A Resource for
Teachers
Soils and Us
VEGETABLE
Finding out about soil and
water, and erosion
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Activity 15 - Part I
Going underground - water moving through soil
Background
What happens when the water cycle goes underground? Students learn about
rainfall, streams, rivers, the sea and evaporation, but are rarely taught about
the ground that absorbs, holds and moves billions of litres of water each year.
When rain falls on the soil, some of it runs off the ground surface and some
infiltrates the soil and filters downward. The water that infiltrates the soil travels
through the small spaces between soil particles and through cracks in the rocks.
Water that continues to filter through the soil may eventually reach the top of a
saturated layer, known as the water table. The layer of rock material storing this
water is known as an aquifer. To demonstrate soil interaction with the water
cycle, have children build their own model of an aquifer.
Equipment –
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one kilogram of pea-size gravel
400 grams of topsoil or potting mix
400 grams of sand
35 grams of granulated sugar
one litre tap water
blue food colouring
a clear plastic tray (24 x 18 x 8 cm)
a watering can
a spray pump that has been cleaned (avoid those pumps that have been
used with cleaners or other chemical products – a clean soap dispenser
pump would be okay)
a paper coffee filter
a rubber band
a metric ruler.
Methodology
1. Pour a three centimetre thick layer of gravel onto the bottom of tray to
represent an aquifer.
2. Slope the gravel so that there is an empty space at one end of the tray 3. this will eventually represent a lake.
4. Place a loose layer of soil over the gravel - about one centimetre thick.
5. Add a few drops of food colouring to the water in the watering can.
6. Gently sprinkle the coloured water onto the soil. This represents rainfall.
7. The water infiltrates the soil and moves laterally (sideways across the tray)
to fill the lake area.
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8. Continue until the water table is three to four centimetres deep.
9. To demonstrate the impact of human activity on an aquifer, place a small
section of coffee filter over the open end of the spray pump hose and
secure with a rubber band.
10. Push the spray pump hose through the soil and into the gravel aquifer model.
11. Work the pump, which will draw water from the aquifer to the surface.
12. Continual pumping lowers the water table.
Key Questions
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What effect does continual pumping have on water wells or water bores?
How can we conserve water?
What are the effects of drought or low rainfall on water supplies?
Activity 15 - Part II
Going underground contamination of groundwater
Background
Many families in New Zealand, especially in country areas, rely on groundwater
for drinking water. We usually can’t see or smell polluted groundwater and once
pollutants do find their way into underground water supplies, they can be difficult
and expensive to remove.
The soil usually absorbs and removes the substance we add to it, but if the soil
layer is thin and the soil texture highly permeable, or the water table too close
to the land surface, we may exceed the soil’s capacity to remove pollutants.
You can demonstrate two types of groundwater contamination with the aquifer
model used in Part I. Point sources are directly identifiable sources of
contamination, such as leaking chemical storage tanks, septic systems, landfills
and spills. Nonpoint source pollution does not enter the groundwater at any
particular spot. Examples are pesticides, fertilisers or acid rain.
Equipment
In addition to the aquifer model and spray pump, you will need coloured sugar in
order to demonstrate nonpoint pollution as follows.
Methodology - Nonpoint source contamination
1. Thoroughly mix 15 drops of food colouring per 40 grams of granulated sugar.
2. Spread the sugar over the surface of a plate and allow to dry for an
hour or so.
3. Prepare the gravel in the aquifer model as before, but this time top with a
layer of sand instead of soil.
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Soils and Us: A Teachers’ Resource
4. Spread 20 grams of coloured sugar over the sand, then sprinkle warm tap
water on the sugar (which will dissolve it).
5. As the water moves through the sand, it will dissolve the sugar and the
colour will mix with the water.
6. Your class will see the ‘polluted water’ move into the aquifer and lake.
7. Draw water out of the aquifer by pushing the spray pump hose into the gravel.
8. Begin spraying the water onto white tissue. The water will be contaminated.
Methodology - Point source contamination
1. Prepare the model with gravel and sand as before, but this time dig a small
hole in the sand.
2. Fill the hole with 10 grams of coloured sugar and cover with sand.
3. Sprinkle with warm tap water.
4. Again, infiltrating water will dissolve the coloured sugar and carry it
downward into the aquifer and lake.
5. Draw water out of the aquifer by pushing the spray pump hose into the gravel.
6. Begin spraying the water onto white tissue. The water will be contaminated.
Key Questions
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Once the ‘pollutants’ have entered into the aquifer, ask your class how they
might clean the contaminated water. It is usually ‘cleaned’ or treated by
pumping it to the surface and treating it using activated charcoal or resins.
However, this is a lengthy and costly process.
Prevention of groundwater pollution is the best and most desirable option.
Reinforce the concept of the water cycle and point out that in reality less than
one percent of groundwater on earth is polluted. Is this a significant amount?
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Activities 16 and 17 - Measuring soil erosion
and how soil erosion occurs
Background
Soil erosion is a serious problem in New Zealand. ‘Accelerated’ erosion is
especially bad as it is caused by bad land management practices, such as
clear felling trees on steep hillsides. Soil erosion also occurs naturally,
especially on steep hillsides during heavy rainfall events such as Cyclone Bola.
During heavy rainfall, soils become waterlogged and lose their strength. On
steep hills they have the potential to slip and slump into the valleys below. The
sediments are then washed into streams, creeks and rivers where they cause
pollution and endanger life within the rivers.
Erosion also results in the loss of topsoil off the land, leading to reduced
pasture production and income for farmers. The loss of soil and rock off the
land is called ‘denudation’. In New Zealand ‘denudation’ is occurring at much
faster rates than soil formation. The steep and mountainous lands are therefore
being gradually ‘lowered’ by natural erosion and from time to time by much
faster erosion due to poor land management practices. The following activities
attempt to simulate rainfall and erosion on hill slopes.
Equipment
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three large plastic deepish-sided trays
plastic ice-cream containers filled with loose topsoil, sand, clay, pasture turf
a tray of seeded plants
a watering can with a fine spray
hair dryer
various wooden blocks or similar to support the trays at various slope
angles.
Methodology
1. This activity is best carried out on a fine day outside, unless you have an
appropriate laboratory or wet area!
2. Transfer the five materials onto the trays.
3. Compare the reaction of different materials to simulated raindrops by
applying water through the watering can.
4. Note the movement of soil, sand and clay particles with the trays flat and
then at increasing tray angles.
● Clay has cohesive strength and sand has frictional strength.
5. Note the effect of vegetation (grass and plants) on slope stability.
6. Estimate the percentage of sediment accumulating at the bottom of the slope
as the tray angles are increased.
7. Simulate high winds blowing over bare soil by using a hair dryer.
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Soils and Us: A Teachers’ Resource
Key Questions
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What is the effect of vegetation on slope stability?
Is a sandy soil stronger than a clayey soil on a hillslope?
At what slope angles do soil, sand and clay begin to slide down the hill?
Is wind a major erosional force?
Why is the turf so strong and intact even on a steep slope under high
rainfall?
How could you measure the amount of material lost from a hillside?
What effect do you think would terraces or drainage systems have on
slope stability?
How can we conserve our hill slope soils?
Wrap up
Group discussions and presentation to the class on hill slope processes,
especially the role of vegetation on stability and the potential of soils on hills to
erode. What are the stabilising factors (resisting forces) and what are the
driving forces of erosion?
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