concentrations. The trees have grown about 30 inches since
planting. Site managers plan to sacrifice one tree either
after or during the next growing season to determine the
extent of root growth.
measurements will also be made as the trees mature. In
the fall of 1997, the surface water control system was replaced due to a summer drought that killed much of the
grass. Site maintenance also involves the prevention of
deer and insect damage. Bars of soap were hung from the
trees to deter deer from rubbing their antlers on the trees.
Some damage was inflicted by an insect larva known as
the poplar leaf caterpillar. This caterpillar lives on poplar
trees and makes its cocoon by rolling itself in a poplar leaf.
A spray containing Bacillus thuringesis, a bacteria that produces toxins specific to various insects, was applied to the
site. This spray has been effective in killing most of the
caterpillars living on the trees.
6.3.3
6.3.4
Results and Status
Costs
The total cost for the installation of 118 deep-rooted and
90 shallow-rooted trees was approximately $25,000. Additionally, installation of the surface water control system
and one year of on-site maintenance totaled about $15,000.
Because the trees had only one full growing season,
very little performance data are available; however more
data are expected in the next growing season. Evapotrans-
66
6.3.5
Contacts
6.4.2
The goal of the project was to assess several landfill
cover designs for controlling water infiltration in a humid
region. Results of the demonstration could be applicable
to various types of disposal materials such as radioactive
waste, uranium mill tailings, hazardous waste, and sanitary waste.
George Prince
US EPA Environmental Response Team
Edison, NJ 00837
(732) 321-6649
6.4 Bioengineering Management: U.S.
Nuclear Regulatory Commission
Beltsville, MD
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
Area of planting:
Date of planting:
6.4.1
Goals
6.4.3
Design
Cover performance was demonstrated in six large-scale
lysimeters with dimensions of 70 ft x 45 ft, a slope grade of
5%, and the bottom of the lysimeters at 10 ft below grade.
For each lysimeter, underlying waste conditions were simulated by applying the contents of 55-gallon steel drums onethird filled with gravel and by tilling the remaining area with
native soil. Table 6-4 summarizes the design type for each
lysimeter.
Bioengineering Management
U.S. Nuclear Regulatory Commission
Beltsville, Maryland
Landfill cover
Cover was installed over a lysimeter
The bioengineering management surface barrier in Lysimeters 1 and 2 was installed in May 1987. The bioengineering
management technique utilizes a combination of engineered
enhanced runoff and vegetation to minimize water infiltration. The covers consisted of 4-ft-wide rows of alternative
aluminum and fiberglass panels with Pfitzer Junipers planted
between the panels at 4-inch widths. The alternating aluminum and fiberglass panels covered over 90% of the surface
layer of the cap. Pfitzer Juniper was chosen in part because
of its drought-resistant characteristics and the success that
the Maxey Flats project encountered using this type of vegetation. The water levels for Plots 1 and 2 were approximately 35 and 75 inches above the bottom of the lysimeters, respectively. The water levels were used to simulate
the water table in the flooded disposal cell.
Pfitzer juniper and fescue
70 m x 45 m test plots
1987-present
Site Description
Three distinct landfill cover concepts were investigated
at the University of Maryland Agricultural Experiment Station, sponsored by the Office of Nuclear Regulatory Research, in Beltsville, MD. The purpose of the full-scale demonstration was to examine and demonstrate various approaches for minimizing water infiltration through landfill
covers. The study, initiated in 1987, evaluated the following
type of design concepts: resistive layer barrier (compacted
clay design), conductive layer barrier (capillary design), and
bioengineering management surface barrier (runoff control/
evapotranspiration design).
In May 1987, reference Lysimeters 3 and 4 were constructed alongside Lysimeters 1 and 2. The reference lysimeters are similar in design to Plots 1 and 2. However, the
cover designs for the reference plots contain only fescue
grass; no impermeable cap was installed. Additionally, Plot
4 was discontinued as a reference lysimeter in February
1988 and converted to a rip-rap surface layer and gravel
drainage layer over a compacted clay layer cover. The cap
design in Lysimeter 5 was a vegetated soil surface layer,
Among the three basic design types in the study, the
bioengineering management design demonstrated the greatest potential for preventing water infiltration and for managing subsidence conditions. The bioengineering management
surface barrier utilizes the evapotranspiration processes of
vegetation and enhancements to runoff to prevent water infiltration to underlying waste. By diverting enough annual
precipitation to runoff and by removing moisture from the
soil profile using evapotranspiration processes, the design
can potentially prevent deep percolation on a yearly basis.
Table 6-4. Design Type and Completion Dates for the Experimental
Covers.
The bioengineering management design is based on a
similar cover design applied at Maxey Flats, KY. In the Maxey
Flats project, a bioengineered cover was constructed by
partially covering fescue grass with an engineered cover of
stainless steel that resulted in no measured percolation of
water through the cover.
Lysimeter
1
2
3
During the Beltsville, MD 9-year study, the bioengineering management surface layer also prevented deep percolation. In addition, this type of design is easily repairable
and involves a minimum amount of materials, equipment,
and labor for construction. Thus, this system provides a
potentially effective approach for addressing damages from
active subsidence conditions.
4
4
5
6
67
Description of
Design
Bioengineering Management
Bioengineering Management
Vegetated Crowned Soil Cover
(reference plot)
Vegetated Crowned Soil Cover
(reference plot)
Rip-Rap over Resistive Layer
Barrier
Resistive Layer over Conductive
Layer Barrier
Vegetation over Resistive Layer
Barrier
Date of Completed
Construction
May 1987
May 1987
May 1987
May 1987
October 1988
January 1990
April 1989
gravel drainage layer, and compacted clay layer over a
gravel capillary barrier. In Plot 6, the cap design consisted
of a vegetated surface layer and gravel drainage layer over
a compacted clay layer.
6.4.4
a rip-rap surface layer cover was installed on Lysimeter 4
in February 1988. Lysimeter 3 continued as a reference
plot throughout the study, although results continuously
indicated deep percolation. For example, deep percolation accounted for 40% of the fate of total precipitation
during 1993-1994.
Monitoring Approaches
For the 9-year study, several water balance parameters
were measured, such as annual precipitation and runoff.
Additionally, neutron probe measurements of soil moisture
in all six lysimeters were taken continuously for 8 years. To
increase the accuracy of the measurements, the neutron
probe apparatus was calibrated with the native soil used for
each of the lysimeters or plots. Instruments were placed
between liners to measure leakage at each lysimeter. Four
liners were used in each plot to create a closed system for
a complete water balance system.
6.4.5
The bioengineering management surface barrier has
been implemented at two sites in Hawaii and New York. At
the Marine Corps Base in Kaneohe Bay, Hawaii, a 14month study was conducted to demonstrate diversion and
removal of annual precipitation by runoff control and evapotranspiration. The Kaneohe Bay study evaluated two types
of designs: 20% enhancement of runoff and 40% enhancement of runoff, along with a conventional soil cover design. The covers with the runoff enhancements used rain
gutters and several native types of vegetation such as
grasses and shrubs (primarily of the genera Acacia and
Panicum). Results of the study demonstrated that the two
design types increased runoff by a factor of 2 to 3 over the
conventional soil design cover. Additionally, the data indicated a reduction in percolation by a factor of 2 to 3 from
the two infiltration control covers over the soil cover, although these differences were not statistically significant.
Finally, statistical tests indicated no advantage of using
the 40% enhancement of runoff over the 20% enhancement of runoff: both produced the same amount of runoff
and percolation.
Results and Status
During the 9-year period (1988-1996), the bioengineered
covers in Lysimeters 1 and 2 experienced no deep percolation. Additionally, the water tables for both plots were eliminated by July 1989. Table 6-5 illustrates the percentage of
rainfall managed by runoff, evapotranspiration, and deep percolation at the two bioengineering management surface barriers. The percent of precipitation associated with evapotranspiration increased annually because of the greater vegetative canopy.
The moisture content of the soil profiles for Lysimeters 1
and 2 decreased annually. Thus, after the water table was
eliminated from both lysimeters, the soil profiles continually “dried out.” However, the soil moisture content for both
lysimeters, although much lower in the soil profile than at
the beginning of the study, still increased with depth. In addition, seasonal cyclical variations in moisture content occurred throughout the study. For example, high moisture
peaks in the soil profile were observed during high incidents
of rainfall events and periods of low evapotranspiration.
In 1993, the New York State Energy Research and Development Authority implemented a bioengineering management system at a low-level waste facility in West Valley, NY. The bioengineered cover was only installed over
one trench at the site, 550 ft x 35 ft, for a total cost of
$70,000. Soil moisture and trench leachate data have
shown no vertical infiltration to date.
6.4.6
A majority of precipitation for reference Lysimeters 3 and
4, with only fescue grass, was managed by evapotranspiration. However, this process was not adequate to prevent the
rise of the water table. Subsequently, both lysimeters were
pumped to prevent the water from overflowing. As a result,
Edward O’ Donnell
NRC Project Manager
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
(301) 415-6265
6.5 Lakeside Reclamation Landfill
Beaverton, Oregon
Table 6-5. Summary of Run-off, Evapotranspiration, and Deep
Percolation From the Bioengineered Plots.
Year
Runoff
Evapotranspiration
Deep Percolation
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
Area of planting:
Date of planting:
Percent of Precipitation
1988
1989
1990
1991
1992
1993
1994
1995
1996
80
74
70
67
63
61
61
58
57
20
26
30
33
37
39
39
39
43
Contacts
0
0
0
0
0
0
0
0
0
68
Lakeside Reclamation Landfill
Beaverton, Oregon
Landfill Cover
NA
Hybrid Poplars and Rye Grass
8 acres
1990-present
6.5.1
Site Description
The hybrid poplar tree cuttings and cool-season grasses
were selected as the type of vegetation to be planted on the
initial demonstration cell cover. Hybrid poplars were selected
primarily because of the research being conducted at the
University of Iowa using these trees for buffer systems. Secondly, the landfill borders the Tualatin River riparian area
populated by both deciduous and conifer trees, thus providing evidence that the site is capable of growing hybrid poplars. Thirdly, the hybrid poplars had a relatively long growing
season, extending from mid-March through November. Finally, hybrid poplars offered the potential for dense tree population, deep root placement, and large quantities of water to
be transpired per tree.
The Lakeside Reclamation Landfill (LRL), located near
Beaverton, OR, is actively receiving nonrecyclable construction demolition debris for disposal on the 60-acre site.
As the waste cells fill to capacity, the owner/operator of
the landfill is required to install covers to fulfill final closure
requirements. In April 1990, a 0.6-acre prototype cap consisting of hybrid poplar trees, primarily cottonwoods, was
installed on a recently filled waste cell. The demonstration
cover was an alternative to using a conventional cover consisting of a geosynthetic membrane and several soil layers. The objective of the initial project was to demonstrate
the application of hybrid poplar trees to effectively prevent
infiltration of water to underlying waste. The prototype plot
was designed to provide the data required to meet regulatory compliance and to provide enough information to close
the entire landfill using this capping technique. The migration of contaminated leachate is a concern to the Oregon
Department of Environmental Quality (DEQ) because the
Tualatin River is located adjacent to the LRL.
In April 1990, approximately 7,455 tree cuttings were
planted on the 0.6-acre site (60 ft x 600 ft), for an average
plant density of 3.4 ft2 per tree. The rows are 42 inches
apart with roughly 1 ft of spacing between the trees in each
row. Three different hybrid poplar varieties were planted in
the prototype cover: DO-1 variety trees from Dula Nurseries, Canby, OR; Imperial Carolina variety from Ecolotree,
Inc., Iowa City, IA; and NE-19 variety from Hramoor Nurseries, Manistee, MI. All tree varieties were available in 5-ft
cutting lengths and were planted at a depth of 40 inches.
The Imperial Carolina and DO-1 variety were also available
in 2-ft cutting lengths and planted at 15-inch depths. No soil
stabilizers, fertilizers, or pre-emergent weed herbicides were
used to plant the tree cuttings.
The waste buried at Lakeside Reclamation Landfill is
restricted primarily to construction demolition debris, tree
stumps, yard wastes, and packaging and crating materials. The contaminants of concern are metals, nitrates, and
phosphorous leachate.
Based on the results of the initial prototype cap, the demonstration was expanded to a 2-acre area in 1991. To date,
the cap has expanded to a total of 8 acres and has demonstrated no deep percolation. For the final closure of the
landfill, the vegetative cover will enclose an area of approximately 40 acres at the Lakeside Reclamation Landfill.
The area of the demonstration was expanded by an additional 1.3 acres in 1991 and a new planting density of 5.2 ft2
per tree. To diversify the cultivar base, 18 new tree varieties
were planted. The new tree cuttings planted were composed
of varieties that had the capability to grow in the Pacific
Northwest region. For the 1992 growing season, a cool-season grass crop, rye grass, had been established on a portion of the demonstration site. The addition of grasses to
the design improved the cover by increasing the evapotranspiration process during the tree’s dormant period, controlling weed growth, and increasing the overall soil stability
during the early periods of tree development. In addition, as
an alternative to grass, another portion of the cover was
mulched with bark chips.
For the LRL landfill, the Oregon DEQ has issued a permit for final closure requirements under Condition 3 of
Schedule C, Solid Waste Disposal Permit No. 214. The
final cover must meet or exceed the mandatory minimum
groundwater quality protection requirements.
6.5.2
Goals
The goal of the initial project was to demonstrate an effective cover design for preventing water infiltration and acquire enough data from the prototype cap to satisfy the
Oregon DEQ. Concurrently, the project must meet or exceed the regulatory requirements for groundwater quality
protection and establish a capping technique for the entire
landfill. Other objectives of installing this cover design were
to provide a low-cost manageable cover that provides a wildlife habitat, stable soils, and a sustainable ecosystem.
6.5.3
6.5.4
Monitoring Approaches
Lysimeters, piezometers, and tensiometers were installed
in May 1990 to collect water samples and measure moisture content in the soil cover. Four instrument “nests” were
placed on the landfill cover. Each instrument nest contained
three suction lysimeters and three ceramic cup tensiometers installed at 1-, 3-, and 5-ft depths. Another instrument
nest was installed off the prototype cap to provide background measurements.
Design
The cover design for the prototype cap is composed of
hybrid poplars (primarily cottonwood trees) and silt loam
soils. The waste cells of the landfill were initially covered
with two layers of silty loam soil at a thickness of roughly 5
ft and graded at a 3% slope. The layer installed over the
waste is approximately 1 ft of compacted silt loam soil that
has a high-clay content. The surface layer consists of
loosely placed loam soil at a depth of 4 ft.
6.5.5
Results and Status
The survival rate for the hybrid poplar trees planted in
April 1990 was greater than 85% and there were no observed losses for the 1991 growing season. The survival
rates for the Imperial Carolina tree variety and the DO-1
tree variety were greater than 90%. However, the NE-19
69
variety had only a 54% survival rate. The low survival rate
was likely due to a December freeze that damaged the cuttings. Despite this relatively low survival rate, the NE-19
variety is desirable to plant because of its longer growing
season. The base diameter of the tree stem and terminal
bud height was measured to estimate tree growth and vigor.
The extent of tree growth was illustrated by comparing measurements in August 1990 to those in March 1992, which
demonstrated a mean height of 6.8 ft and 12.7 ft, respectively.
LRL for incorporation into the cap. To date, monitoring data
has indicated that moisture has only penetrated to a maximum depth of 4 feet.
6.5.6
Contacts
Howard Grabhorn
Owner/Operator
Lakeside Reclamation, Grabhorn, Inc.
Beaverton, OR 97006
(503) 628-1866
Weekly tensiometer data were collected to determine the
moisture content of soils at various locations both on and
off the 0.6 acre cover. The moisture content is described in
general terms, “wet” or “dry.” The tensiometers were not
calibrated to the variable soil conditions because of the
mixture of repacked soils in the landfill material. From September to December 1990, the tensiometer indicated moisture content, at the 1-ft horizon, fluctuated considerably from
saturated to very dry depending on precipitation, solar intensity, air temperature, relative humidity, ground cover, and
shade at the soil surface. In addition, during the growing
season, there was no apparent impact on the tensiometer
reading at the 3- and 5-ft depths, implying no change in the
moisture content. Significantly less water was present at
the 5-ft soil profile in 1991, than in 1990. In addition, moisture content observed in December 1991 indicated the soil
profile had more storage capacity available than was available in measurements taken the previous year.
Wesley M. Jarrell
Professor
Oregon Graduate Institute of Science & Technology
P.O. Box 9100
Portland, OR 97291
(503) 690-1183
Louis A. Licht
President
Ecolotree, Inc.
102A Oakdale Hall
Iowa City, IA 52319
(319) 358-9753
Mark F. Madison
Agricultural Engineer
CH2M Hill
Portland, OR
(503) 235-5002 Ext. 4453
Samples of the soil water were acquired through suction
lysimeters as part of the instrumental nest monitoring. Only
one set of samples was screened in May 1990 for nutrients.
Nitrate concentrations in this set of lysimeter samples did
not measure above the EPA’s Maximum Contaminant Level
(MCL).
6.6 Alternative Landfill Cover
DemonstrationSandia National
Laboratories Albuquerque, NM
During 1993 and 1994, the Oregon Graduate Institute of
Science and Technology conducted a field experiment at
LRL to investigate leachate production potential under a
grass cap and a hybrid poplar cap. The soil moisture was
monitored in 25 wells (20 in the hybrid poplar plot and 5 in
the grass plot) using a neutron probe and later a capacitance probe. The depths of the wells varied from 32.5 to 59
inches. Annual precipitation was 37 inches for 1993 and 59
inches for 1994. In general, the results of the 2-year study
indicate that the average soil moisture under the grass cover
was higher than that under the hybrid poplar cover. Additionally, the vertical soil moisture profile study revealed that
soil moisture varied less under the poplar trees than under
the grasses.
Site name:
Location:
Alternative Landfill Cover Demonstration
Sandia National Laboratories
Albuquerque, New Mexico
Media:
Landfill Cover
Primary contaminant Covers were installed over lysimeters
and maximum
with no waste or contaminants
concentration:
Type of plant:
Wheat Grass, Indian Ricegrass, Alkali
Sacaton, Sand Dropseed and FourWing Saltbrush
Area of planting:
10 m X 10 m test plots
Date of planting:
1995-1996
6.6.1
Currently, the cap has been expanded to 8 acres and consists of 25 varieties of hybrid poplars (cottonwoods). The
diversity in tree species prevents a single disease from destroying the entire tree population and allows for a broader
growing season for the cap. In addition, another 25 varieties
of hybrid poplars are being evaluated in a greenhouse at
Site Description
The Alternative Landfill Cover Demonstration (ALCD) is
a large-scale field test comparing final landfill cover designs at Sandia National Laboratories, located on Kirtland
Air Force Base in Albuquerque, New Mexico. The demonstration is testing innovative landfill covers using currently
70
and the climate conditions of the area. The Albuquerque
climate is an arid/semiarid environment with average annual rainfall of 20.6 cm/yr. Based on the results, a 90-cmthick monolithic soil cap was installed. The origin of soil
was from on-site cut excavations. The bottom 75 cm was
placed in 15-cm-deep lifts and compacted, while the top
15 cm was loosely placed topsoil. The type of vegetation
used in the design was based on the premise of extending
the evapotranspiration process of the plants over as much
of the growing season as possible. Therefore, the vegetation consisted of an optimum mixture of species (such as
grasses, shrubs, or trees) and an optimum blend of cool
and warm weather plants. The vegetation drill-seeded for
the cover was composed of native species, primarily
grasses such as crested wheat grass, Indian ricegrass,
alkali sacaton, sand dropseed, and four-wing saltbrush.
accepted EPA cover designs as baselines. Two conventional and four alternative cover designs were constructed
in 1995 and 1996 and are currently being monitored. The
two traditional cover designs are a RCRA Subtitle D Soil
Cover and a RCRA Subtitle C Compacted Clay Cover, and
the four alternatives are Geosynthetic Clay Cover, Capillary Barrier, Anisotropic Barrier, and Evapotranspiration
Cover. Of the four alternatives, the Evapotranspiration
Cover utilizes vegetation, along with soil texture and depth,
as the primary mechanism to minimize infiltration of water
into underlying waste. To a lesser degree, the Capillary
Barrier and the Anisotropic Barrier are also designed to
enhance evapotranspiration using vegetation and encourage water storage for the prevention of water infiltration.
In addition, the ALCD includes a side study to assess eight
different enhancement techniques to the vegetation planted
on several of the covers.
6.6.2
A side study was established to assess enhancements
to induce the growth of vegetation seeded on the ET Cover.
Twenty-four 10-m x 10-m test plots were installed alongside the cover. Accordingly, there are three sets of eight
different surface augmentations for statistical analysis. Soil
moisture is being sampled at a depth of 1.2 m to evaluate
the effect of the surface enhancements on evapotranspiration.
Goals
The purpose of the demonstration is to evaluate the various cover designs based on their respective water balance
performance, reliability and ease of construction, and cost
for arid and semiarid environments. Also, the data from the
demonstration will be available for validation of EPA’s HELP
(Hydrologic Evaluation of Landfill Performance) model.
6.6.3
The Capillary Barrier is designed to use the difference in
hydraulic conductivity of the two soil layers under unsaturated flow conditions to cause water to be retained in the
upper soil layer. The cover design of this capillary barrier
was composed of four primary layers: surface layer, upper
drainage layer, barrier layer, and lower drainage layer. In
general, the cover design consisted of the barrier layer
and lower drainage layer forming the capillary barrier. The
upper drainage layer, composed of pea-gravel and sand,
served as both a filter to prevent clogging and allowed for
lateral water movement. The surface layer, 30 cm of topsoil, is placed to provide a medium for growth of vegetation and enhance evapotranspiration. It also protects the
barrier soil layer from desiccation and protects against
surface erosion.
Design
All of the test covers are installed and instrumented in a
side-by-side demonstration. Each of the plots are 100 m x
13 m, crowned in the middle with a constructed 5% slope
for each layer. Hence, the slope lengths are 50 m each and
sloping to the west and east. The western slope of each
cover is monitored under ambient or passive conditions.
For the eastern slope, a sprinkler system is utilized to facilitate additional precipitation to the test plots, providing
hydrological stress to the various covers. This system represents peak or worse case precipitation events.
The two conventional cover designs were installed to provide a baseline for comparison among the four alternatives.
The Soil Cover design satisfies the minimum requirements
set forth for
The Anisotropic Barrier is composed of layered capillary
barriers that function to limit downward movement of water while enhancing lateral movement. The cover design
consists of four layers: top vegetation layer, soil layer, interface layer, and sublayer. The top vegetation layer is 15
cm thick and is composed of topsoil and pea gravel (gravel
to soil mixture is 25% by weight). The vegetation is for
encouraging the evapotranspiration processes of the vegetation, while the pea gravel is primarily for minimizing erosion effects. The soil layer is 60 cm of native soil and functions for water storage and a rooting medium for vegetation. The interface layer serves as a drainage layer to laterally divert water that has infiltrated the soil layer. Both
the interface layer and sublayer function as bio-barriers to
prevent roots and burrowing animals from intruding into
the underlying material.
RCRA Subtitle D landfills, which are typically municipal
solid waste landfills. The second baseline cover is a Compacted Clay Cover designed to meet the RCRA Subtitle C
requirements for hazardous waste landfills. Among the four
alternative cover designs, the Geosynthetic Clay Cover is
the most similar in design and function to the traditional
compacted clay cover. The Geosynthetic Clay Cover design is identical to the compacted clay cover except for the
clay barrier layer which consists of a manufactured sheet, a
geosynthetic clay liner (GCL).
The Evapotranspiration (ET) Cover consists of a single,
vegetative soil layer constructed with an optimum mix of
soil texture, soil thickness, and vegetation cover. The cover
is engineered to increase water storage and enhance evapotranspiration to minimize the infiltration of water. The design
of the cover was based on the results of a computer model
6.6.4
Monitoring Approaches
Continuous water balance and meteorological data are
being collected for all six covers. In addition, periodic mea-
71
surements are being taken to produce data on vegetation.
Continuous water balance data include soil moisture status, soil temperature, runoff and erosion, and percolation
and interflow. At a weather station installed at the ALCD
site, meteorological data is being collected for the following
parameters: precipitation, air temperature, relative humidity, wind speed and direction, and solar radiation. Finally,
several attributes of vegetation are being measured seasonally, such as biomass, leaf area index, and species composition.
According to Stephen Dwyer, the site manager, the
amount of percolation through the Capillary Barrier can be
attributed to the initial design of the cap. The fine layer of
the cover was installed below the surface to protect against
desiccation, freeze-thaw cycles, etc. Unfortunately, this
design specification does not allow for maximum enhancement of evapotranspiration processes. This concept is further illustrated by the Anisotropic Barrier, designed with the
fine layer at the surface, which seems to be performing adequately to date.
6.6.5
6.6.6
Results and Status
Monitoring data will be collected for a minimum of 5 years
after construction of the covers. Additionally, the data will
be made available on a yearly basis. Table 6-6 displays the
results from the first year of collecting data for the six cover
designs (May 1997 through March 1998).
The individual construction cost for each cover is presented in Table 6-7. These values only represent construction costs and do not include instrumentation equipment,
monitoring provisions, or other items associated with cover
testing.
6.6.7
Table 6-6. Summary of Percolation and Precipitation Rates From
May 1997 Through March 1998 for the Six Cover Designs.
Description
Percolation (L)
Precipitation (L)
6,724
380,380
1.77
RCRA Subtitle C
Compacted Clay
Cover
46
380,380
0.01
Contact
Stephen F. Dwyer
Sandia National Laboratories (MS 0719)
P.O. Box 5800
Albuquerque, NM 87185
(505)844-0595
Percolation/
Precipitation (%)
RCRA Subtitle D
Soil Cover
Costs
Table 6.7 Construction Costs for the Final Landfill Covers
Geosynthetic Clay
Liner Cover
572
380,380
0.15
Description
m 2)
Evapotranspiration
Cover
80
380,380
0.02
804
380,380
0.21
63
380,380
0.02
RCRA Subtitle D Soil Cover
RCRA Subtitle C Compacted Clay Cover
Geosynthetic Clay Liner Cover
Evapotranspiration Cover
Capillary Barrier
Anisotropic Barrier
Capillary Barrier
Anisotropic Barrier
72
Cost ($/
52.42
157.58
92.89
72.66
96.45
74.92
Appendix A
Glossary
Accumulation coefficient: The ratio of a contaminant
concentration in biomass to the contaminant concentration in soil.
Alpine pennycress (Thlaspi caerulescens): A potentially useful, but slow-growing and low- biomass
hyperaccumulator plant.
Application(s): Something that is done by a
phytoremediation technology; e.g., remediation of TCE in
groundwater.
Bioaccumulation coefficient: The ratio of metal concentration in the plant ( g metal/g dry weight tissue) to the
initial solution concentration of the metal (mg metal/L), for
rhizofiltration of metals (Salt et al. 1995).
Bioconcentration factor (Bv): The concentration in
aboveground plant parts (on a dry-weight basis) divided
by the concentration in the soil for organic compounds
(Paterson et al. 1990).
Biogeochemical prospecting: Exploration for mineral deposits through analysis of metal concentrations in plants
that might indicate underlying ore bodies.
Constructed wetlands: Artificial or engineered wetlands
used to remediate surface water or waste water.
Eastern cottonwood (Poplar) (Populus deltoides): A
widely studied tree with potential for hydraulic control,
phytodegradation, and phytovolatilization.
Ecosystem restoration: “The process of intentionally altering a site to establish a defined, indigenous ecosystem.
The goal of this process is to emulate the structure, function, diversity, and dynamics of the specified ecosystem.”
(Parks Canada. http://parkscanada.pch.gc.ca/natress).
Forensic ecology: Investigation of a site to determine
the history and causes of the current flora and fauna.
Forensic phytoremediation: Investigation of a naturallyrevegetated site to establish that remediation has occurred
or has begun to occur.
Geobotanical prospecting: The visual study of plants
as indicators of the underlying hydrogeologic and geologic
conditions.
Geobotany: The use of plants to investigate the underlying geology, especially related to metal ores.
Halophyte: A salt-resistant-plant; one that will grow in
saline soil. Salt cedar is an example.
Humification/fixation: The incorporation of contaminants into biomass in soil.
Hydrologic control: The use of plants to rapidly uptake
large volumes of water to contain or control the migration
of subsurface water. Synonym: Phytohydraulics.
Hyperaccumulators: Metallophytes that accumulate an
exceptionally high level of a metal, to a specified concentration or to a specified multiple of the concentration found
in other nearby plants. Alpine pennycress is an example.
Indian mustard (Brassica juncea): A potentially useful and relatively high biomass hyperaccumulator plant that
can accumulate metals and radionuclides.
Indicator plants: Plants with a metal concentration in
the aboveground biomass that reflects the soil concentration of the metal. Bladder campion is an example.
Land reclamation: The revegetation of eroded or
nonvegetated land to decrease erosion of the soil or to
increase the beneficial uses of the soil.
Evapotranspiration cap (or cover): A cap composed of
soil and plants, engineered to maximize evaporation and
transpiration processes of plants and the available storage
capacity of soil to minimize infiltration of water. Synonym:
Water-balance Cover
Metal-tolerant plants: Plants that can grow in metalrich soils without accumulating the metals.
Fibrous root: A root system that has numerous fine roots
dispersed throughout the soil.
Phreatophyte: A deep-rooted plant that obtains water
from the water table.
Metallophytes: Plants that can only grow in metal-rich
soils.
A-1
Phytoaccumulation: The uptake and concentration of
contaminants (metals or organics) within the roots or
aboveground portion of plants.
Phytodegradation: The breakdown of contaminants
taken up by plants through metabolic processes within the
plant, or the breakdown of contaminants external to the
plant through the effect of compounds (such as enzymes)
produced by the plants. Synonym: Phytotransformation.
Phytoextraction: The uptake of contaminants by plant
roots and translocation into the aboveground portion of
the plants, where it is generally removed by harvesting the
plants. This technology is most often applied to metal-contaminated soil or water. See also: Phytoaccumulation.
Phytoextraction coefficient: The ratio of metal concentration in the plant ( g metal/g dry weight tissue) to the
initial soil concentration of the metal ( g metal/g dry weight
soil), for phytoextraction of metals (Nanda Kumar et al.
1995).
Phytoinvestigation: The examination of plants at a site
for information about contaminant presence, distribution,
and concentration.
Phytoremediation: The direct use of living green plants
for in situ risk reduction for contaminated soil, sludges, sediments, and groundwater, through contaminant removal, degradation, or containment. Synonyms: Green remediation,
Botano-remediation.
Rhizodegradation: The breakdown of a contaminant in
soil through microbial activity that is enhanced by the presence of the root zone. Synonyms: Plant-assisted degradation, Plant-assisted bioremediation, Plant-aided in situ biodegradation, Enhanced rhizosphere biodegradation.
Rhizofiltration: The adsorption or precipitation onto plant
roots or absorption into the roots of contaminants that are
in solution surrounding the root zone.
Rhizosphere: The zone around plant roots that has significantly higher microbial numbers and activity than in the
bulk soil.
Root concentration factor (RCF): The concentration
in the roots divided by concentration in external solution,
for non-ionized organic compounds taken up by plants with
nonwoody stems (Ryan et al. 1988; Paterson et al. 1990).
Root exudates: Chemical compounds such as sugars
or amino acids or that are released by roots.
Stem concentration factor (SCF): The concentration
in the stem divided by the concentration in the external
solution, for non-ionized organic compounds (Ryan et al.
1988; Paterson et al. 1990).
System: The overall picture; e.g., a constructed field plot
that uses mechanisms within technologies for a particular
application.
Tap root: A root system that has one main root.
Phytoremediation cap (or cover): A cap consisting of
soil and plants, designed to minimize infiltration of water
and to aid in the degradation of underlying waste.
Technology: A combination of processes and mechanisms; e.g., phytoextraction, rhizodegradation (BP uses and
favors this terms).
Phytoremediation natural attenuation: Intrinsic
bioremediation processes (in soil or groundwater) enhanced
by the presence of naturally-occurring plants.
Transpiration stream concentration factor (TSCF):
The concentration in the transpiration stream divided by
the concentration in the external solution, for organic compounds (Paterson et al. 1990).
Phytostabilization: Immobilization of a contaminant
through absorption and accumulation by roots, adsorption
onto roots, or precipitation within the root zone of plants.
Phytovolatilization: The uptake and transpiration of a
contaminant by a plant, with release of the contaminant or
a modified form of the contaminant to the atmosphere from
the plant.
Process, mechanism: Something that a plant does; e.g.,
uptake, transpiration.
Vegetative cap (or cover): A long-term, self-sustaining
cap of plants growing in and/or over materials that pose
environmental risk; a vegetative cover reduces that risk to
an acceptable level and requires minimal maintenance. Two
specialized types of vegetative caps are the Evapotranspiration Cap and the Phytoremediation Cap.
Vegetative soil stabilization: The holding together of
soil by plant roots to decrease wind or water erosion or
dispersion of soil.
A-2
Appendix B
Phytoremediation Database
Project Name/
Location
Size
of Area
Primary
Contaminant
Media and
Properties
Vegetative
Type
Aberdeen Pesticide
Dumps Site - NC
7 acres
VOCs, Pesticides/
Herbicides
Groundwater
Hybrid Poplar
and ground cover
grasses
Aberdeen Proving Grounds
J - Field Toxic Pits Site - MD
1 acre
Halogenated Volatiles
Soil and groundwater
Hybrid Poplar
Groundwater
Poplar
ACME landfill - NC
Date
Planted
March 1996
AGI - WA
~ 2 acres (proposed)
Phenols
Surface water
Wetland plants
Not planted
yet
Agricultural Cooperative - WI
~ 0.3 acres
Pesticides, herbicides;
VOCs
Soil and groundwater
Hybrid poplar
May 1996
Agricultural Cooperative - WI
0.9 acres
Ammonia
Soil
Hybrid poplar
and grass
June 1997
Aluminum Manufacturing
Facility - SC
6 acres
Heavy metals
Soil and groundwater;
sandy clay
Hybrid poplar
May 1992
Aluminum Processing
Facility - NV
6 acres
Saline wastewater
from cooling tower
Dune sand
Hybrid willows
May 1997
Amargosa Desert Research
Radionuclides
Amarillo - TX
Amboer Road - OR
~ 5 acres
Chlorinated Solvents
Groundwater and soil
Hybrid and native
poplar
April 1999
Anderson - SC
17 acres
Heavy Metals
Groundwater and soil
Hybrid poplar,
Grasses
1993
Annette Island Site - AK
Each plot 100
square ft.
Semi-volatile Petroleum
Products
Native soil
Fescue;, ryegrass,
and clover
Summer
1998
Anonymous - KS
~ 100 acres
TPH
Cottonwood, hybrid
poplar
Spring
1998
Artemont, CA
Municipal and hazardous
waste
Various trees
DOE Facility - OH
Uranium, cesium,
strontium
Groundwater and
wastewater
Sunflowers
November
1997
Other, Heavy Metals
Subsurface soil
Hybrid poplar
1993, 1994
Army Ammunition
Plant - IL
Barje Landfill - Slovenia
10 acre
B-1
Project Name/
Location
Size
of Area
Barrow Site - AK
Each plot is ~
square ft.
Bayonne - NJ
Primary
Contaminant
120
Media and
Properties
Vegetative
Type
Date
Planted
VOCs and Semi-volatiles
Soil. Soils marine beach
Mix of grasses and
legumes planned
for spring 1999
Spring 1999
1,000 square ft.
Heavy metals
Soil to 15 cm bgs
Indian Mustard
Spring 1997
Beltsville, MD
70’ x 45’ x 10’
Radioactive - low level
waste
Pfitzers junipers
1987
Bluestem Landfill #1 - IA
3 acres
Leachate
Subsurface soil
Hybrid poplar and
grass
1994
Bluestem Landfill #2 - IA
5 acres (2 test plots)
Subsurface soil
Hybrid poplar and
grass
1994
Pesticides, herbicides,
dyes
Soil, sediments
Trees and wetland
plants
Spring 1999
Bofors-Nobel - MI
British Steel South Wales, UK
18 hectares
Coke oven effluent
Effluent
Soil based reed
bed
Bunker Hill - ID
1,050 acres
Heavy metals
Soil
Mix of herbaceous
species
1998-2001
C-H Plant Area - TX
27 acres
Salt, metals (possible
radionuclides
Groundwater
Trees
Planned
1999
Calhoun Park - SC
0.5 acre
Non-halogenated
Semi-volatiles
Ground 1 to 5’ bgs
Native trees and
shrubs
Campion Site - AK
Each plot is ~
square ft.
Volatile and Semi-volatile
Petroleum Products
Soils will be more fully
characterized fall 1998
Mix of grasses
and legumes
Summer
1998
Cantrall - IL
1-3 acres
Pesticides/Herbicides
Soil and groundwater
Hybrid poplars
1992
Carswell AFB (former) - TX
1 acre
Halogenated volatiles
Groundwater
Eastern Cottonwood April 1996
Brookhaven National
Labs - NY
~ 1/4 acre
Radionuclides
Landscaping soils to
~
6 inches in depth.
Redroot Pigweed
Chernobyl - Ukraine
Radionuclides
Soil, groundwater, water
Sunflowers, Indian
Mustard
Chevron Facility
No. 129-0334 - UT
Volatile petroleum products
Groundwater
Hybrid poplar trees
(DN 34)
375
Chevron Site - CA
30 acres, 90 acres
Volatile petroleum products
Soil at root level,
groundwater, wastewater
Fescue, Cowpeas,
cattails
Chevron Station
No. 7-7992 - CO
300 x 15 ft. area
(120 poplar trees)
Volatile petroleum products
Groundwater
Hybrid poplar trees
Explosives
Soil at root level
Parrot Feather
Silty sand soils
Ryegrass, Bermuda
grass
Childerburg - AL
City of Glendale
Landfill - AZ
1 acre (2 test covers)
City of Madras WWTP
Reuse - OR
1257.5 acres
Wastewater; nitrogen;
phosphorus
WWTP Sludge Lagoon - OR
4,200 acres
Heavy metals, PCB’s
Old sewage sludge
applied to grazing land
Natural Treatment
System (NTS) - OR
Land-applied wastewater
Wastewater
Poplars
City of Woodburn WWTP - OR 7 acres
Other (Wastewater reuse)
Soil
Hybrid Poplar
B-2
Turf grass
May 1998
April 1996
April 1995
Already
planted
1990
1995
Project Name/
Location
Size
of Area
Primary
Contaminant
Media and
Properties
Vegetative
Type
Date
Planted
Closed Disposal Facility - IL
26 acres
Petroleum Hydrocarbons
Surface water
Groundcover plants
in combination with
phreatophyte trees
Closed Terminal - RI
~ 1 acre
PAHs
Surface soils
Indigenous plants/
trees
Coffin Butte Landfill - OR
14.4 acre overland
polishing system
Landfill leachate
Leachate
Grass, hay, and
native trees
Existing
Columbus - OH
1 acre
Volatile petroleum
Hydrocarbons
Soil
Hybrid poplars;
ground cover
1997
Craney Island Fuel
Terminal - VA
~ 180’ x 100’ (4
plots with replicates
TPH, PAHs
Soil
Fescue, rye, clover
Dearing (Cherokee
County) - KS
1 acre
Heavy metals
Soil
Hybrid poplars
March 1995
Chevron Bulk Facility
100-1838 - OR
0.6 acres planted
with grass
Volatile petroleum
products
Surface soil
Tall fescue
April 1995
Fuel Hydrocarbons
Soil
Variety of grasses
and clovers.
Corvallis - OR
Delaware Solid Waste
Authority - DE
Diashowa Paper Mill - WA
Dieners - CA
5 hectares
Heavy metals
Soil, groundwater and
effluent are affected
Brassica sp.
1995
Dorchester - MA
~ 1,200 square ft.
Lead
Soil
Indian Mustard
June 1997
Eagle Flat - TX
Radionuclides - Low
level waste
Semiarid prairie
East Ravendale Yorkshire, UK
240 square miles
BOD, suspended solids
WWTP Effluent
Soil based reed
bed
1991
Edward Sears Site
(Superfund) - NJ
1/3 acre, 0.5 acre
Halogenated volatile
petroleum products
Soil and groundwater
Hybrid poplar,
willow
December
1996
Farm Service Facility - MN
0.4 acres
Ammonia
Soil
Hybrid poplar and
grass
May 18,
1998
Farmer’s Loop Site - AK
Multiple plots, each
~ 10’ x 10’, 2-3’ deep
Semi-volatile petroleum
products
Soil
Ryegrass and
red fescue
Summer
1995
Nitrogen
Soil and groundwater
Hybrid poplars
May 1996
Fertilizer Plant Site - SD
Fly ash Landfill
Covercap - MO
6 acres
Fly ash Landfill - MO
Hybrid poplars
Fly ash
Forest Grove WWTP - OR
5 acres
Chevron Terminal
No. 129-0350 - UT
~ 5 acres
Former Farm Market - WI
Former Farm Market - IL
Hybrid poplars
Soil
Hybrid poplar and
grass
Volatile petroleum products
Groundwater and vadose
zone are contaminated
Hybrid poplars,
fescue, alfalfa
1 acre
Pesticides, nitrates, and
ammonium
Soil, groundwater
Hybrid poplars
Spring 1992
1 acre
Pesticides, nitrates, and
ammonium
Silt loam with clay
Hybrid poplars
Spring 1992
B-3
May 1994
Project Name/
Location
Size
of Area
Primary
Contaminant
Media and
Properties
Vegetative
Type
Date
Planted
Former Farm Market - SC
1 acre
High salt concentrations
Surface soil
Various trees and
grasses
Spring 1993
Former Farm Market - IL
1 acre
Pesticides, nitrates,
ammonium
Silt loam soils
Hybrid poplars
Spring 1992
Former Fertilizer Facility - OK
4 acres
Nitrates and ammonium
Soil and groundwater
Prairie grasses
Irrigation
est. 1990
Former Fertilizer Facility - NC
1 acre
Ammonium and nitrate
nitrogen
Groundwater
Hybrid poplar
Spring 1992
Former Fertilizer Facility - NJ
1 acre
Ammonium and nitrate
nitrogen
Soil and groundwater
Hybrid poplar
and Australian
willows
Fall 1992;
spring 1992;
spring 1994
Former Municipal
Landfill - NY
3 acres border area
Heavy metals
Groundwater
Hybrid poplars
June-July
1998
Petroleum Processing
Facility - PA
2 acres
TPH in fill soil;
BTEX
Ash and cinder with soil
fill
Hybrid poplar and
hybrid willows
June 1996
Standard Oil Facility
No. 100-1348 - WA
5 acres
Volatile petroleum
products
Perched aquifer
Hybrid poplars
(DN 34)
April 1995
Former Truck Depot - LA
0.5 acres
TPH in fill soil; BTEX
Groundwater and soil
Hybrid poplar,
hybrid willows
June 1995
Fort Carson (Landfills
5 and 6) - CO
20-40 acres
Municipal/mixed waste
Fort Lewis Army Base - WA
~ 10 acres
Chlorinated solvents
Groundwater
Hybrid poplar
Proposed for
Spring 2000
Grasses and
legumes
September
1997
Fort Richardson - AK
Fort Riley - KS
Soil
4,800 square ft.
Fort Wainwright - AK
Semi-volatile petroleum
products
Contaminated sediments
Pesticides/Herbicides, Other Soil and groundwater
Gardner Avenue - CY
2.5 acres
Great River Regional Waste
Authority - IA
6 acres
Green II Landfill - OH
30 acres
Greenbelt Project - WY
Pesticides/Herbicides,
Heavy metals
Felt Leaf willow
Soil, groundwater
Leachate and subsurface
soil
Hybrid poplar
and grass
1997-1998
VOCs and other organics
in leachate
Leachate and soil
Hybrid poplar
and hybrid willows
Fall 1998
40 acres
Wood preservatives
Soil
Poplar, herbaceous
Greenhouse studies of
Phytoremediation
~ 20’ x 4’ per
experiment
Heavy metals, halogenated
volatiles
Soil to 15’ bgs, groundwater Gamagrass, poplars March
willows
1995-1997
Grundy County Landfill - IA
5 acres, 2 acres
Leachate
Subsurface soil
Hybrid poplar
Gulfcoast Site
1,800 square ft.
Volatile petroleum products
Soil to 6”, groundwater
Sorghum, Cowpeas
Sweet clover, rye
Hanford Barrier - WA
1 acre
No waste
Hawaii
~ 1 acre
Pesticides
Groundwater
Koa (native
plant)
No waste
Groundwater
Vegetative cover
Hill Air Force Base - UT
Hillsboro Landfill
Wetlands - OR
54 acres
B-4
1993, 1994
June 1998
Project Name/
Location
Size
of Area
Primary
Contaminant
Media and
Properties
Vegetative
Type
Date
Planted
Hollola Landfill Hollola, Finland
3 hectares
Ash, oily waste
ICI - Billingham, UK
5 hectares
COD
Effluent
Soil based reed bed 1990
ICI Explosives Americas
Engineering - MO
~ 3.2 acres
Explosives, fertilizers
Surface soils, surface
water, and groundwater
Willows, ninebark
and cypress
IMC Global Limited - Ontario,
Canada
100 acres
Leachate
Poplar
IMC, Port Maitland Canada
100 acres
Indianapolis - IN
1-1.5 acres
Pesticides/Herbicides
Industrial Landfill - TN
3 acres
February
1996
Hybrid poplar
Spring 1998
Groundwater
Hybrid poplar
1995
Volatile organic compounds
and thallium
Groundwater
Hybrid poplar with
grass
Spring 1998
Iowa
Heavy metals, pesticides/
herbicides
Soil 0 to 48 cm
Knotweed,
crabgrass
Army Ammunition Depot - IA
Explosives
Wastewater, soil and
pond water
Wetland and
terrestrial
Spring 1997
Subsurface soil
Hybrid poplar and
grass
1992, 1993
Jackson Bottoms
Wetland - OR
Johnson County Landfill - IA
9 acres
Halogenated volatiles,
heavy metals, other
Juniper Utility Co. WWTP
Effluent Reuse - OR
64.5 acres
Wastewater; nitrogen;
phosphorus. Secondary
effluent biosolids
Kaiser Hill - CO
15 to 46 acres
Radionuclides
Shallow groundwater
and surface streams
Native cottonwoods; Not yet.
Hybrid poplar
Kauffman and Minteer - NJ
1) 50’ x 300’ and 2)
30’ x 30’
TCE; DCE
Soil and groundwater
Black willow; hybrid
poplar
April 1998
Keyport Naval Warfare
Facility - WA
~ 8 acres
Chlorinated solvents,
PCB’s
Groundwater
Hybrid poplars
Site in
preparation
Klamath Falls site - OR
12 acres (1994)
Halogenated semi-volatiles
Shallow soil
Hybrid poplar and
grass
1994, June
1995
Heavy metals
Soil
Alpine pennycress
September
1997
Soil and water
Hybrid poplar,
grass
April 1990
Kurdjaly - Bulgaria
Ryegrass and
Kentucky
bluegrass
Lakeside Reclamation
Landfill - OR
0.6 acres; 8 acres
Landfill leachate
Lamb-Weston Food
Processing Reuse - OR
>5,000 acres
Food processing
wastewater
Lanti Landfill - Finland
17 hectares
Ash
Liquid Fertilizer Plant - ND
~ 0.25 acres
Nitrogen
Soil and groundwater
Hybrid poplar
May 10,
1996
Los Banos - CA
0.5 hectares
Heavy metals
Clay, loam
Indian Mustard,
Fescue, Trefoil,
Brassica sp.
1991
Magic Marker Site - NJ
0.25 acre, 1 acre,
4500 square ft. study
area
Heavy metals
Shallow soil
Indian Mustard
(Brassica juncea)
June 1997
B-5
Grass, wheat,
barley, corn, alfalfa
Project Name/
Location
Size
of Area
Primary
Contaminant
Media and
Properties
Vegetative
Type
Date
Planted
Magnesite Processing
Plant - WA
1 acre
Ammonia
Soil
Hybrid poplar
(5 varieties)
May 19-20,
1995
Manchester Site - UK
2 pilot beds each
3m x 5m
Starch factory effluent
Effluent
Soil based reed
bed
1995
Manufacturing Facility - MI
0.5 acres
Halogenated volatiles
Groundwater, silty
clay soil
Hybrid poplar
June 1996
Manufacturing Facility - WI
2 acres
TPH in fill soil
Ash and cinder with
soil fill
Hybrid willow
June 1996
Matoon - IL
3 acres
Nitrate nitrogen
soil
Hybrid poplar
1994
Maxey Flats - KY
Radioactive - low level
waste
Metal Plating Facility - OH
Heavy metals, halogenated
volatiles
Soil
Indian Mustard
Milan Army Ammunition
Plant (MAAP) - TN
Demonstration
scale
Explosives (TNT, RDX,
HMX, TNB); BOD5,
nutrients
Groundwater
Grass, sweetflag,
parrotfeather
Military site
Feasibility study test
cells
Explosives
Soil
Proprietary
Mill Creek Correctional
Facility - OR
3.5 acres
Nitrogen
Groundwater and soil
Native black
cottonwood; Hybrid
poplar (total of
8,500 planted
perpendicular to
groundwater flow)
Mississippi Site
~ 3 acres
Volatile and nonvolatile
petroleum products
Soil and groundwater
Under consideration Spring 1999
Groundwater
Hybrid poplars
May 24,
1997
Monmouth Site - NJ
Montezuma West - OR
April-May
1996
May 1997
Radionuclides
~ 1 acre
Monticello - UT
Chlorinated volatiles
Radionuclides
Moonachie - NJ
2 acres - 46 trees
Volatile petroleum products; Groundwater
Halogenated volatiles
Hybrid poplars;
DN 34 (Populus
deltoids x P. nigra)
May 28,
1997
MS Service Station - NJ
~ 0.3 acre
GROs
Groundwater
Phreatophyte trees
Spring 1999
Kennedy Space Center - FL
3 acres
Halogenated volatiles,
volatile petroleum products,
heavy metals
Soil and groundwater
Hybrid poplars
and grass
March - April
1998
NCASI Test Cells - MI
New Hampshire Landfill - NH
Halogenated volatiles
Nitrogen Contaminated
Site - MN
2.3 acres
Nitrate and ammonia
Soil and groundwater
Hybrid poplars,
and grass
May 1993
Nitrogen Products
Site - AK
Six buffer areas
(total 5 acres)
Pesticides, herbicides
Soil
Hybrid poplar
March 5-10
1995
Northeast Site
2000 square ft.
Volatile petroleum products
Soil to 2’ bgs,
groundwater
Perennial Warm
Season Grass,
Sorghum
May 1993
Nu-Glo Site - OH
0.5 acres
TCE, PCE
Soil and groundwater
Hybrid poplar
and willows
Fall 1998
B-6
Project Name/
Location
Size
of Area
Primary
Contaminant
Media and
Properties
Vegetative
Type
Date
Planted
Oconee - IL
1-3 acres
Pesticides/Herbicides
Groundwater and soil
Alfalfa, corn, and
Hybrid poplar
Ohio Location
~ 2 acres
Volatile petroleum products
Soil and shallow
groundwater
Hybrid poplars; rye
grass
May 1997
Ohio Site
25’ x 100’
Volatile petroleum products
Shallow groundwater
Hybrid poplars
April 1997
Oil Refinery - Perth,
Australia
10-100 square meters
Semi-volatile petroleum
products
Soil
Rye, legumes,
fescue, sedges
Sept 1998 Dec 1999
Gas Station - OR
75 square ft.
VOCs and semi-volatiles
groundwater
Hybrid poplars
(DN-34)
May 1997
OREMET Titanium, Inc. - OR
5 acres
Wastewater
Hybrid poplar
1995
Osage River Riparian
Buffer - MO
0.15 acres
Streambank soil
Hybrid poplar and
willow
June 1997
Osh Kosh - WI
9 square meters
Soil 0 to 30 cm
Fungi,
chrysanthemum p.
sordida
Palmerton - PA
25 square meters
Heavy metals
Soil at 12 cm
Campion, alpine
pennycress
Palo Alto - CA
1 acre
Heavy metals
Groundwater
Tamarisk,
Eucalyptus
Semi-volatile petroleum
products
Soil
Hybrid poplar
Paper Industry
Nov 1997
Petroleum Company - KS
0.5 acres
Petroleum Hydrocarbons
Soil
Hybrid poplars
April 1998
Phytoremediation of Soils
from Argonne West - ID
Greenhouse
Cesium 137, Cr, Hg, Zn,
Ag, Se
Soil
Hybrid willow,
canola, Brassica
April 1998
Piketon DOE Facility - OH
5 acres
Halogenated volatiles
Shallow and deep
groundwater
Hybrid poplars; rye
grass
Spring 1999
(planned)
Pipeline Site OBJ - MO
1.25 acres
Petroleum Hydrocarbons
Soil and groundwater
Alfalfa,
Phreatophyte trees
Sept 1998
Poppy Lane - AK
Volatile petroleum products, Soil, root level
Heavy metals
Port Hueneme
Willow, Elderberry,
Alder, Cottonweed
Groundwater
Eucalyptus
Prineville Golf Course
Reuse - OR
160 acres
BOD and TSS
Wastewater
Turf grass
1993
Rail Tie Yard - TN
0.75 acres
Semi-volatile petroleum
products
Soil
Hybrid poplar and
grass
1997
Red Mud Coastal
Restoration Project - LA
Test area ~ 2 acres
Iron Sesquioxides
Sludge
Grass, alfalfa,
willow, black locust
Fall 1992
Refinery - CA
7,000 square ft.
Hydrocarbons
Soil
Grasses
1995
Reliable Plating Site - OH
0.5 acres
TPH in fill soil; BTEX in
groundwater
Excavated soil
Hybrid poplars and
hybrid willows
June 1995
Residual Petroleum Waste
Remediation
0.8 acres
Petroleum Hydrocarbons
Soil
Hybrid poplar and
alder
1999
(Proposed)
Reuben Gaunts - Leeds - UK
450 square meters
Dyes, Residual OPs, COD
Effluent
Soil based reed
bed
1997
Riparian Buffer, Grande
Ronde Valley - OR
Various, most are a
few acres
Agricultural runoff
Grande Ronde River
Poplar and other
1997-1998
B-7
Project Name/
Location
Size
of Area
Primary
Contaminant
Media and
Properties
Rocky Mountain Arsenal - OR
74 acres
Radionuclides - low level
waste
Saginaw Mill - WA
~ 5-8 acres
Formaldehyde
No contaminant
Savannah Ricer Site - SC
Halogenated volatiles
Sludge Lagoon - CT
Herbicides, metals
Date
Planted
Trees
Groundwater
Sandia National Laboratories - 40’ x 300’ for each
NM
cover
Vegetative
Type
Poplar, Alder, and
Native willow
Various
Groundwater
Loblolly Pine,
grasses
March 1999
Hybrid poplar
SRSNE - CT
2 acres
Halogenated volatiles
Groundwater
Hybrid poplar
May 18,
1998
Tama County Landfill - IA
12 acres, 14 acres
Leachate
Shallow soil
Hybrid poplar
1993-1995
Tanfield Lea - Newcastle - UK
1,800 square meters
Heavy metals, TDS, COD
Leachate effluent
Soil based reed
bed
1997
Tennessee Site
2 acres
Volatile and nonvolatile
petroleum products
Soil and groundwater
Hybrid poplar
May 1997
Texaco - WA
18 acres
Petroleum Hydrocarbons
Soil
Grass and clover
Texas Land Treatment
Facility - TX
22 acres
PAHs and O&G
Residuals in soils
Groundcover plants
(grasses)
Proposed
4th Qtr 1998
Texas Site
5-10 acres
Volatile petroleum products
Soil
Grasses
Spring 1999
Halogenated Volatiles
Groundwater - 2-3’ depth
185 Willow Trees
Planted
1996
Surface water, soil,
groundwater
Hybrid poplars
and grass
May 1998
Thiokol Corp.
Tippee Beef Facility - IA
1.5 acres
Trucking Terminal - NJ
~ 1 acre
Volatile petroleum products
Shallow soil and
groundwater
Hybrid poplars
June 1998
Twin Cities Army Ammunition
Plant - MN
Two demo areas
0.2 acres
Heavy metals for
both sites
Soil
Corn, white
mustard
2 yr. demo
is May-Oct
1998 & 1999
Union Carbide - TX
1 acre
RCRA K-waste,
semi-volatiles
Sludge
Mulberry, grasses
hackberry
Unknown - NJ
Non-halogenated
Semi-volatiles
Soil
Alfalfa, switch and
bluestem grass
Unknown - NJ
Heavy metals
Soil, rocky, root level
Ragweed, Hemp,
Dogbane, Musk,
Nodding, Thistle
Sludge
Poplars
Unknown - MD
Unknown - ID
Upper Plant Area - NJ
0.1 acre
Upper Silesia, Poland
US Generating - OR
>5,000 acres
USA Waste-Chambers
Development - VA
10 acres
Heavy metals,
Radionuclides
Soil
GROs and DROs
Groundwater
Alfalfa,
Phreatophyte trees
Heavy metals
Clay and silt, 0-20 cm
Cereals, Potatoes
Heat (cooling water)
Grass, wheat, barley
corn, alfalfa
Subsurface soil
B-8
Sept 1998
Hybrid poplar
1995
Project Name/
Location
USA Waste Riverbend
Landfill - OR
Size
of Area
14.3 acres irrigated
Primary
Contaminant
Media and
Properties
VOCs, Heavy metals,
Other
Leachate
Vernon Brincks Site - IA
Volunteer Army Ammunition
Plan - TN
Vegetative
Type
Date
Planted
Hybrid poplar,
Grass
1992
Hybrid poplar
1991
Explosives
Water
Heavy metals
Soil
Hybrid poplar
1991, 1993
Whitewood Creek - SD
300’ buffer strip
Whyalla Site - Australia
8 pilot beds ~ 3m x
Coke oven effluent
10m; field = 2 hectares
Effluent
Soil based reed
bed
1993
Widen - WV
<1 acre
Volatile petroleum products
Soil and groundwater
Hybrid poplar
1994
Nitrate-Nitrogen,
ammonium-nitrogen
Groundwater and soil
Hybrid poplar
Wilmington - NC
Wisconsin Site - WI
17 acres
BTEX and TPH
Soil
Species under
consideration
Spring 1999
Woodlawn Landfill - MA
~ 21 acres
Halogenated volatiles and
metals
Groundwater
Hybrid poplars
Pending
approval
YPLMO - Edinburg, UK
2,000 square meters
Surfactants, petroleum,
hydrocarbons, organics
Groundwater
Soil based reed
bed
1997
B-9
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C-6
Appendix D
Common and Scientific Names of Referenced Plants
Common Name Listed First
Common Name
Scientific Name
Medicago sativa
Nitella
Alyssum wulfenianum
Phaseolus coccineus L.
Phaseolus vulgaris cv. “Tender Green”
Cynodon dactylon
Betula nigra
Robinia pseudoacacia
Salix nigra
Silene vulgaris
Andropogon gerardi Vit.
Andropogon scoparius
Schizachyrium scoparius
Buxaceae
Buchloe dactyloides
Elymus canadensis
Brassica napus
Typha latifolia
Quercus falcata
genus Trifolium
Agrostis tenuis cv Goginan
Agrostis tenuis cv Parys
Populus deltoides
Populus
Malus fusca Raf. Schneid
Agropyron desertorum (Fisher ex Link) Schultes
Taxodium distichum
Lemna minor
Populus deltoides
Achillea millefolium
Salix alaxensis
Festuca ovina var. duriuscula
Festuca rubra cv Merlin
Festuca arundinacea Schreb.
Aicanescens
Bouteloua curtipendula
Bouteloua gracilis
Agrotis tenuis
Zoysia japonica
Armoracia rusticana
Populus deltoides X nigra DN-34, Imperial California;
Populus charkowiieensis x incrassata;
Populus tricocarpa x deltoides
Alfalfa
Algae stonewort
Alyssum
Bean
Bean, bush
Bermuda grass
Birch, river
Black locust
Black willow tree
Bladder Campion
Bluestem, big (prairie grass)
Bluestem, little
Bluestem, little
Boxwood
Buffalo grass
Canada wild rye (prairie grass)
Canola
Cattail
Cherry bark oak
Clover
Colonial bentgrass
Colonial bentgrass
Cottonwood, Eastern, tree
Cottonwood (poplar)
Crab apple
Crested wheatgrass (Hycrest)
Cypress, bald
Duckweed
Eastern Cottonwood tree
European milfoil / yarrow
Felt leaf willow
Fescue, hard
Fescue, red
Fescue, tall
Four-wing saltbrush
Grama, side oats (prairie grass)
Grama, blue
Grass, cool season (colonial bentgrass)
Grass, warm season (Japanese lawngrass)
Horseradish (roots)
Hybrid poplar tree
D-1
Common Name
Scientific Name
Agropyron desertorum (Fisher ex Link) Schultes
Sorghastrum nutans
Brassica juncea
Oryza sativa subsp. indica
Zoysia japonica
Datura innoxia
Hibiscus cannabinus L. cv. Indian
Leucaena leucocephala
Pueraria lobata
Chenopodium
Lespedeza cuneata (Dumont)
Schizachyrium scoparius
Pinus taeda (L.)
Prosopis
Panicum miliaceum L.
Morus rubra L.
Brassica juncea
Arabidopsis thaliana
Quercus falcata
Quercus virginiana
Maclura pomifera (Raf.) Schneid
Myriophyllum aquaticum
Thlaspi rotundifolium
Thlaspi caerulescens
Hydrocotyle umbellata
Populus
Populus
Populus deltoides X nigra DN-34, Imperial California;
Populus charkowiieensis x incrassata;
Populus tricocarpa x deltoides Populus charkowiieensis x
incrassata
Liriodendron tulipifera
Festuca rubra cv Merlin
Phragmites
Oryza sativa L.
Sporobolus wrightii
Armeria maritima
Spartina alterniflora
Sporobolu cryptandrus
Glycine max (L.) Merr, cv Davis.
Mentha spicata
Saccharum officinarum
Sorghum vulgare L.
Helianthus annuus
Panicum virgatum
Festuca arundinacea Schreb.
Datura innoxia
Armeria maritima
Nicotiana tabacum
Eichhornia crassipes
Myriophyllum spicatum
Azolla pinnata
Agropyron trachycaulum
Agropyron smithii
Salix nigra
Salix alaxensis
Hycrest, crested wheatgrass
Indiangrass (prairie grass)
Indian mustard
Indian ricegrass
Japanese lawngrass
Jimson weed
Kenaf
Koa haole
Kudzu
Lambsquarter
Legume
Little bluestem (prairie grass)
Loblolly pine
Mesquite
Millet, Proso
Mulberry, red
Mustard, Indian
Mustard weed
Oak, cherry bark
Oak, live
Osage orange
Parrot feather
Pennycress
Pennycress, Alpine
Pennyworth
Poplar, cottonwood
Poplar
Poplar, hybrid
Poplar, yellow
Red fescue
Reeds
Rice
Sacaton, alkali
Sea pink; wild thrift
Salt marsh plant
Sand dropseed
Soybean
Spearmint
Sugarcane
Sundangrass
Sunflower
Switchgrass (prairie grass)
Tall fescue
Thornapple (or jimson weed)
Thrift (wild); sea pink
Tobacco
Water hyacinth
Water milfoil
Water velvet
Wheat grass, slender
Wheat grass, western (prairie grass)
Willow tree, black
Willow tree, felt leaf
D-2
Scientific Name Listed First
Scientific Name
Common Name
Achillea millefolium
Agropyron desertorum (Fisher ex Link) Schultes
Agropyron smithii
Agropyron trachycaulum
Agrostis tenuis cv Goginan
Agrostis tenuis cv Parys
Aicanescens
Alyssum wulfenianum
Andropogon gerardi
Andropogon scoparius
Arabidopsis thaliana
Armeria martima (var. halleri)
Armoracia rusticana
Azolla pinnata
Betula nigra
Bouteloua curtipendula
Bouteloua gracilis
Brassica juncea (B. Juncea)
Brassica juncea (L.) Czern
Brassica napus
Buchloe dactyloides
Buxaceae
Chenopodium
Cynodon dactylon
Datura innoxia
Eichhornia crassipes
Elymus canadensis
Festuca arundinacea Schreb.
Festuca ovina var. duriuscula
Festuca rubra cv Merlin
Glycine max (L.) Merr.
Helianthus annuus
Hibiscus cannabinus L. cv. Indian
Hydrocotyle umbellata
Lemna minor
Lespedeza cuneata (Dumont)
Leucaena leucocephala
Liriodendron tulipifera
Maclura pomifera (Raf.) Schneid
Malus fusca Raf. Schneid
Medicago sativa
Mentha spicata
Morus rubra L.
Myriophyllum aquaticum
Myriophyllum spicatum
Nicotiana tabacum
Nitella
Oryza sativa L.
Oryza sativa subsp. indica
Panicum miliaceum L.
Panicum virgatum
Phaseolus coccineus L
Phaseolus vulgaris cv. Tender Green
Phragmites
European milfoil; yarrow
Hycrest crested wheatgrass
western wheatgrass (prairie grass)
slender wheatgrass
Colonial bentgrass
Colonial bentgrass
four-wing saltbrush
Alyssum
big bluestem
little bluestem prairie grass
mustard weed
Sea pink; wild thrift
horseradish
water velvet
river birch
side oats grama (prairie grass)
blue grama (prairie grass)
Indian mustard
Indian mustard
canola
buffalo grass
includes boxwood
lambsquarter
Bermuda grass
jimson weed or thornapple
water hyacinth
Canada wild rye (prairie grass)
tall fescue
hard fescue
red fescue
soybean
sunflower
Kenaf
pennyworth
duckweed
a legume
Koa haole
yellow poplar
osage orange
crab apple
alfalfa
spearmint
red mulberry
parrot feather
water milfoil
tobacco
algae stonewort
rice
Indian ricegrass
proso millet
switchgrass (prairie grass)
Bean
bush bean
reeds
D-3
Scientific Name
Common Name
Pinus taeda (L.)
Populus
Populus charkowiieensis x incrassata
Populus deltoides X nigra DN-34, Imperial California
Populus tricocarpa x deltoides
Prosopis
Pueraria lobata
Quercus falcata
Quercus virginiana
Robinia pseudoacacia
Saccharum officinarum
Salix alaxensis
Salix nigra
Schizachyrium scoparius
Silene vulgaris
Sorghastrum nutans
Sorghum vulgare L.
Spartina alterniflora
Sporobolu crypandrus
Sporobolus wrightii
Taxodium distichum
Thlaspi caerulescens
Thlaspi rotundifolilum
Trifolium (genus)
Typha latifolia
Zoysia japonica
Loblolly pine
Poplar, cottonwood
hybrid poplar
hybrid poplar tree (eastern cottonwood)
a hybrid poplar tree
mesquite
Kudzu
cherry bark oak
live oak
black locust
sugarcane
felt leaf willow
black willow tree
little bluestem prairie grass
bladder campion
indiangrass (prairie grass)
sudangrass (prairie grass)
salt marsh plant
sand dropseed
Sacaton, alkali
bald cypress
Alpine pennycress
Pennycress
clover
cattail
Japanese lawngrass (warm season grass)
D-4
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