TRANSFORMATION IN THE EFFLUENT RECHARGE Denise A

TRANSFORMATION IN THE EFFLUENT RECHARGE Denise A
NITROGEN TRANSFORMATION IN THE SUBSURFACE
DURING EFFLUENT RECHARGE
by
Paula Denise Santerior
A Thesis Submitted to the Faculty of the
DEPARTMENT OF HYDROLOGY
AND WATER RESOURCES
In Partial Fulfillment of the Requirements
For the Degree of
MASTER OF SCIENCE
WITH A MAJOR IN HYDROLOGY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1992
2
STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of the
requirements for and advanced degree at the University of Arizona and
is deposited in the University Library to be made available to
borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special
permission, provided that accurate acknowledgement of source is made.
Requests for permission for extended quotation from or reproduction of
this manuscript in whole or part may be granted by the head of the
major department of the Dean of the Graduate College when in his or
her judgement the proposed use of the material is in the interest of
scholarship. In all other instances, however, permission must be
obtained by the author.
Signed:
APPROVAL BY THESIS DIRECTOR:
This thesis has been approved on the date shown below:
L. G. Wilson
Hydrologist
Date
3
ACKNOWLEDGMENTS
I would like to thank Dr. Gray Wilson for his constructive
criticism and willingness to help me finish this work under a very
tight schedule. Thanks also to Dr. Martha Conklin for being on my
committee.
I would thank Tucson Water for providing me with the data I needed
to complete this thesis and Mr. Bruce Johnson for being on my
committee.
4
TABLE OF CONTENTS
LIST OF FIGURES 6
LIST OF TABLES 7
ABSTRACT 8
CHAPTER 1: INTRODUCTION 9
CHAPTER 2: OBJECTIVES 15
CHAPTER 3: LITERATURE REVIEW 17
3.1 Nitrogen Transformation 17
3.2 Column Studies 20
3.3 Field Studies 22
3.4 University of Arizona Studies 25
CHAPTER 4: DESCRIPTION OF STUDY AREA 28
4.1 Tucson Physiographic Environment 4.1.1 Tucson Hydrogeological Setting 4.1.2 Tucson Groundwater Quality 28
28
30
4.2 Sweetwater US&R Facility 4.2.1 Construction 4.2.2 Operation 31
31
33
CHAPTER 5: MATERIALS AND METHODS 36
5.1 Overview 36
5.2 Sample Collection 45
5.3 Sample Analysis 46
CHAPTER 6: RESULTS AND DISCUSSION 52
6.1 Seasonal Trends 52
6.2 Wet/Dry Cycle Trends 65
6.3 Nitrogen Species Reactions 6.3.1 Nitrogen Transformation Processes 6.3.2 Nitrogen Balance Equations 79
79
80
6.4 Discussion of Error 84
CHAPTER 7: CONCLUSIONS 87
CHAPTER 8: RECOMMENDATIONS 90
5
REFERENCES 91
APPENDIX I 94
APPENDIX II 97
6
LIST OF FIGURES
Figure 4
Page
1.1 Sweetwater Underground Storage and Recovery Facility 11
3.1 Nitrogen Transformation Process 18
3.2 Nitrate Concentration Through Profile in Mini-Basins 27
4.1 Nitrate as Nitrogen for April, 1992 Samples 32
4.2 Water Table Elevations for WR-069 A 35
5.1 High-pressure Vacuum Lysimeter 37
5.2 Infiltration Basin RB-001 B 39
5.3 Well Construction of WR-199 A 44
5.4 Sample HACH DR/700 Calibration Curves 49
6.1 Response to MW-7 and MW-8 to Flooding 54
6.2 Nitrate Distribution in Lysimeters Through Profile 58
6.3 Nitrogen Species in WR-199 A 61
6.4 Chloride Concentration in WR-199 A 63
6.5 Depth to Water and Specific Conductance in WR-199 A 64
6.6 Nitrate Concentration Through Profile-North Lysimeters 67
6.7 Nitrate Concentration Through Profile-South Lysimeters 68
6.8 Depth to Water in 1'4W-7 and MW-8 69
6.9 Nitrate Concentration at 2.5 foot Depth 71
6.10
Nitrate Concentration at 5 foot Depth 72
6.11
Nitrate Concentration at 10 foot Depth 73
6.12
Nitrate Concentration at 17 foot Depth 74
6.13
Nitrate Concentration at 40 foot Depth 75
6.14
Nitrate Concentration at 60 foot Depth 76
6.15
Nitrate Concentration at 80 foot Depth 77
7
LIST
OF
TABLES
Table #
Page
5.1
WR-199 A Lithologic Description 41
5.2
April 12 - May 1 Sampling Events 47
6.1
Tabulated Basin RB-001 B Cycles and Infiltration Rates 53
6.2
Analysis of Point 520 B Samples 56
6.3
Analysis of WR-199 A Samples 60
6.4
Nitrate, Nitrite and Sulfate Results 66
8
ABSTRACT
The transformation of nitrogen in treated effluent during
recharge at the Sweetwater Underground Storage and Recovery
Facility was examined. The procedure consisted of obtaining
samples of percolating water and groundwater below a 3.3 acre
infiltration basin, carried out over the recharge season October
29, 1991 - May 23, 1992 and during an entire 4 day wetting/6 day
drying cycle. Nitrate concentrations decreased in the profile and
in groundwater throughout the season. During the wetting/drying
cycle the nitrate level decreased from a maximum of over 20 mg/L
NO3 -N at a depth of 5 feet to about 10 mg/L NO3 -N at 17 feet.
-
-
Below 17 feet, the nitrate level remained approximately constant.
A 36% reduction in total nitrogen concentration was observed
between source water and groundwater. Results suggest that soil
aquifer treatment (SAT) may be useful in reducing the total
nitrogen concentration of effluent.
9
Chapter 1
Introduction
Historically, water supplies in the Tucson Basin, Arizona were
derived from withdrawals the Santa Cruz River, which was perennial in
the days of the Spanish missionaries, to the overdrafting of
groundwater that exists today. The City of Tucson (COT), being a part
of the Tucson Active Management Area, is required by the Arizona
Department of Water Resources (ADWR), under the provisions of the
Groundwater Management Act (GMA) of 1980, to show that it will be able
to meet its projected customer demand for water for the next 100 years.
The existing supply of groundwater must also be maintained at safe
yield, that is where pumpage of water equals recharge, by the year
2025.
Tucson Water, the city's water utility, has assumed the
responsibility for meeting this requirement for its customers.
Achievement of the GMA objectives will require the careful management
of existing resources and the ability to find new, supplementary
supplies. One new supply of water will be Colorado River water
delivered via the Central Arizona Project (CAP) canal. CAP water will
be introduced over the next few years, during which the pumping of
groundwater by Tucson Water will be greatly reduced. To manage
existing resources, Tucson Water has implemented several innovative
programs such as a highly successful water conservation effort that is
administered through its Public Information Office. In Tucson, water
users are encouraged to refrain from watering their lawns during peak
usage periods. In this situation of course, customers are using
10
potable groundwater for irrigation purposes. Additionally, Tucson
Water has evaluated artificial recharge as a primary water management
program (CH2M Hill et al., 1988).
In 1984, Tucson Water initiated the Demonstration Recharge
Project, which has since evolved into the Sweetwater Underground
Storage & Recovery (US&R) Facility (Figure 1.1). The project was begun
to determine the feasibility of recharging and recovering secondary and
tertiary effluent to supply nonpotable water to high volume irrigators,
among these being private and municipal golf courses and school
districts (CH2M Hill et al., 1988). The program has reduced pumpage of
native groundwater resources and has become an important part of Tucson
Water's water conservation program. Although the purpose of the
Sweetwater US&R Facility is to provide an adequate and reliable source
of nonpotable water, suitable for turf irrigation, it also serves as an
experimental field site to evaluate renovation of wastewater through
soil aquifer treatment (SAT).
The Sweetwater US&R Facility is located on a site west of the
Santa Cruz River, across from Pima County's Roger Road Wastewater
Treatment Plant and Tucson Water's Roger Road Reclaimed Water Treatment
Plant (Figure 1.1). The recharge method comprises a series of
infiltration basins surrounded by high capacity extraction wells.
During the low water demand period of the winter months, filtered
secondary effluent from the reclaimed water treatment plant is pumped
to these basins where it infiltrates into the subsurface and reaches
the groundwater table. Thus, it is stored in the aquifer and mixes
11
m GRAVEL ROAD
LYSIMETER
RECHARGE BASIN
SWEETWATER US&R FACIUTY
PAVED ROAD
•
MONITOR WELL
EPHEMERAL STREAM.
BASIN SLOPES
0 EXTRACM ON WELL
OUTLET STRUCTURE
A
RECHARGE AND
"441" EXTRACTION PIPES
Figure 1.1
SOURCE WATER
RECHARGE BASIN
DEMONSTRATION RECHARGE
PROJECT (No longer
in existence)
\ SAMPLING POINT
Sweetwater US&R Facility
12
with the native groundwater. In the high demand summer months, the
stored water beneath the basins is recovered through the extraction
wells and delivered to customers needing on the reclaimed water
delivery system. This program of more efficient water use has proven
to be successful.
Continued success of this recharge program, along with the
anticipated expansions of additional ones, will require that certain
state permit requirements be met. A critical requirement is chemical
water quality. The Aquifer Protection Permit (APP) No. P-101998,
issued by the Arizona Department of Environment Quality (ADEQ), has set
an Aquifer Quality Limit (AQL) of 10 mg/L nitrate reported as nitrogen
(NO3 -N) in all but one of the monitoring wells surrounding the
-
Sweetwater US&R Facility. Additional requirements contained in the APP
that Tucson Water must comply with are: volume recharged and removed
from the aquifer, analytical methods for hazardous and non-hazardous
substances, and sampling and reporting frequency to the ADEQ.
Although the quality of effluent supplied from the treatment plant
is non-potable, natural processes in the vadose zone and aquifer, i.e.
(SAT), can improve water quality to meet these requirements. During
SAT, sewage effluent is applied to spreading basins at a high rate and
allowed to percolate into the soil. Chemical and microbial processes
occur in the subsurface which improve water quality by removing
suspended solids and pathogenic organisms from water and reducing
concentrations of phosphates and nitrates (Bouwer, 1989). The
principal inorganic constituent of concern in the domestic wastewater
13
treatment process is the nitrogen species, nitrate (NO3
-
). This is of
importance during artificial recharge because high levels of the
ammonium ion (NHe) present in treated effluent may result in increased
levels of NO3
-
in groundwater. Negative health related consequences
such as methemoglobinemia (blue baby syndrome) have been linked to the
presence of high levels of nitrate in drinking water (Todd, 1980).
Facilities in Los Angeles (Welsh, 1989), and Phoenix (Bouwer et
al., 1974, 1980, 1984) have been constructed and operated for the
artificial recharge of groundwater. An important finding observed at
the 23rd Avenue Project in Phoenix was the occurrence of NO3
-
peaks in
the profile. These peaks, with concentrations as high as 20-30 mg/L
NO3 -N, were detected with the arrival of renovated water that had
-
infiltrated at the beginning at the flooding period (Bouwer and Rice,
1984). Similar results were observed at the Flushing Meadows Project,
also in Phoenix (Bouwer et al., 1974). An Australian study, which used
both primary effluent and a mixture of primary and secondary effluent
in amended sand basins, also observed these peaks (Ho et al., 1992).
The explanation for the occurrence of the nitrate peak was the
fact that the fate of NH4 during infiltration is influenced by two
simultaneous processes: adsorption and nitrification (Lance, 1975).
Adsorption of NH4 onto the negatively charged clay and organic
colloids in the soil occurs during the wetting cycle. NH4 and other
cations compete for exchange sites on the clay until it becomes
saturated. Since the nitrification of NH4 to NO3
-
requires an
adequate supply of oxygen, NH4 is held in the soil until the dry cycle
14
begins and oxygen becomes available (Lance, 1972). Thus, after
nitrification, NO3
-
present from the previous flooding period is
leached through the soil in a concentrated peak.
Previous SAT research by the University of Arizona in mini-basins
at the Sweetwater US&R Facility has raised important questions
concerning trends in NO3
-
movement through the soil profile. During
their tests, University researchers observed a NO3
-
peak in the upper
vadose zone shortly after flooding began (University of Arizona and
University of Colorado, 1992). It is unknown, however, if this peak
moves through the profile into groundwater or if it is damped out at
depth in the vadose zone. A related point of interest for both
technical and institutional reasons is the possibility of seasonal
changes in NO3
-
concentrations in groundwater resulting from numerous
wet-dry cycles.
The investigation reported in this thesis was conducted in Basin
RB-001 B at the Sweetwater US&R Facility. The research focused on
evaluating the overall fate of nitrogen in effluent applied to this
basin during the 1991-92 recharge season and changes in nitratenitrogen in the vadose zone during a wetting-drying cycle in April,
1992. Possible mechanisms governing the fate of nitrogen are
presented, together with a discussion of institutional implications.
15
Chapter 2
Objectives
Although the current literature provides evidence that nitrate
reduction takes place during SAT, many questions remain concerning
short and long-term changes during recharge. This is of importance to
Tucson Water because it must meet requirements of the APP, and ensure
the long-term, effective operation of its Sweetwater US&R Facility.
The research described herein has therefore focused on the fate of
nitrogen species during recharge of effluent at the Sweetwater US&R
Facility during the recharge season of 1991-92. Of particular
importance is the fate of nitrate concentrations in effluent during
deep percolation through the vadose zone and recharge to the
groundwater. The specific objectives are to:
1.
Determine the fate of nitrate in the vadose zone underlying
Basin RB-001 B by observing trends in the nitrogen species nitrate and
nitrite (NO2 ) as filtered secondary effluent percolates through the
-
vadose zone, as well as nitrate, nitrite, and ammonia concentrations in
groundwater during one wet-dry cycle.
2.
Study possible processes affecting the fate of nitrogen by
examining sulfate ( 504 ) and bicarbonate (HCO3) concentrations in the
-
subsurface.
3. Study the seasonal fate of nitrogen species nitrate, nitrite,
kjeldahl nitrogen, and ammonia in wastewater during recharge in Basin
RB-001 B throughout the 1991-92 season. Calculate a total nitrogen
mass balance between source water and groundwater and account for
16
changes to the nitrogen species through processes occurring in the
vadose zone.
4. Study the hydraulics of the infiltration basin, including
infiltration rates; infiltration volume; impeding layers in the vadose
zone that promote perched groundwater; and the effects of perched
groundwater on infiltration and the fate of nitrogen during high rate
effluent recharge.
These objectives focus on Tucson Water's operational goal and
commitment to supply reclaimed water for nonpotable purposes. The
results described here will also seek to determine the feasibility of
using SAT in the future as a possible treatment process, relative to
nitrogen species, for potable water. The results of this study will
also be of interest to other communities in Arizona considering
recharge and recovery of treated effluent.
17
Chapter 3
Literature Review
3.1 Nitrogen Transformations in the Subsurface
Recharging treated municipal effluent is becoming an important
technique used to replenish groundwater supply and as an additional
treatment method to improve water quality (Crites, 1985). Secondary
treatment generally calls for the removal of pathogenic organisms,
suspended solids, organic matter, and reduction in the nutrient
concentration (Bouwer, 1991). The technique of SAT is based upon the
premise that nutrients contained in treated effluent can be transformed
and reduced to an acceptable level, depending on the ultimate use of
the water. Physical, biological, and chemical processes within the
soil control the fate of nutrients, including nitrogen.
Figure 3.1 (after Lance, 1972) illustrates nitrogen
transformations that occur when treated sewage effluent is applied to
land. Nitrogen present in sewage is primarily in the form of NH4 + .
Once applied to land, it can be transformed in the soil, percolate into
the groundwater or be lost to the atmosphere through denitrification.
Among the more significant mechanisms involved are biological
denitrification, volatilization of ammonia, adsorption of ammonium ion,
fixation of ammonia by the organic fraction, and removal of nitrogen by
vegetation (Lance, 1972). The importance of each process has been the
subject of field and column studies over the past twenty years.
The specific series of reactions in which certain microorganisms
oxidize ammonium (NH4 + ) to nitrite (NO2 - ) or NO2 - to nitrate (NO3-)
18
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(Delwiche, 1970) is known as nitrification. In effluent, nitrification
can occur by the reaction (Robertson and Cherry, 1990):
NH4 + 202 =* NO3
-
+ 2 1-1+ + H20.
Denitrification is the process in which NO2
-
(Equation 3.1)
and nitrate NO3
-
are
reduced to gaseous compounds such as molecular nitrogen (N 2 ) or nitrous
oxide (N20) (Delwiche, 1970). Denitrification is the primary mechanism
for the removal of nitrogen from effluent.
The principal form of nitrogen contained in soils is the highly
oxidized nitrate ion. Denitrifying bacteria, under anaerobic
conditions, are able to use nitrate as electron acceptors in the
process described by the equation:
C6H1206 + 4NO3
-
=* 2N2 + 6 1-1 20 + 6002 .(Equation 3.2)
The stochiometric ratio of 6:4 carbon to nitrogen translates to 0.7 mg
carbon/mg nitrate-nitrogen. Lance, (1972) found the three conditions
most difficult to meet when trying to achieve the denitrification of
secondary effluent to be (1) oxidation of the ammonium ion to nitrate,
(2) the passage through an anaerobic zone after nitrification, and (3)
the provision of an adequate energy source in the anaerobic zone for
the denitrifying bacteria.
Biological denitrification can be divided into two categories,
depending on the mediating bacteria. Heterotrophic bacteria are able
to use organic carbon as the electron donor and NO3
-
as the electron
acceptor in their metabolic process. In groundwater, the organic
carbon level must be equal to or greater than the NO3 -N concentration
-
(Korom, 1992). An important by-product of denitrificat ion is HCO 3 , as
-
20
the following equation (Kolle, 1983):
5C + 4NO3 - + 2H20 =*2N2 + 6HCO3 - + CO2 (Equation 3.3)
Depending on the geochemical composition of the aquifer, HCO3 - may act
to buffer the acid produced during the nitrification of N14+ (Robertson
and Cherry, 1990).
In autotrophic denitrification, bacteria use reduced inorganic
species such as Mn 2 +, Fe 2 +, and HS
-
as electron donors (Korom, 1992).
According to the reaction (Kolle, 1985):
5FeS2 + 14NO3 - + 4H+ =* 7N2 + 10SO4 2- + 5Fe 2 + + 21-120, (Equation 3.4)
904 2- is produced in amounts almost equal to the NO 3 - input. Each type
of denitrification reaction may occur simultaneously or independently
at any given site. The production of HCO3 - and/or SO4 2- will be useful
in determining the extent of denitrification.
3.2 Column Studies
To study the mechanisms involved in nitrogen transformation in
more detail requires monitoring all nitrogen species closely. This can
be accomplished by constructing soil columns that simulate field
conditions. In addition to the added degree of monitoring capability,
different theories designed to improve nitrogen removal can be tested
and the results obtained in a much shorter period of time than in the
field. The use of soil columns cannot always be regarded as an
accurate representation of field conditions and the results should,
whenever possible, be compared with field results.
21
The presence or absence of organic carbon has been studied under
laboratory conditions to determine its role in the denitrification
process. In columns containing soils taken from the Flushing Meadows
rapid infiltration basins, Lance and Whisler (1976) found a 12 percent
increase in nitrogen removal with the addition of 150 mg/L methanol and
almost complete denitrification with 150 mg/L organic carbon (contained
in dextrose). This shows that the organic carbon content of secondary
effluent can be a limiting factor in denitrification. Later work by
Lance et al. (1980) also found greater loss in nitrogen using primary
effluent compared with secondary because it contained a greater organic
matter content. The recharge and recovery of primary effluent, even
for use in turf irrigation, will require further treatment due to
public health concerns.
Barley and Bermuda grass were grown in columns containing sand
taken from the bed of the Salt River, at a site used for rapid
infiltration. At infiltration rates of 10 to 12 cm/day, nitrogen
removal was increased by 10 to 18.5% while plants were growing as
opposed to bare soil. It was postulated that this increase was a
result of nitrogen uptake by the plants and not from stimulation of
denitrificat ion (Lance and Whisler, 1978).
A comprehensive study using a silt loam soil was conducted in a
Virginia wastewater treatment facility to evaluate the extent of
denitrification by controlling wastewater disposal operations and
environmental factors including effluent loading rates (Degen et al.,
1991).
The laboratory aspect of the investigation found the soil
22
horizon (surface vs. subsurface) to be the most significant treatment
factor. For effluent at 10 ° C and 20 ° C, surface soils produced 2.7 and
4.1 times as much NO3 - -N as the subsurface soils. It was expected that
the higher permeability of the surface soils would diminish the
saturated soil conditions and increase nitrification. Nitrate
concentration in the leachate was also found to be at the highest
levels using the highest loading rates.
3.3
Field Studies
Currently, there are no reliable numerical groundwater quality
models on nitrate transport available due to limited knowledge of these
processes (Kinzelbach et al., 1990). Accordingly, the ultimate goal of
field studies is to evaluate the geochemical and biological processes
that facilitate denitrification. This can best be achieved by
establishing a network of subsurface water-sampling devices. The fate
of nitrogen can then be determined by sampling water percolating
through the vadose zone and in groundwater for nitrogen species.
Among the earliest and most comprehensive feasibility studies for
the renovation of wastewater through SAT was the Flushing Meadows
project near Phoenix, Arizona using effluent from Phoenix's 91st Avenue
Treatment Plant. The objectives of the study were the removal of the
total nitrogen content and the improvement of water quality of treated
effluent in order to meet irrigation water standards. Various
parameters, including different flooding and drying schedules and
23
different basin floor covers, were studied to determine their effect on
water quality.
Bouwer et al. (1974) found that after a 2 day flooding and a 4 day
drying cycle, ammonium was completely converted to nitrate but after a
2 to 3 week flooding and 2 week drying cycle, the nitrate level in the
well water sample was almost zero. They postulated that oxygen was
available in the soil profile during the shorter wet/dry cycle but the
upper one meter of soil becomes anaerobic during the longer cycle. The
nitrogen remains in the ammonium form, and is adsorbed by the clay and
organic matter in the soil. During the subsequent drying cycle, the
adsorbed ammonium becomes nitrified to nitrate, which is then
denitrified in anaerobic regions to produce nitrogen gas and oxides of
nitrogen. Some nitrate will remain in the upper region, producing a
nitrate spike during the next wet cycle.
Later results by Bouwer et al. (1980) followed the same pattern,
giving further support to this theory. In 1972, after 4 years of basin
operation at maximum hydraulic loading (400 ft/yr), they also found an
increase in the ammonium (NH4+-N) concentration in the renovated water
from almost zero to about 20 mg/L. The higher NH4 4- -N concentration
resulted in a 30% reduction in the amount of nitrogen removed. They
assumed that more NH 4 4- -N was adsorbed in the soil during flooding than
could be nitrified during drying.
In 1975, based on results obtained from column studies, the
flooding schedule was changed from 2 weeks flooding and 2 weeks drying
to 9 days flooding and 12 days drying. They hoped to achieve
24
equilibrium between the amount of NH4+-N in the effluent and the amount
of adsorbed NH 4 +-N that was nitrified during drying. By 1977, samples
from a monitor well showed average concentrations of 2.8 mg/L NH4+-N,
6.25 mg/L NO3 - -N, 0.58 mg/L organic N, trace amounts of NO2 - -N, and a
65% reduction in the nitrogen content of the effluent. During further
studies at the 23rd Avenue project, SAT reduced the nitrogen
concentration in the effluent from 18 mg/L (mostly ammonia) to 5.6 mg/L
(mostly nitrate) (Bouwer and Rice, 1984).
The field aspect of the Virginia wastewater treatment site found
that loading rate played an important role in nitrogen transformation.
Degen et al. (1991) observed that a loading rate of 0.9 cm/day produced
the lowest NH4 4- -N, and NO3 - -N values. They also found N20 production
increases with loading rate. A model based on laboratory data was
developed to compare with field results. At the lowest loading rate
(0.9 cm/day), the model predicted an anaerobic effluent applied at 48
hour periods would result in the highest level of denitrification. The
model significantly underestimated the amount of denitrification
observed in the field study. They postulated that there must be a
combination of aerobic followed by anaerobic conditions in the soil for
the NI 4 +-N to be denitrified to NO3 - -N (D4gen et al., 1991). This
work agrees with theory postulated by Lance.
By comparison, a 1986 study by Robertson and Cherry focused on the
behavior of NH4+-N, and NO3 - -N from a septir, ystem plume. The soil
was rich in carbonate minerals, contained a high organic carbon content
(foc = 0.025), and the depth to water was only two to 15 m. The
25
ammonium level was reduced from 58 mg/L NH4+-N to less than 1 mg/L
during the 5 to 10 day residency of the effluent in the four m thick
unsaturated zone (Robertson and Cherry, 1990). Denitrification
occurred in the plume 10 to 70 m downgradient of the discharge point.
A loss of 45 mg/L NO3 - -N from the discharge point to the depleted zone
corresponded to increases in HCO3 - and SO4 2- concentration. The
authors postulated denitrification was accomplished in an anaerobic
environment, with organic carbon naturally present in the aquifer as
the energy source (Robertson and Cherry, 1990).
These studies show denitrification and renovation of wastewater
can be achieved with the aid of SAT. Information obtained in other
studies will be used to compare with results obtained at Sweetwater.
In addition, theories regarding the nitrogen transformation process
will be tested here by attempting to observe similar patterns of
behavior.
3.4
University of Arizona Studies
The University of Arizona, in cooperation with Tucson Water, the
Salt River Project (SRP), and the University of Colorado, began a study
at the Sweetwater US&R Facility to gain insight into the hydrological,
chemical, and microbial processes occurring during wastewater
reclamation. The motivation for this study was interest in the use of
SAT for potable water reuse. Among the concerns were:
26
1.
The level of dissolved organic carbon (DOC) after secondary
and tertiary treatment may cause the production of trihalomethanes
(THM's) after chlorination. THM's are thought to be carcinogenic.
2.
High levels of nitrate in reclaimed water after recharge.
3. Recovered effluent may contain pathogenic organisms.
To study the fate of chlorination byproducts and nitrogen
compounds during SAT, two mini-basins, each 144 square feet, were
constructed within RB-001 B at Sweetwater US&R Facility. One minibasin was flooded with chlorinated secondary effluent and the other
with filtered, chlorinated secondary, i.e., tertiary effluent. Seven
suction lysimeters were installed at depths up to 20 feet to assess
depth-wise changes in DOC, total organic halide (TOX), and the nitrogen
compounds, ammonia, nitrate, and nitrite.
Results of the nitrogen compound studies showed that nitrate
values were very high just after the flooding cycle began and peaked in
the upper part of the profile (University of Arizona and University of
Colorado, 1992). Later, nitrate values decreased in the profile
(Figure 3.2). Again, this follows the same trend observed by Bouwer.
The average NH4 -1- -N concentration of 14 mg/L in secondary effluent and 7
mg/L after tertiary treatment was reduced to less than 1.0 mg/L at a
depth of 15 feet. Samples of a perched groundwater region, occurring
at about 15 feet below land surface, resulted in an average 16 mg/L
NO3 - -N. Desire to gain a better understanding of the nitrogen
transformation process deeper in the subsurface resulted in the need
for further study.
27
NITRATE-N CONCENTRATION (mg/I)
• 50
100
150
I
I
LJ
LJ
LJ
LL
10
15
0
_J
LJ
I-
a_
20 -
LJ
11111 NOVEMBER 21, 1990
>seee< NOVEMBER 26, 1990
NOVEMBER 28, 1990
25
Figure 3.2 Nitrate—N Concentration through
Profile During Mini—Basin Flooding
Wetting)
Drying
Drying
200
28
Chapter 4
Description of Study Area
4.1 Tucson Physiographic Environment:
Tucson Geological Setting
4.1.1
The Tucson Basin is a broad, northwesterly sloping valley
approximately 1,000 square miles in area. It is bounded on the north
and east by the Tortolita, Santa Catalina, Tangue Verde, and Rincon
Mountains, on the south by the Empire, and Santa Rita Mountains, and on
the west by the Tucson, Black, and Sierrita Mountains. The Santa Cruz
River flows northward from Mexico and drains the Tucson Basin with its
tributaries, the Rillito Creek, Pantano Wash, Tanque Verde Wash, and
Canada del Oro Wash.
The Tucson Basin is considered typical of the Basin and Range
Lowlands Hydrogeologic Province of Southern Arizona. The region is
characterized by alluvial valleys and basins filled with sediments
eroded from the surrounding mountain ranges. The geologic formations
of the Tucson Basin form a single aquifer distinguished by age and
lithology. The primary formations within the Basin are the sedimentary
units consisting of the Fort Lowell formation of the Quaternary age and
the Tinaja beds of the Tertiary age, and pre-basin sedimentary and
volcanic rock of the Pantano formation also of the Tertiary age (CH2M
Hill et al., 1988).
The recent alluvium forms the modern flood plain and stream
channel and terrace deposits which occur along streams and washes.
Data obtained from drillers logs show the deposits to be a few feet
thick along small washes to almost 100 feet thick along the largest
29
stream channels (CH2M Hill et al., 1988). Along the Santa Cruz River,
the thickness of this unit ranges from 30 to 80 feet. The alluvial
material consists of coarse sand and gravel which forms the most
permeable unit of the basin. There is a larger silt fraction deposited
along the Santa Cruz River than Canada del Oro and the Rillito River
(Davidson, 1973). This unit is unsaturated throughout the entire
basin.
The Fort Lowell formation crops out at the basin edge and valley
floor. It is comprised of a series of heterogeneous deposits of
unconsolidated to weakly lithified interbedded clayey silts, sandy
silts, sands, and gravels. This unit is thicker near the center of the
basin and thinner near the edge of the mountain ranges. Average
thickness of this unit in the Tucson Basin is 350 feet and the average
saturated thickness is 100 feet (CH2M Hill et al., 1988).
Underlying the Fort Lowell formation are the Tinaja beds, which
are comprised of the upper, middle, and lower beds. The upper Tinaja
beds consist of unconsolidated to poorly indurated, lenticular,
interbedded clayey silt, sandy silt, sand and gravel. They range in
thickness from about 50 feet to over 1,000 feet with the average
thickness being 700 feet (CH2M Hill et al., 1988). The high value of
porosity and permeability of the upper Tinaja and Fort Lowell formation
create the principal water bearing unit in the Tucson Basin. The
middle and lower Tinaja beds consist primarily of clayey silt, cemented
sand, and mudstone. These beds are fully saturated, but do not yield
large amounts of groundwater to wells.
30
The climate of Tucson is semiarid with an average precipitation of
12 inches per year in the center of the basin to 25 inches in the
mountainous areas (Davidson, 1973). Most rains follow a seasonal
pattern of long, light rains in the winter months and intense,
convective monsoonal downpours during the summer. Because of this,
streamflow usually only occurs during the spring thaw in the mountains
and to a much greater extent after a summer thunderstorm. Annual flows
are highly variable from year to year and location to location, but
average 10,000 ac-ft per year at the downtown Tucson gaging station
(United States Geological Survey, 1981). Groundwater flows in a
northwesterly direction out of the basin, roughly paralleling the Santa
Cruz River. Depth to groundwater ranges from about 20 feet near Tanque
Verde Creek to over 200 feet in the center of the basin (Tucson Water,
1992).
4.1.2
Tucson Groundwater Quality
The quality of groundwater in the Tucson basin is a result of
natural and artificial recharge, the composition of the minerals
comprising the basin fill, and chemical reactions within the aquifer.
Shallow groundwater, found at depths less than 700 feet, generally
contains less than 500 mg/L dissolved solids, and the principal ions
are calcium, sodium, and bicarbonate (Laney, 1972). Calciumbicarbonate and calcium-sodium-bicarbonate have been found to be the
most common types of groundwater found in the Tucson Basin (Laney,
1972).
31
The principal potential sources of groundwater contamination are
sewage effluent recharge, urban runoff, and irrigation runoff.
Historically, the City of Tucson has sold sewage effluent for crop
irrigation, with excess volume being released into the Santa Cruz River
channel (Pima Associations of Government, 1983). The highest levels of
NO3 -N in the Tucson Basin generally occur near the Santa Cruz, with
-
levels from Tucson Water groundwater samples ranging from 1.7 to 35
mg/L (Figure 4.1). The source of the high nitrate levels in the area
along the Santa Cruz can be traced to past agricultural activity and to
the discharge of effluent from Roger Road and Ina Road treatment
plants. Nitrate levels in groundwater from other parts of the Tucson
Basin is generally less than the EPA drinking water standard of 10 mg/L
NO3 -N (Laney, 1972).
-
4.2 Sweetwater US&R Facility
4.2.1 Construction
Sweetwater US&R Facility is located in Section 20, 21, 28, and 29
of Township 13 South, Range 13 East adjacent to the western bank of the
Santa Cruz River (Figure 4.1); the approximate boundary of the Facility
is outlined. The final design for the Sweetwater Facility was approved
by Tucson Water in February, 1989 and construction activities began
June, 1989. Sweetwater US&R Facility consists of four infiltration
basins, three extraction wells, and ten monitor wells used for water
quality and water level monitoring.
The four basins, designated RB-001 B, RB-002 B, RB-003 B, and RB004 B, range in size from 2.8 to 5.2 acres for a total of approximately
32
WI(-204A
17 7
WR-203A
3.5
20
21
WR-202A
2.0
T. 13 S
Sweetwater US&R
Facility
29
WR 5A W 69A
•
161
12
W 199A
WR-201A
1.6
WR-198A
378
R. 13 E
SCALE 1 inch = 1500 Feet
Figure 4.1 Nitrate as Nitrogen (mg/L)
Concentration for April, 1992 Samples
28
33
14 acres (Figure 1.1). In order to increase infiltration rates over
those observed in the Demonstration Project, the basins were excavated
to depths of between ten and fifteen feet below land surface where a
layer of coarse sediments occurs. The recent alluvial deposits here
are between 27 to 32 feet thick and consist of silty and clayey sands
and gravels. In addition, there is a discontinuous clay-rich layer at
approximately 15-16.5 feet below the basin floor (Cline, 1992). This
layer causes an extensive region of perched groundwater after the basin
is flooded.
Ten monitor wells are located around the facility and within the
basins. The wells were constructed using 6" diameter steel casings,
gravel packed within a 12" diameter borehole, grout sealed from land
surface to various depths, and perforated at various depths and
intervals.
Each is equipped with a submersible pump and sampling
outlet. Tucson Water is required under its operating permit to sample
the wells periodically and meet mandated quality standards.
4.2.2 Operation
Secondary effluent from the Roger Road Water Wastewater Treatement
Facility is sent to Tucson Water Reclaimed Water Treatment Plant (RWTP)
for further filtration and chlorination (Figure 1.1). The additional
treatment consists of filtration through coarse anthracite and fine
sand to remove the suspended particulate matter, and finally the
addition of chlorine for disinfection purposes. This tertiary effluent
is then distributed to nonpotable water users. Effluent that is
34
delivered to the Sweetwater Facility has undergone the filtration step
only.
After filtration, delivery to the Sweetwater US&R Facility is made
via an underground pipeline, and constant head is maintained at the
basin inlet by float activated sensors. During the demonstration phase
and the later operational phase, a wetting/drying cycle was established
in order to maximize infiltration rates. This is accomplished by
rotating the flooding between Basins RB-001 B - RB-004 B. This varies
depending on the quantity of algae that develops and the size of the
basin. Generally, the basins are flooded to a depth of 1.25 to 1.75
feet, measured at the basin inlet, for three to six days and dried for
seven to fourteen days. Flooding in each basin continues until algal
growth appears which could clog basin bottoms and decrease infiltration
rates.
In addition to monitoring chemical constituent levels, Tucson
Water also measures water table elevations, volume recharged to the
basins, and volume recovered by the extraction wells. The water table
elevation in the vicinity of Sweetwater US&R Facility, as measured by
well WR-069 A, has fluctuated between approximately 2130 and 2160 feet
Above Mean Sea Level (A.M.S.L.), which is about 100 to 130 feet below
land surface, since the Sweetwater US&R Facility has been in operation
(Figure 4.2). Storage balance is defined as the volume of recharged
water available for recovery to meet demand for non-potable use. It is
calculated as recharged volume minus recovered volume. Under the APP,
the volume in storage must not exceed 6,500 ac-ft.
35
(>i33m/1voiN) ]wnioA
0
0
K")
1
0 0
N
1
1
1
1
1
1
1
.............................
.................... ••-•'
..... • .....
....
o
O)
— co
.,,
1
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1 1
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36
Chapter 5
Methods and Materials
5.1 Overview
In order to observe the transformation of nitrogen as it
infiltrates from the basin surface, through the subsurface, and into
the groundwater, water sampling points at several locations must be
used. During this study, source water samples were taken downstream of
the Tucson Water RWTP APP compliance point 520B (Figure 1.1). Samples
from the vadose zone were obtained from a series of 14 ceramic, highpressure vacuum lysimeters. Groundwater samples were obtained from a
monitoring well.
The vadose zone was sampled using in situ pore-liquid samplers
(Figure 5.1). These units consist of a porous ceramic cup mounted onto
a dual-chambered tube. A pressure-vacuum line extends from land
surface, through a solid plug to the upper chamber. A discharge line
also extends from land surface through the plug, but terminates at the
base of the upper, sample collection chamber. The upper and lower
chambers are separated by a plug containing a one-way check valve. The
check valve prevents sample loss through pressurization. A small hand
pump is used to apply a vacuum of 70-80 kPa within the sampler. The
pressure gradient across the porous cup forces water through the pores
within the ceramic material. To collect the sample, the hand pump is
used to apply a positive pressure to the upper chamber, forcing the
sample to the surface through the discharge line.
SOLID PLUG
CHECK
VALVE
DISCHARGE
UNE
CHECK
VALVE
SOLID
PLUG
PRESSUREVACUUM
LINE
PRESSURE
SEAL
TRANSFER
CHAMBER
"0"RING
PRESSURE
SEAL
POROUS
CUP
Figure 5.1 High Pressure-Vacuum Lysimeter
38
The lysimeters were installed during the summer of 1990. The
general method for the installation of suction samplers is given in
Appendix I. Two transects of samplers were installed, each containing
seven lysimeters (Figure 5.2). The depths used to sample the soil
profile in each transect are 2.5, 5, 10, 17, 40, 60, and 80 feet below
the surface of the basin. The layout and depths were selected to
obtain a profile of water quality changes in the vadose zone,
especially between the surface and the perched layer. Careful
installation of the samplers is required in order to insure the soil
surrounding the porous cup remains as undisturbed as possible and a
good hydraulic connection can be made using a silica flour slurry.
After installation, each sampler was tested for air leaks and extra
tubing connected to each line to facilitate sampling.
The University of Arizona began sampling the lysimeters during the
initial wet cycle of the 1991-92 recharge season. Subsequent samples
were taken at approximately the same time after the dry cycle began.
Appendix II contains the results of the major ion analysis. Initially
it was hoped that the lysimeters could be used to sample ammonia and
kjeldahl nitrogen in addition to nitrate and nitrite. Literature on
vadose zone sampling methods suggests that a ceramic cup, such as used
in this experiment, would be inappropriate (Dorrance et al., 1991).
Adsorption, when a dissolved ion or molecule becomes attached to the
surface of solid, may occur between the ceramic material and the NH4+
ion. If this occurs, inaccurate mass-balance estimations would result.
39
40
To test this, a lysimeter similar to those installed at the
Sweetwater Facility was set in a container of ammonium nitrate. A
vacuum was applied and the lysimeter was allowed to collect sample
overnight. The next day, the source water and sample were collected.
Results of ion chromatography analysis found 23.9 mg/L NH4+-N in the
source water and 10.9 mg/L NE4+-N, in the lysimeter sample, a loss of
over 50%. At the time the sample was collected, an odor of ammonia was
also observed, suggesting that volatilization of the sample also
occurred. At the pH of the solution, most nitrogen would be in the
NH3-N form, and it was most likely that NH3-N was volatilized
(Freney et al., 1983). Based on these results, it was decided that
ammonia would only be measured from source water and groundwater
samples.
In August, 1991, a new monitor well, WR-199 A, was constructed in
the southwest corner of RB-001 B, near the lysimeters. Table 5.1
summarizes the lithological log from samples collected during the
drilling of WR-199 A. The depth from 0 - 15 feet is considered to be
recent alluvium based on the high gravel and sand content. From 15 to
35 feet, the percentage fines greatly increases, again showing the
presence of a clay-rich region at a depth of about 15 feet.
Figure 5.3 shows the construction details of well WR-199 A. The
well is perforated from 86 to 206 feet. It is equipped with a Grundfos
40520-7 2 HP submersible pump, with the intake set at 131.7 feet. In
order to sample the uppermost portion of the water table, a series of
"packers" were installed just below the intake. The packers consist of
Table 5.1 WR-199 A Lithologic Description Depth
To
From
0
Major Components
4'
Particle Size
of Sand & GravelDistributionRoundness
1595% Volcanic
0 % Ipeous(Phaneritic)
— % Gneiss & Schist
.__Z% Metasediments
(Quartzite, etc.)
_% Sedimentary
(lithified)
Chemical Precipitates
_ %
60% Gravel
_
35 % Sand
5 % Fines
Well Rounded
Rounded
___X_
Subrounded
Subangular
_
Angular
CementationWell
Poor
Mod. X
None
Comments: Hole collared in bottom of Basin #1 sooty Mn oxide coatinz small (less than or enual to ll subrounded cobblers
from 5 - 15'. Cobblev channel deposit.
Formation Description: yellowish to lizht brown. purp lish - medium tray. silty/clavey, coarse sandy. cobbley gravel.
Depth
To
From
15
35
Particle Size
Major Components
of Sand & GravelDistributionRoundness
95% Volcanic
3 % Igneous(Phaneritic)
%
Gneiss & Schist
% Metasediments
(Quartzite, etc.)
—% Sedimentary
.....K% Gravel
_23.% Sand
_
_A% lines
X
_
Well Rounded
Rounded
Subrounded
Subangular
Angular
(lithified)
—% Chemical Precipitates
Cementation Well
Poor
Mod.
None X
Comments: Increase in lizht tan latite Doran . Traces of dendritic Mn oxide on anzular to subanzular gravels. Basin fill with
no cobbles. Noticeable increase in fines in this 15' interval.
,
Formation Description: purplish-yellowish tan to tannish lizht zrav. silty/clayey coarse sandy zravel.
Table 5.1 con t
42
,
Depth
From
To
35
Major Components
Particle Size
of Sand & GravelDistribution
80100% Volcanic
Igneous(Phancritie)
_ %
—% Gneiss & Schist
_% Metasediments
Roundness
42% Gravel
Well Rounded
Rounded
____
____
55 % Sand
_1% Fines
Subrounded
Subangular
X
(Quartzite, etc.)
Angular
_% Sedimentary
(lithified)
—% Chemical Precipitates
CementationWell
Mod.
Poor X _None
Comments: Spotty Mn oxide coating gravel. Gravels oomorised of rhvolite porphry. latitc porphry and rbyolitie tuft
Formation Description: yellowish, reddish tan to light purplish medium gray, silty, gravelly coarse sand.
Depth
From
To
Particle Size
Major Components
of Sand & GravelDistributionRoundness
ao
_100% Volcanic
Gravel
70 % Sand
% Igneous(Phaneritie)
5 % Fines
% Gneiss & Schist
% MetasedimentsX
95
Well Rounded
Rounded
Subrounded
Subangular
Angular
(Quartzite, etc.)
% Sedimentary
(lithified)
%
Chemical Precipitates
Cementation Well
Poor
Comments: Sandy horizon in basin fill gravels. Trace Mn oxide stain Atoneable decrease).
_Formation Description: purplish light to medium gray, silty, gravelly, coarse sand.
Mod.
None X
Table 5.1 con't Depth
To
From
95
43
Major Components
Particle Size
of Sand & GravelDistributionRoundness
210100% Volcanic
_ %
Igneous(Phaneritic)
_ % Gneiss & Schist
___% Metasediments
(Quartzite, etc.)
_% Sedimentary
42% Gravel
Well Rounded
55 % Sand
— Rounded
3 % FinesSubrounded
—
X
—
Subangular
Angular
(lithified)
_% Chemical Precipitates
Cementation
Well
Poor
Mod.
None X
Comments: Sandy horizons from 110 - 135' and from 150- 160'. Visible Mn oxides on travels down to approximately 160'.
Traces of Fe oxide stain.
Formation Description: yellowish tan to purplish lieu stray, silty, suavely coarse sand to sandy travel.
44
CASING DIAMETER
16"
DEPTH TO TOP OF
CASING SEGMENT
LAND SURFACE
CEMENT
COMMENTS
SURFACE
Top of 6" casing 4 ft. above
land surface.
CASING DEPTH
35
90 Ft. blanh 6 jn. casino.
Sand cement
grout
71.4 Ft. Sand cement grout.
Centralizers every 40 Ft.
4 Ft. fine sand
71.4
75.4
86
otiO
Top of sand seal
Top of grovel pock
Top of screen
oc,'
0 00
o 0 o
,°0 c
1/4 - 116 Tachna gravel from
Grundfos 2 HP
Intake at
0 .c ol---131.69 ft below
6 a° 1 top of casing
col
cc%
coo
75 4 Ft. to bottom of hole.
c o 'f. ,Pump
.
120 Ft. perforated 6" casing.
I
\ iI I
c c
3" long, 1/8" wide saw cut perfs.
00
00
1 ,0D
00
Sealed on bottom.
i-
to °
r:
206
210
:
U0
1
OC.
I
C C0
LC,
0
c
0°0
0 C'
o 0„,
0p
-0 o
.' -c, 0
Do G
L C
OCO
1
BOTTOM OF CASING
C COO C
c0 o D
Figure 5.3 Well Contruction Details for WR-199 A
45
4 inch "food grade" neoprene disks, 1/4 inch thick and 6 1/8 inch in
diameter in between 5 inch steel washers. WR-199 A was used in this
study to sample the percolated water once it had reached the water
table. Analyses for NO3 , NO2 , NH4+, and kjeldahl nitrogen were
-
-
performed on these groundwater samples.
Several small monitor wells were installed in RB-001 B for the
purpose of observing depths to water at various locations throughout
the basin, including the perched layer. These wells were constructed
out of 2 inch, schedule 40 PVC. Water level measurements in these
wells demonstrated that there is horizontal flow in the perching layer
from RB-002 B to RB-001 13 while RB-002 B is being flooded. For this
study, two of the wells located near the lysimeters, MW-7 and MW-8,
were used to measure the depth to water of the perched layer. The
total depth of MW-7 is 18 feet below land surface with perforations
from 8-18 feet. MW-8 is 25 feet deep, with perforations from 15-25
feet. Figure 5.2 shows the layout of the lysimeters, shallow monitor
wells, and WR-199 A within RB-001 B.
5.2 Sample Collection
Groundwater samples were collected from monitor well WR-199 A with
the assistance of Tucson Water personnel. The well was pumped for 30
minutes at an average flow rate of 35 gpm. This insured that at least
3 well volumes were purged from the region surrounding the well. This
is required under the APP to insure that the samples collected are
representative of the aquifer water quality. Samples for all nitrogen
46
species, ammonia, kjeldahl nitrogen, nitrate, and nitrite analysis were
collected at the well head in 125 mL Nalgene® bottles. The bottles
were transfered to coolers and were either taken for immediate
analysis.
Samples from the lysimeters were collected by the University of
Arizona during the 1991-92 recharge season. One day before the
lysimeters were to be sampled, they were purged of any residual water
in the cup. A vacuum of approximately 70 kPa was applied via the
pressure-vacuum line with a hand pump. This line and the sample line
were then clamped off. Approximately 24 hours later, pressure was
applied to the pressure-vacuum line, forcing the sample to the surface
through the discharge line. Samples were collected in 125 mL Nalgene®
bottles which were rinsed thoroughly prior to use. The bottles were
also transfered to coolers and were either taken for immediate analysis
or frozen. Table 5.2 summarizes the sample collection activities for
the wetting/drying cycle April 12 - May 1, 1992. The wet cycle was 4
days, and the subsequent drying period was 10 days in duration.
5.3 Sample Analysis
Samples collected by the University of Arizona were analyzed on an
ion chromatograph. The analytical method used to measure the
concentrations of NO3 , NO2 , and SO4
-
-
-
in the lysimeter samples during
the April 12 - May 1, 1992 wetting/drying cycle was a HACH model DR/700
Colorimeter. The principle behind the colorimetric analytical method
is as follows: a known volume of sample, 10 mL in this case, is added
47
Table 5.2
Date/Time
April 12 - May 1 Sampling Events
Event
Sampler
Basin cycle
4-12
RB-001
B
4-13/800
WR-199
A sample
4-20/700
R 8 -001 B
4-20/730
Lysimeter
sample
Univ
of Arizona
Wet
4-20/1630
Lysimeter
sample
Univ
of Arizona
Wet
4-21/730
Lysimeter
sample
Univ
of Arizona
Wet
4-22/2300
Pond water pH sample
Univ
of Arizona
Wet
4-23/2230
Pond water pH sample
Univ
of Arizona
Wet
4-24/640
RB-001
4-24/715
Pond water pH sample
4-24/730
Lysimeter
4-27/730
B
Dry cycle begins
Dry
Tucson Water
Wet Cycle begins
Dry
Wet
Dry cycle begins
Dry
Univ
of Arizona
Dry
sample
Univ
of Arizona
Dry
Lysimeter
sample
Univ
of Arizona
Dry
5-1/730
Lysimeter
sample
Univ
of Arizona
Dry
5-1/830
WR-199
A sample
Tucson Water
Dry
48
to a borosilicate glass sample cell. A powder pill containing the
specific reagent is added to the cell and allowed to react for a
specified period of time. A chemical reaction occurs causing the
sample to change colors. A blank cell containing milli-Q water is
placed in the colorimeter to zero the absorbance reading. The sample
cell is then placed in the colorimeter and a specific wavelength of
light is shown through the cell. The colorimeter electronically reads
the absorbance and transmittance of the beam of light through the cell.
Based on a set of internally or externally determined calibrations
contained in the filter module inserted in the back of the colorimeter,
the unit returns a value for the concentration of the constituent. The
specific steps for each analysis are slightly different.
Nitrate analysis was made according to HACH method number 8039,
cadmium reduction method (HACH, 1991). After the powder pillow was
added to the sample, it was shaken vigorously for one minute and
allowed to stand undisturbed for five minutes. The chemical reaction
that takes place is the reduction of nitrates to nitrites by the
cadmium metal. The nitrite ion reacts in an acidic medium with
sulfanilic acid to form a diazonium salt. This salt couples to
gentistic acid in the powder pillow to turn the solution an amber
color. With this method, the HACH DR/700 contains an internal
calibration. To test the accuracy, a set of standards, 0, 10, 20, and
30 mg/L NO3 -N, where prepared and measured for concentration. A
-
sample result of the calibration (Figure 5.4a) shows the internal
calibration to be accurate over the entire range of 0-30 mg/L NO3--N.
49
1
.0
—
a. Nitrate Calibration Curve
-
0.8 —
-
Best fit = 0.0173x + 0.0351
-
0.2 —
--
0.0
o
1
i
i
1
I
I
I
I
I
I
10
20
Concentration (mg/L)
1
i
i
30
b. Sulfate Calibration Curve
1 .0
0.8
Best fit = 0.0113x + 0.0203
0.2
0.0
10
20
30
40
50
60
70
80
90
Concentration (mg/L)
Figure 5.4 Sample HACH DR/700 Calibration Curves
100
50
This test was re-performed after analyzing 14 samples. Possible
interferences with this method include high levels of nitrite, the
presence of ferric iron, chlorine concentrations above 100 mg/L, and
very high or low pH values. None of these potential interferences are
likely in this study.
Nitrite analysis was made according to HACH method number 8040,
diazotization method (HACH, 1991). After the powder pillow was added
to the sample, it was inverted to mix and allowed to stand undisturbed
for ten minutes. In the chemical reaction that takes place here, the
nitrite ion reacts with sulfanilic acid to form a diazonium salt
causing the solution to turn a pink color. With this method, the HACH
DR/700 is also internally calibrated in the range 0 to 0.350 mg/L NO2N. A calibration check was also performed every 14 samples. Possible
interferences with this method include high levels of nitrate and the
presence of cupric, ferrous, ferric, mercurous, silver, bismuth,
antimonous, lead, auric, chloroplatinate, and metavanadate ions. None
of these potential interferences are likely in this study.
Sulfate analysis was made according to HACH method number 8051,
SulfaVer 4 method (HACH, 1991). After the powder pillow was added to
the sample, it was inverted several times to mix and allowed to stand
undisturbed for five minutes. During the chemical reaction that takes
place, sulfate ions react with barium in the SulfaVer 4 Sulfate
Reagent, forming an insoluble barium sulfate. The resulting turbidity
in the cell is proportional to the sulfate concentration. With this
method, a set of prepared standards must be used to calibrate the HACH
51
DR/700. The standards used to create a standard curve were 0, 25, 50,
and 100 mg/L. Figure 5.4b is a sample calibration curve for sulfate.
Since this method only allows concentrations up to 100 mg/1, samples
over that level were diluted with milli-Q water until they fit in the
calibrated range. Possible interferences are silica above 500 mg/1,
calcium above 20,000 mg/L as CaCO3, chloride above 40,000 mg/L as Cl,
and magnesium above 10,000 mg/L as CaCO3
.
Analyses for nitrate and nitrite from samples obtained from WR-199
A were made by Tucson Water-Water Quality Laboratory according to the
guidelines of EPA method number 300.0. The analytical method employed
is ion chromatography, which measures the concentrations of anions
present in a water sample. The sample is injected into a stream of
carbonate-bicarbonate eluent and passed through ion exchangers. Anions
are separated on the basis of their relative affinities for a low
capacity, strongly basic anion exchanger. The separated anions then
pass through a strongly acidic cation exchanger (suppressor) where they
are converted to their highly conductive acid form and the eluent is
converted to weakly conductive carbonic acid. The separated anions in
their acid form are measured by conductivity. They are identified on
the basis of retention time as compared to standards. Quantification
is made by measurement of peak height of the conductivity signal.
Measurements of kjeldahl nitrogen in the source water and WR-199 A
samples were made at an outside laboratory, BC Laboratory in
Bakersfield, CA.
52
Chapter 6
Results and Discussion
6.1
Seasonal Trends
On October 29, 1992 a summer drying period ended and the second
full year of operation began at Sweetwater US&R Facility. During the
recharge season, October 29, 1991 to May 23, 1992, fourteen
wetting/drying cycles were completed. The total volume recharged to
Basin RB-001 B was 664.25 acre-feet. The average infiltration rate for
RB-001 B was 3.1 ft/day. Table 6.1 summarizes the basin cycles, volume
recharged, and infiltration rates for the 1991-92 recharge season.
The horizontal and vertical movement of percolating water through
Basin RB-001 B was studied to determine hydraulic characteristics of
the basin. During the first wetting cycle of the season, the response
of water table elevations in shallow monitor wells within the basins
were measured. Figure 6.1 shows the response of MW-7 and MW-8 when the
basin was flooded. The water level in MW-7 began to rise about 10
hours after flooding began, with MW-8 lagging behind in both response
time and levels. Even though the basin floor was not completely
submerged at this time, there was considerable horizontal flow,
probably due to the impeding clay layer. Infiltration rates appear to
have been retarded by the perched layer. The initial intake rate in
the mini-basin flooded with tertiary effluent, previously located in
the southeast corner of RB-001 B, was 20 feet per day (University of
Arizona and University of Colorado, 1991). This is compared with the
observed rate of 4.5 feet per day, averaged over the entire basin,
53
TABLE 6.1 Tabulated Basin Cycles and Infiltration Rates
Basin RB-001 B
Basin Area = 3.3 Acres
Start Date
06/17/91
10/29/91
11/02/91
11/07/91
11/12/91
11/27/91
11/29/91
12/06/91
12/11/91
12/23/91
12/26/91
01/11/92
01/17/92
01/29/92
02/05/92
02/19/92
02/24/92
03/05/92
03/11/92
03/20/92
03/24/92
04/03/92
04/08/92
04/20/92
04/24/92
05/03/92
05/06/92
05/13/92
05/16/92
End Date
10/29/91
11/02/91
11/07/91
11/12/91
11/27/91
11/29/91
12/06/91
12/11/91
12/23/91
12/26/91
01/11/92
01/17/92
01/29/92
02/05/92
02/19/92
02/24/92
03/05/92
03/11/92
03/20/92
03/24/92
04/03/92
04/08/92
04/20/92
04/24/92
05/03/92
05/06/92
05/13/92
05/16/92
05/23/92
Cycle
SUMMER DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
WET
DRY
*Volumes of water discharged during
dry cycles (ex. for flushing lines)
are assigned to preceding wet cycle.
Total Days
for Cycle
4.1
5.1
5.3
14.4
2.7
6.5
5.1
11.7
2.8
16.2
6
12
7
14
5
10
6
13
4
10
5
12
4
9
3.04
7
2.92
7
Infiltration
Rate
(Ac-Ft)
(Ft/Day)
Volume
Recharged
(Mgal)
20.01
61.43
4.54
21.66
66.50
3.80
12.5
38.38
4.31
17.13
52.59
3.12
13.85
42.52
4.60
17.31
53.14
2.68
20.69
63.52
2.75
15.63
47.98
2.91
18.04
55.38
2.80
13.13
40.31
3.05
13.32
40.89
2.48
13.08
40.16
3.04
9.99
30.67
3.06
10.02
30.76
3.19
54
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-
-
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lliiiimE111111,IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
OEn
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55
during the first flooding cycle.
In this section, the results of the SAT study at the Sweetwater
US&R Facility during 1991-92 recharge season are presented. The first
samples were collected during the first wet cycle in October, 1991 and
subsequent dry cycles from the suction lysimeters and well WR-199 A.
The lysimeter samples were analyzed for major cations, anions, and
alkalinity by the University of Arizona Soil, Water, and Plant Analyses
Laboratory. The results of the seasonal study are presented in
Appendix II.
Source water samples were collected by Tucson Water throughout the
recharge season at the APP compliance point 520E (Figure 1.1). The
520E samples were analyzed for NO3 -N, NO2 -N, and NH4+-N by Tucson
-
-
Water Water Quality Laboratory, and kjeldahl nitrogen by BC
Laboratories, Bakersfield, California. This data is presented in Table
6.2. The concentrations of the nitrogen species remained approximately
constant from October, 1991-May, 1992. In addition, two source water
samples were collected by the University of Arizona, directly from RB001 B near the basin inlet. The nitrate concentration in each source
water sample was 5.08 mg/L NO3 -N taken 10/30/91 and 4.49 mg/L NO3 -N
-
-
taken 1/16/92. These values are close to the 520E samples of 3.9 and
4.4 mg/L NO3 -N taken on 11/06/91 and 1/16/92 respectively. Point 520E
-
can therefore be considered a good estimate of the source water
entering RE-001 B.
On four occasions, (11/6/91, 1/6/92, 2/19/92, and 4/1/92) the
kjeldahl nitrogen sample for 520E failed to pass quality control (QC)
56
Table 6.2 Source Water Samples
ANALYSIS OF 520 8 SAMPLES (All units are mg/L as N)
DATE
N O 2-N
N O 3-N
NH3-N
TKN
TOTAL-N
06/12/91
3.2
2.5
9.5
13.0
18.7
11/06/91
2.4
3.9
8.6
FAILED QC
25.3
(19)
01/16/92
4.5
4.4
6.6
FAILED QC
27.9
(19)
02/19/92
2.5
2.1
<0.10
FAILED QC
23.6
19.6
19.7
03/04/92
2.1
1.5
11
(19)
16.0
04/01/92
2.2
1.5
16
FAILED QC
(16)
Where:
N O 2-N = Nitrite
N O 3-N = Nitrate
NH3-N = Ammonia
TKN = Total Kjeldahl Nitrogen (NH3-N +
TOTAL-N = N O 2-N + N O 3-N + TKN
Organic Nitrogen)
Where <0.1 means detected below that level, but not quantifiable
Where NO.1 means none detected above that level
57
requirements. Two basin source water samples were collected and both
had a concentration of 19.0 mg/L kjeldahl nitrogen. On the three
occasions 520B samples did not pass QC, the value of 19.0 mg/L kjeldahl
nitrogen was substituted in order to estimate the total nitrogen. The
4/1/92 kjeldahl nitrogen sample resulted in a value less than that of
ammonia. On this occasion, the ammonia concentration was used to
approximate the concentration of total nitrogen.
The lysimeter data consistently shows that the nitrate
concentration is initially high in the upper profile and then decreases
with time (Appendix II). The first pulse of the season may reflect the
leaching of nitrate generated during the summer drying period (Figure
6.2). Figure 6.2 shows that the seasonal NO3 -N concentrations, as
-
represented by the average from the six sampling periods from November,
1991 - March, 1992 (Appendix II), reached a peak at depths above 20
feet. Below 20 feet, the concentration decreased then flattens out to
an approximately constant value.
Wilson et al., (1992) attributed
this trend to the process described earlier, that is, sorption of
source-water ammonia on the soil exchange complex during flooding, as
upper subsurface environment becomes anaerobic; conversion of ammonia
to nitrate during subsequent drying cycle as air re-enters profile; and
leaching of the nitrate seen as a "pulse" during the next wetting
cycle.
The seasonal trend for vadose zone samples taken during dry cycles
show the nitrate concentration decreasing after each wet/dry cycle in
each lysimeters through the profile (Figure 6.2). In fact, based on
58
NITRATE-N CONCENTRATION (mg/I)
10
20
30
40
I
I
I
I
t
I
I III
1
IIIIIIIII
, 50
1
I
Illil OCTOBER 30, 1991
xxxxx MARCH 13, 1992
xxxxx AVERAGE
Figure 6.2 Nitrate—N Concentration through Profile
for October 30, 1991, March 13, 1992 and Average
for Six Data Sets (North Lysimeters)
59
nitrate data presented in Appendix II, there appears to have been an
average decrease in total nitrogen amounting to 46% in the vadose zone
region below the 17 foot lysimeter. (This assumes that nitrogen is
entirely in the nitrate form.) This implies that either SAT has
improved since the recharge cycle began or that river flows from the
Santa Cruz River influenced the water quality around the Sweetwater
US&R Facility. Infiltrating river water would dilute concentrations of
nitrate, nitrite, chloride and other species in the vadose zone and at
depth. Specific data on the times and volumes of streamflow in the
Santa Cruz River were unavailable at the time of thesis submission.
The groundwater samples were analyzed for NO3 -N, NO2 -N, and
-
-
NH4+-N by Tucson Water Water Quality Laboratory, and kjeldahl nitrogen
by BC Laboratories, Bakersfield, California. Table 6.3 summarizes the
concentrations of nitrogen species in WR-199 A. In groundwater samples
from WR-199 A, the highest nitrate values occurred in November, just
after the recharge season began (Table 6.3, Figure 6.3). This may
reflect leaching of nitrate remaining in the profile after the 1990-91
recharge season. The level decreased throughout the fall and winter,
and appears to be leveling off near the end of the season. Toward the
end of the season, there was a loss in total-N amounting to 36% during
recharge of the source water. The nitrate concentration was, however
above the 10 mg/L N O3 -N level stipulated in the APP. Throughout the
-
season, the nitrite, ammonia, and kjeldahl concentrations in WR-199 A
were low and constant.
Inasmuch as chloride is a conservative tracer, it is informative
60
Table 6.3 Groundwater Samples
ANALYSIS OF WR-199A SAMPLES (Ali units are mg/L as N)
DATE
NO2N
NO3N
NH3N
TKN
TOTAL-N
09/12/91
NO.1
15
0.038
0.93
15.93
10/22/91
NO.1
22
<0.1
0.25
22.25
11/07/91
NO.1
23
<0.1
0.33
23.33
01/10/92
NO.1
16
<0.1
0.12
16.12
02/06/92
NO.1
16
0.14
NO.2
16
03/03/92
NO.1
14
<0.1
NO.2
14
04/13/92
NO.1
13
<0.10
N1.0
13
04/27/92
NO.1
12
<0.1
NO.2
12
05/01/92
NO.1
13
<0.1
NO.2
13
05/12/92
NO.1
14
<0.1
PENDING
14
06/09/92
NO.1
12
<0.10
PENDING
12
Where:
NO2-N = Nitrite
NO3-N
Nitrate
NH3-N = Ammonia
TKN = Total Kjeldahl Nitrogen (NH3-N +
NO2-N + NO3-N + TKN
TOTAL-N
Organic Nitrogen)
Where <0.1 means detected below that level, but not quantifiable
Where NO.1 means none detected above that level
61
0
le
.—
n
tto
tzrt
1
•'Cf)
I
0
ct.1
1
I
1
i
o
VON 'N SV NOI1V2i1N33N00
62
to compare trends in concentrations of this ion in groundwater with
trends in nitrate levels. In WR-199 A, the chloride concentrations
decreased throughout the recharge season (Figure 6.4), paralleling the
behavior of nitrate concentrations. The reason for decreasing chloride
levels appears to be attributable to dilution. Two possible sources
for groundwater dilution are source water and river water, i.e.
recharging from flood events in the adjoining Santa Cruz River. The
chloride levels in river water are well below concentrations observed
in groundwater from well WR-199 A. Similarly, Figure 6.4 also shows
that chloride concentrations in source water were generally lower than
values observed in groundwater. Thus either or both of these sources
could be a factor in reduced nitrate levels in groundwater.
Lysimeter NL-80 also showed a slight decrease in chloride
concentration. Since the surface of the Santa Cruz River is at
approximately the same elevation as the surface of RE -001, infiltrating
river water would move horizontally above the clay layer. The chloride
concentration as measured at NL-17 fluctuates during this period,
suggesting that dilution from the Santa Cruz River may have affected
constituent concentrations.
Specific conductance (EC), a measure of total salinity, is another
parameter reflecting dilution of groundwater during recharge. The EC
of source water is about 800 micromhos per cm (Appendix II) EC values
in recharging river water are expected to be even lower. Figure 6.5
shows that changes in EC values in groundwater samples from well WR-199
A correspond to changes in water levels. Inasmuch as the packer
63
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CN
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IIIIII[11- 1 -1-1[111-1-T-iirmliiiiiiIrilim
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-
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100
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100
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j[I,IIIII
mtil,
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•-•-•-•-•
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+ +4 +-
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=1000
z
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--
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1 1 0 -_
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-
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i iii,
50
DAYS AFTER START
1400
iiiiiillimiiiiiiiiii
200
150
100
OF 1991-92 RECHARGE SEASON
Figure 6.5 Depth to Water and Specific
Conductance in Well WR-199 A
(1991-1992 RECHARGE SEASON)
65
assembly in well WR-199 A was designed to promote sampling of water
near the water table, the observed data reflect dilution from either or
both surface sources and not from native groundwater.
6.2
Wet/dry Cycle Trends
Table 6.4 summarizes the results of the nitrate, nitrite, and
sulfate analyses performed on lysimeter samples by the HACH method.
Figures 6.6 and 6.7 show the variations of nitrate concentrations in
the north and south lysimeters respectively. The wet cycle was 4 days
and the dry cycle 10 days during the period April 20-May 1. The total
volume delivered to RB-001 B was 40.16 acre-feet and the average
infiltration rate was 3.04 feet/day during this wetting cycle (Table
6.1).
Four sampling events are shown on each graph, representing the
background (i.e. the end of the previous dry cycle) (4-20), the start
(4-21) and end (4-24) of the wet cycle, and one week into the dry cycle
(5-1). These graphs show a nitrate peak at the beginning of the wet
cycle which moves through the profile during the cycle.
The water levels in MW-7 and MW-8 (Figure 6.8) indicate that there
was a perched layer during the background sample collection on April
20. This probably remained from the previous wetting cycle and the
sample collected reflects background because the lysimeter collected
the sample over the previous twenty-four hour period. The depth to
water in MW-7 was about 15 feet when sampling began and rose to within
2 feet by 24 hours. During flooding, lateral flow in the perched layer
66
Table 6.4
Depth
2.5
5
10
17
40
60
80
Depth
2.5
5
10
17
Nitrate, Nitrite & Sulfate Results from April 20-May 1 Sampling
(All Units mg/L)
North Lysimeters
South Lysimeters
4-20-92 (7:30 am)
Nitrate-N Nitrite-N Sulfate
Nitrate-N Nitrite-N Sulfate
5.8
.027
108
8
.251
150
13.5
.011
106
190
22.4
.011
4.6
.008
260
3.5
.007
180
4.3
.052
230
4.1
.006
180
5.7
.003
220
5.7
207
.005
5.3
.005
140
7.3
.007
230
5.2
.004
124
7.5
180
.012
4-20-92 (4:30 pm)
Nitrate-N Nitrite-N Sulfate
Nitrate-N Nitrite-N Sulfate
11.6
.017
230
6.5
.263
180
9.5
.005
200
22.2
.005
190
3.9
.004
170
4
.005
170
3
.054
190
.007
190
3.2
4-21-92 (7:30 am)
Depth
2.5
5
10
17
40
60
80
Nitrate-N Nitrite-N
13.2
.027
7.2
.009
5.7
.007
3
.058
4.8
.006
5.7
.006
5
.005
Sulfate
Depth
2.5
5
10
17
40
60
80
Nitrate-N Nitrite-N
6.9
.037
.017
9.1
14.7
.015
8.4
.064
3.2
.008
5.9
.005
4.6
.004
Sulfate
164
156
163
162
264
196
191
Depth
2.5
5
10
17
40
60
80
Nitrate-N Nitrite-N
.008
4.9
10.7
.022
3.3
.017
2.5
.08
4.3
.005
5.7
.006
.008
5.1
Sulfate
Nitrate-N
10.2
20.9
3.3
2.8
4.9
Nitrite-N
.298
.008
.008
.007
.007
6.7
4.9
.006
.008
Sulfate
4-24-92 (7:30 am)
Nitrate-N
22.6
6.8
9
10
5
6
5.7
Nitrite-N
.218
.016
.029
.011
.003
.008
.015
Sulfate
156
155
154
151
185
187
174
Nitrate-N
3
12.3
2.2
5.6
7.1
3.5
Nitrite-N
.045
.023
.007
.013
.007
.005
.004
Sulfate
Nitrite-N
.128
.016
.012
.013
.009
.007
.008
Sulfate
4-27-92 (7:30 am)
5.2
5-1-92 (7:30 am)
Depth
2.5
5
10
17
40
60
80
Nitrate-N Nitrite-N 3.8
.008
8.9
.02
3.9
.009
4
.07
.01
6.2
5.8
.006
6.4
.008
Sulfate
Nitrate-N
4.5
12.8
5.5
6.1
6.3
5.7
5.5
67
Nitrate as N (mg/L)
o
0
_
-
10
I
15
20
,,,,,,,iiiiilitijiiiiIIIII,IIIIimi,,,tittli
-^
-
10-:
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-
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-
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-
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40=
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o
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-
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am
Geeee 4-20 17:30
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11-111-0-•-• 5-1
7:30
-
60=
^
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70=
-
-
80 -
Figure 6.6
Nitrate Concentration Through Profile
North Lysimeters
25
68
Nitrate as N (rng/L)
10
111111111111
15
20
10 —
20
30 —
40
aeeao 4-20 17:30 ami
4-21 7:30 am
••••-• 4-24 7:30 am
••••••• 5-1 7:30 am
DISO8-0
0 50 —
25
111111111111111111111111111111111111
60 —
70 —
80
Figure 6.7 Nitrate Concentration Through Profile
South Lysimeters
69
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(pej) Ja4om oq. 1_4daa
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0
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70
caused water levels to rise in the upper vadose zone in the region of
the 2.5, 5, 10, and 17 foot lysimeters, before surface water flooded
the overlying soil surface.
On average, the nitrate concentration was higher in the south
lysimeters than in the north. This result has consistently been
observed during the entire season. The samplers are identical, except
for NL-2.5 which is a stainless steel unit. The observed difference
may be due to spatial variability in the geochemical composition in the
upper subsurface.
Figures 6.9 - 6.15 show the nitrate concentration at each depth
vs. time since flooding began. At the 2.5 and 5 foot depths, the
initial nitrate values were the highest in the profile, especially in
the 5 foot samplers. In this upper vadose zone, the high levels of
ammonia present in source water (Table 6.2) have undergone
nitrification to nitrate, and to a lesser extent nitrite. Nitrate
concentrations increased in NL-2.5 to a peak during the first 48 hours
of the wet cycle, then decreased for the next 48 hours. Nitrification
may have been slowing down at this point with ammonia being sorbed.
Nitrification continued to decline during the drying cycle.
In NL-5 and SL-5, nitrate values decreased during most of the
flooding cycle, suggesting additional ammonia sorption. During the dry
cycle, nitrate levels increased again, suggesting nitrification. This
may be the region where the nitrate that leaches in subsequent wetting
cycles is formed.
At the 2.5 and 5 foot depths, the observed behavior in the north
71
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78
lysimeter did not parallel the south lysimeter. As these lysimeters
are over 50 feet apart, spatial variability in soil properties may
explain the difference between the results. In addition, flow at the
2.5 foot depth is mainly vertical and changes in nitrate reflect
vertical flow. The water level in the perched water table was above
these lysimeters throughout most of the wet cycle.
By a depth of 10 feet, very little nitrite was found and the most
measurable nitrogen species present was in the form of nitrate. In NL10 and NL-17, the nitrate peak occurred at approximately the same time
that flooding ended, suggesting leaching from the upper regions. The
nitrate pulse was probably a result of the sorption of source-water
ammonia in the soil during the previous flooding cycle and subsequent
leaching of the nitrate wave after nitrification during the dry cycle.
Between 10 and 17 feet, the nitrate level had significantly decreased
in the north lysimeters suggesting denitrification was occurring in the
saturated, perched layer. Nitrate values were lower in the 17 foot
samplers. The nitrate trends reflect both lateral and vertical flow at
these depths.
The observed behavior in the south lysimeter paralleled the north
lysimeter, although the nitrate values in SL-17 were higher than those
in SL-10. The water level in the perched water table was above these
lysimeters during all of the wet cycle and part of the dry cycle. The
reactions at 10 and 17 feet can therefore be considered to have taken
place in a saturated zone.
At 40, 60 and 80 foot depths, there was almost no evidence of a
79
nitrate pulse moving through this region of the profile, although at an
infiltration rate of 3.1 feet/day, percolating water may not have
reached these depths. The nitrate concentrations were constant through
this wetting/drying cycle suggesting that denitrification is not taking
place. The levels of nitrate observed here, about 5 mg/L NO3 -N, were
-
however less than those observed in WR-199 A, which were over 10 mg/L
NO3 -N. Possible reasons include the following: denitrification,
-
dilution from infiltrating river water, spatial variability, or
differences resulting from analytical measurement techniques, i.e.
between the HACH method and the ion chromatograph method.
Comparison of this wet/dry data to the previous University of
Arizona SAT studies conducted at the Sweetwater US&R Facility show
that: (1) a nitrate peak moves though the profile to a depth of 17 feet
after flooding begins and below which it is either diluted or
denitrified, (2) the maximum concentrations of nitrate in the upper
vadose zone (2-10 feet) were much higher in the 1990-91 recharge season
(Figure 6.2) than those observed during the April 20 - May 1, 1992
wet/dry cycle (Figures 6.6 and 6.7), and (3) denitrification probably
takes place within the perched water table.
Nitrogen Species Reactions
6.3
6.3.1
Nitrogen Transformation Processes
Inasmuch as saturated and unsaturated zones create different
environments for biological and chemical reactions to occur, it is
important to differentiate between the zones. During flooding, the
80
reactions between land surface and 17 feet were considered to be taking
place in a saturated zone created by a clay layer occurring at
approximately 17 feet. The region below the clay layer may have
remained basically unsaturated throughout the flooding cycle. When the
soil surface is unsaturated, oxygen is available for the oxidation of
ammonia to nitrate. An anaerobic zone exists in the saturated zone in
which denitrification can take place.
6.3.2
Nitrogen Mass Balance
In an effort to determine the fate of nitrogen during the
artificial recharge of effluent, a mass balance was estimated at
various locations within the system. Using data from source water
(Point 520E and basin inlet), and well WR-199 A samples, the mass of
each nitrogen species was determined. The average total nitrogen
calculated from various times during the 1991-92 recharge season
(Tables 6.2 and 6.3) was 23.2 mg/L at point 520E and 14.8 mg/L in WR199 A. The results show an average loss of nitrogen to be 8.4 mg/L, or
36% from the ponded water to the water table. Since one object of this
study was to determine and measure losses, it is important to discuss
the processes which will cause the losses. The following are nitrogen
sinks which may have existed during recharge at Sweetwater US&R
Facility: volatilization, adsorption, fixation, metabolic uptake, and
denitrificat ion.
Volatilization
The volatilization of ammonia increases with increasing pH above
8.5 (Lance, 1972). Algal growth in a basin tends to increase the pH
81
levels in surface waters during the daylight hours by the uptake of
carbon dioxide during photosynthesis. Although algal growth appears
after flooding begins, Tucson Water (1992) found no correlation between
ponded water pH and algal growth.
Volatilization of ammonia requires
considerable air-water contact (Lance, 1972). This could only occur
during the flooding cycle, when effluent is ponded in the basin.
Several factors are most important in regulating this process. With
the usual pH of effluent, 7.5 to 8, less than 10 percent of the
nitrogen occurs in the gaseous ammonia form and most in the ammonium
ion form.
To test for the volatilization of ammonia during flooding, the pH
of the ponded water was measured on two occasions at night and once
during the morning. A daytime pH reading of surface water in RB-001 B
was 7.70, while nighttime readings were 7.05 and 7.75. Thus, the
effect of algal growth on pH of surface water in the basin was
insignificant during the late stages of the wetting cycle. Similarly,
the loss of ammonia through volatilization is assumed to be
insignificant.
Adsorption of Ammonia
Adsorption of NH 4+ into the clay fraction of the soil matrix is
possible since the clay fraction is negatively charged. The cation
exchange capacity measures the ability of a soil to sorb positively
charged particles. At RB 001 B, the maximum clay content of the soil
-
occurs between 15-16.5 feet (Cline, 1992). Below 17 feet, the soil is
mostly sands and gravels (Cline, 1992). If nitrification is not
82
complete by this depth, ammonium ions sorb to the clay. Eventually,
the surface of the clay will become saturated and the exchange capacity
will be greatly reduced. Thus, the total nitrogen which reaches the
groundwater will be lower than that in the source water. During the
dry cycle, nitrification will form nitrate or nitrite. When the next
flooding cycle begins, a nitrate pulse can be measured moving through
the profile. Over an entire season, adsorption of the ammonium ion
will not contribute to the reduction of the total nitrogen
concentration.
Fixation of Ammonia
Fixation of ammonia by the organic fraction of a soil, can result
in nitrogen compounds resistant to leaching and decomposition (Lance,
1972). The organic fraction of the soil beneath the basin is very low.
The SAT study, however, observed an increase in organic fraction at the
surface as the 1990-91 recharge season progressed (University of
Arizona and University of Colorado, 1992). This may be caused by an
accumulation of dead algae on the basin floor together with organic
matter from the source water.
Metabolic Uptake
NH4+ removal by metabolic uptake is another contributor to total
nitrogen reduction because it can be readily incorporated into cell
tissue by microbial protoplasm (Lance, 1972). According to Lance
(1972), the average carbon:nitrogen (C:N) ratio of microbial protoplasm
is about 10:1. The average Dissolved Organic Carbon (DOC) of tertiary
effluent is about 11 mg/L (University of Arizona and University of
83
Colorado, 1992). Thus, the uptake of nitrogen from tertiary effluent
applied to RB-001 B is estimated according to the relationship:
(C/N)
tertiary
= 11/N = (10/1)
protoplasm
or N = 1.1 mg/L.
In other words, microbial uptake would account for a total-N loss of
only about 1 mg/L at maximum. This assumes that the release of carbon
dioxide by microbial respiration is insignificant, Additionally, the
nitrogen incorporated into microbial tissue may be released or recycled
upon death and decomposition of the microorganisms (Lance, 1972).
Denitrification
Denitrification is the process which will most likely produce the
greatest loss in applied nitrogen. At the Sweetwater US&R Facility,
the three requirements, stated earlier, for achieving denitrification
are met. Oxidation of NH4 to NO3
-
is possible in the aerobic zone of
the upper unsaturated zone, between the basin floor and the saturated
zone created by the clay layer. The requirement of an anaerobic zone
is created by the perched water table and e
-
donors could be found in
the organic content of the soil or effluent or inorganic constituents
in the soil.
If the denitrifying bacteria use organic carbon as the energy
source, depending on pH, HCO3
-
production will result from this
reaction (Equation 3.3). Based on Equation 3.3, each mole of NO3
-
denitrified would produce 2.5 moles of HCO3. So, the 8.4 mg/L loss of
total-N would result in about a 21 mg/L increase in HCO3. Data in
Appendix II does not show a significant trend for HCO3
-
production over
84
the season, except for NL-2.5 which showed slightly higher levels.
Since this type of soil is naturally low in organic fraction,
denitrification through the oxidation of organic compounds may not be a
significant process contributing to total nitrogen loss.
A reaction more likely to occur under the conditions found at
Sweetwater US&R Facility, would be denitrification by bacteria which
use an inorganic carbon source as the electron donor. This process
would cause the production of SO 4 2- (Equation 3.4). The average
content of total iron is <0.211 mg/L (Tucson Water, 1992) and the iron
content in the soil has not been determined. Data in Appendix II and
in Table 6.4 show that the sulfate concentration level increases in the
40, 60, and 80 foot lysimeters, below the suspected region of
denitrification, i.e. between the 10 and 17 foot depths. The sulfate
values between the 10 and 17 foot lysimeters are among the lowest
observed in the profile. As previously suggested, the denitrification
occurring at the Sweetwater US&R Facility is probably due to a
combination of heterotrophic and autotrophic bacteria.
6.4
Discussion of Error
The water sampling network established at RB-001 B has the ability
to sample source water, percolating water, and groundwater. This
system therefore has the advantage of being able to closely monitor
transformation processes during SAT. There were however several
important problems, sampling, analytical, and logistical, encountered
during this experimental procedure. The biggest difficulty in trying
85
to determine a mass balance and to estimate losses resulted from the
various sources of data used. Point 520E and WR-199 A samples were
analyzed by Tucson Water Water Quality Laboratory using the method of
IC; lysimeter samples were analyzed by the University of Arizona by IC
and the HACH method. The handling of the samples was also different as
the University of Arizona samples were frozen for a period of time
before analysis and the Tucson Water samples were not.
The process of sampling the vadose zone itself presents some
difficulties. As previously mentioned, ceramic-cup lysimeters will
absorb positively charged ions, such as NH 4 + into the matrix. In an
ideal monitoring system, a lysimeter would be able to take a
representative sample of the vadose zone without interfering in the
process.
The determination of species concentrations in the vadose zone
requires that lysimeter sample volumes be truly representative of the
surrounding region being sampled (Starr et al., 1991). It is unknown
if the sample volume taken, 250 mL per lysimeter, is a representative
elemental volume (REV). As pointed out by Angle et al. (1991), the
sample composition may not represent the depth from which it is taken,
but is a composite of the soil solution contributing to the sample. In
turn, the soil solution composition varies depending on the amount of
vacuum applied, sample volume, sample time, and moisture content.
In addition, lysimeter samples assume uniform distribution at a
certain depth with no preferential flow occurring. In RB-001 B, the
samplers are located over a distance of about 90 feet and therefore the
86
samples do not represent a truly vertical profile. There is also the
potential for spatial variation in physical properties. Additionally,
the two transects are separated by 15 feet, so two samples taken at the
same depth may not show similar results. There may also have been
preferential flow from leakage down the boreholes constructed for the
samplers.
87
Chapter 7
Conclusions
Over the past three years, Tucson Water has operated the Sweetwater
US&R Facility and continues to meet its goal of providing water suitable
for turf irrigation. The Facility has also presented a unique setting
for observing the nitrogen transformation process and for evaluating SAT
during artificial recharge of treated effluent. The specific purposes
of this study were to study nitrogen transformation in tertiary effluent
after application to basin RB-001 B; to monitor subsurface changes in
nitrate concentration in the vadose water and groundwater during the
1991-92 recharge season of October 29, 1991 - May 23, 1992; to determine
a subsurface nitrogen mass balance; to observe subsurface changes during
one wet/dry cycle in April, 1992; and to study how the physical features
of RB-001 B affect basin operation and SAT.
The water samples used in this study were collected from tertiary
treated source water, the vadose zone, and groundwater. Source water
samples were collected from Tucson Water's RWTP at the APP compliance
point 520B. These samples were analyzed for the complete nitrogen
series. Vadose zone samples were collected using suction lysimeters
distributed in the profile from 2.5 feet to 80 feet below the basin
floor. Inasmuch as ammonia sorbs on the ceramic samplers used during
the study, lysimeter samples were analyzed only for nitrate and nitrite.
Groundwater samples were collected from a monitor well installed in the
basin and were analyzed for the complete nitrogen series.
88
The results of the seasonal study show that nitrification losses
take place during SAT. The nitrogen mass balance from source water to
groundwater showed an overall loss in total nitrogen of 36%. Throughout
the season, a decrease in nitrate was observed in the lysimeters and in
well WR-199 A. Of all possible processes contributing to the loss,
denitrification and dilution are most likely contributors.
Seasonal trends in other parameters were also observed, including
chloride, depth to water in the basin, specific conductance, and
infiltration rate. Chloride concentration, EC, and depth to water, as
measured in well WR-199 A, all decreased through the season. Because
the trend is constant, the decreases were probably due to dilution from
source water, and to a lesser extent from river flows occurring during
the season. It is possible that lateral flow of river water above the
impeding layer may have caused dilution in the perching layer also.
Since concentrations of chloride, nitrate, and EC are low in river
water, flow events would contribute to total reduction in nitrogen
concentration due to dilution. Source water concentrations of chloride
and EC were low, also showing dilution.
For the April, 1992 wet/dry cycle study, a nitrate peak was
observed to move through the profile until it reached a region of
perched groundwater. At this point, denitrification took place, and the
peak was not observed below 17 feet. In the lower profile, nitrate
levels remained approximately constant during the wet and dry cycle.
Between 10 and 17 feet, sulfate levels were low, but increased at the
40, 60, and 80 foot depths.
89
The infiltration rates observed during the April, 1992 wet cycle
were lower than those observed during the tests conducted in the minibasins in 1990. The nitrate concentrations, however were much lower in
this study compared with the 1990 University of Arizona SAT study
(University of Arizona and University of Colorado, 1992). This shows
that the clay impeding layer, although it may slow infiltration, aids in
the nitrogen removal process. Both studies show the nitrate pulse that
occurs in the upper profile just after flooding begins. This pulse is a
result of sorbed ammonia remaining in the soil from the preceding dry
cycle and confirms the work by Bouwer, Lance, and others.
If the observed trend of decreasing nitrate concentration in
groundwater continues, Tucson Water will be able to meet the
requirements of the APP, and the Sweetwater US&R Facility will continue
to provide an alternative source of water for irrigational purposes for
many years to come. It has been shown that SAT is a feasible treatment
process for reducing total nitrogen during recharge of tertiary
effluent, not only for Tucson, but for other communities with similar
soil conditions. This implies that recovered water can be used for
unrestricted irrigation, whereas additional treatment might be required
if recovered water is used for drinking water.
90
Chapter 8
Recommendations
Although this study presented a comprehensive evaluation of
nitrogen transformation during one wet/dry cycle at Sweetwater US&R
Facility, further study at the site is recommended. To improve the
ability to trace movement of all nitrogen species, including NH4+,
piezometers or stainless steel lysimeters could be installed from the
surface to depths of up to 20 feet, i.e. in the perching region.
Provided there is no loss of ammonia by sorption or by volatilization,
this may yield more insight into the reactions taking place above and
within the clay layer.
In addition, WR-199 A should continue to be monitored to determine
if nitrogen trends continue from season to season. Investigation into
reasons behind the seasonal decline of all nitrogen species in
groundwater samples from WR-199 A may prove useful for the long-term
operational goals and permitting requirements for the Sweetwater US&R
Facility.
91
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Subsurface Environment During Wastewater Effluent Recharge",
Report Submitted to: The City of Tucson, February, 1992.
Angle, J. S., M. S. McIntosh, and R. L Hill, "Tension Lysimeters
for Collecting Soil Percolate," In: Groundwater Residue Sampling
Design, Edited by R. G. Nash and A. R. Leslie, American Chemical
Society, pp. 290-299, 1991.
Bouwer, H., J. C. Lance, and M. S. Riggs, "High-rate Land
Treatment II: Water Quality and Economic Aspects of the Flushing
Meadows Project", Journal Water Pollution Control Federation, Vol.
46, No. 5, May, 1974.
Bouwer, H., R. C. Rice, J. C. Lance, and R. G. Gilbert, "Rapidinfiltration Research at Flushing Meadows Project, Arizona",
Journal Water Pollution Control Federation, Vol. 52, No. 10,
October, 1980.
Bouwer, H., and R. C. Rice, "Renovation of Wastewater at the 23rd
Avenue Rapid Infiltration Project", Journal Water Pollution
Control Federation, Vol. 56, No. 1, January, 1984.
Bouwer, H., "Ground Water Recharge of Effluent", In: Artificial
Recharge of Ground Water, Edited by A. I. Johnson and D. J.
Finlayson, ASCE, New York, pp. 249-282, 1989.
Bouwer, H., "Ground Water Recharge with Sewage Effluent", In:
Water Pollution Research and Control, Kyoto, Japan, 1990, Part 4,
Edited by P. Grau, et al., ASCE, Pergamon Press, Oxford, p. 20092108, 1991.
CH2M Hill, Errol L. Montgomery & Associates, and Dr. L. Gray
Wilson, Tucson Recharge Feasibility Assesment, Phase A: Task 5,
Hydrogelogic Evaluations for Recharge Sites, March, 1988.
Cline, D. J., "Tracer Experiments Using Bromide Ion and Two
Bacteriophages During Soil Aquifer Treatment Studies", M. S.
Thesis, University of Arizona, Tucson, AZ, 1992.
Crites, R. H., "Micropollutant Removal in Rapid Infiltration", In:
Artificial Recharge of Groundwater, Edited by T. Asano,
Butterworth Publishers, Boston, p. 579-608.
Davidson, E. S., "Geohydrology and Water Resources of the Tucson
Basin, Arizona", United States Geological Survey: Water-Supply
Paper 1939-E, 1973.
Degen, M. B., R. B., Reneau, Jr., C. Hagedorn, and D. C. Martens,
Denitrification in Onsite Wastewater Treatment and Disposal
Systems, Virginia Water Resources Research Center Bulletin 171,
113 p., November, 1991.
92
Delwiche, C. C., "The Nitrogen Cycle", Scientific American, Vol.
223 No. 3, September, 1970.
Dorrance, D. W., L. G. Wilson, L. G. Everett, and S. J. Cullen,
"Compendium of In Situ Pore-Liquid Samplers for Vadose Zone", In:
Groundwater Residue Sampling Design, Edited by R. G. Nash and A.
R. Leslie, American Chemical Society, pp. 300-331, 1991.
Freney, J. R., J. R. Simpson, and O. T. Denmead, "Volatilization
of Ammonia,", In: Gaseous Loss of Nitrogen from Plant-Soil
Systems, Edited by J. R. Freney and J. R. Simpson, pp. 1-32,
Martinus Nijhoff/ Dr. W. Junk Publishers, 1983.
EACH Corpoation, Instructions for Use of DR/700 Colorimeter, 1991.
Ho, G. E., R. A. Gibbs, K. Mathew, and W. F. Parker, "Groundwater
Recharge of Sewage Effluent Through Amended Sand", Water Resources
Research, Vol. 26, No. 3, March, 1992.
Kinzelbach, W. K. H., P. J. Dillon, and K. H. Jensen, "State of
the Art of Existing Numerical Groundwater Quality Models of the
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Sources, Part I, Edited by D. D. DeCoursey, Rep., ARS-81, pp.
307-325, Agricultural Resource Services, U. S. Dept. of
Agriculture, Beltsville, Md., 1990.
Kolle, W., O. Strebel, and J. Bottcher, "Formation of Sulfate by
Microbial Denitrification in a Reducing Aquifer", Water Supply,
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Kolle, W., P. Werner, O. Strebel, and J. Bottcher,
"Denitrification by Pyrite in a Reducing Aquifer" (In German), Vom
Wasser, Vol. 61, No. 1, January, 1983.
Korom, S. F., "Natural Denitrification in the Saturated Zone: A
Review", Water Resources Research, Vol. 28, No. 6, June, 1992.
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Pollution Control Federation, Vol. 44, No. 7, July, 1972.
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Soil", Journal of the Irrigation and Drainage Division, September,
1975.
Lance, J. C., and F. D. Whisler, "Stimulation of Denitrification
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1976.
Lance, J. C., R. C. Rice, and F. D. Whisler, "Effects of
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93
Lance, J. C., R. C. Rice, and R. G. Gilbert, "Renovation of
Wastewater by Soil Columns Flooded with Primary Effluent", Journal
Water Pollution Control Federation, Vol. 52, No. 7, July, 1980.
Laney, R. I., "Chemical Quality of the Water in the Tucson Basin,
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Robertson, W. D., and J. A. Cherry, "Subsurface Behaviour of
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Aquifer", Submitted to: International Journal of Applied
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Starr, J. L., J. J. Meisinger, and T. B. Parkin, "Experiences and
Knowledge Gained from Vadose Zone Sampling", In: Groundwater
Residue Sampling Design, Edited by R. G. Nash and A. R. Leslie,
American Chemical Society, pp. 279-289, 1991.
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Basin and Avra Valley, Pima County, Arizona, 1990", June, 1992.
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Submitted to: The City of Tucson, March, 1992.
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Johnson and D. J. Finlayson, ASCE, New York, pp. 170-185, 1989.
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City of Tucson, April, 1992.
94
APPENDIX I
95
Lysimeter Installation Procedure (after Cline, 1992)
1.
Auger one foot below target depth for gravel strata and six inches for sand
a. measure depth below land surface to bottom of hole (BOH) and to auger bit
b. record depths to BOH and auger bit
2. Pull auger flights up to target depth
a. record (measure) depth to auger bit
measure to SOH (check for caving)
c. record depth to BOH and distance between
b.
BON and auger
3. Tremie 1/8 inch screened material into bottom
recorded auger bit depth
a. measure to BON
bit
of hole as a slurry up to the
b. continue until slurry is at target depth
4. Pull auger flights up 6 inches
a. record (measure) depth to auger bit
b. measure to BON (check for caving)
c. record depth to
BON
and distance between
5. Lower sampler into bottom of hole
a. measure and record depth to top
b. measure and record depth to BOH
c. record depth to porous cup
6.
BOH and auger
bit
of sampler reservior
Test sampler unit and connections for leaks
a. maintain a vacuum on input line while output line is clamped off
b. release clamp from output line and apply pressure to input line
7. Tremie 1/8 inch native screened material into bottom
slurry up to the recorded auger bit depth
a. measure and record depth to top of slurry (BOH)
b. determine if slurry is over top of porous cup
of hole as a
8. Pull auger flights up 6 inches aboyer recorded top of sampler
a. record (measure) depth to auger bit
b. measure and record depth to top of slurry (BOH)
c. pour 1/4 inch screened native material (dry) through top of
auger stem up to recorded auger bit depth
d. measure and record depth to BOH
9. Pull auger flight up 6 inches
a. record (measure) depth to auger bit
b. meassure and record depth to BOH and (check for caving)
and record distance between BOH and auger bit
10. Tremie 1 gallon bentonite chips (dry) into bottom of hole to form 6 inch plug
a. measure and record depth to top of bentonite plug
b. pour Milli-Q water through top of auger stem until top of bentonite surface just glistens
96
Lysimeter Installation Procedure con't
11. Pull auger flights up two feet
a. record (measure) depth to auger bit
b. meassure and record depth to BON and record distance between BON and auger bit
12. Pour 1/4 inch screened material (dry) to auger bit depth and tamp
13. Repeat step 12 up to land surface
97
APPENDIX II
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