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Groundwater Resources: How does soil type affect the movement of ground water?
Groundwater is water that moves through soil and rocks below Earth's surface. When
groundwater moves through soil, the structure of the soil affects its movement. The three particles that
determine soil structure are sand, silt, and clay. Sand particles can be seen with the naked eye and
range in width from 0.05 to 2.00 mm. A microscope must be used to see silt particles, which are
between 0.002 and 0.05 mm in width. An electron microscope is required to clay particles, which are
less than 0.002 mm wide.
Soil type depends on how much of each kind of particle makes up the soil. There are 13 types
of soil. Groundwater moves through the spaces between soil particles. These spaces are called pores.
Not all soils have the same amount of pore space. Porosity depends on the number and size of
pores soil. You can find the volume of pore space in a soil by determining the volume of water its
pores are capable of holding. The volume of pore space divided by the volume of a soil sample
determines its porosity. This measurement is given as a percentage of the volume of the soil.
The pores in soil are all connected, allowing water to pass through them. This means that soil is
permeable. The ease with which water moves through the soil is called permeability. A soil with
high porosity and many small pores can hold a lot of water, but it takes longer for the water to enter all
the pores. This means that when a soil has higher porosity it has lower permeability.
A soil's porosity and permeability are important to many people. Farmers must be aware of
these soil conditions so they can determine the best way to grow crops. Builders also consider these
conditions in the area they plan to build. Environmentalists are concerned with porosity and
permeability when there is a risk that a pollutant might be introduced into the groundwater supply.
In this Virtual Lab, you will investigate how different types of soil hold water. You will collect data use it
to make observations about the relationship of soil porosity to soil permeability.
Objectives:
· Determine the porosity of various types of soil.
· Define the relationship between porosity and permeability.
· Describe how a soil's porosity and permeability affect the movement of groundwater.
Procedure:
1. To select a type of soil, change the percentages of sand, silt, and clay by sliding the bars on
the Soil Meter.
2. Click the Test This Soil button to move 100 mL of the soil mixture to the funnel.
3. Click the Pour Water button to begin pouring water on the soil. The water will automatically when the
first drop of water begins to form at the bottom of the funnel.
4. Observe the readout that displays how much water was poured. This is the volume of water soil held.
5. To find the porosity of the soil, divide the volume of water the soil held by the total volume of soil.
Multiply the decimal by 100 and type this percentage in the Table.
In the formula below, V w stands for the volume of water and V s stands for the volume of soil.
Vw /Vs x 100 = % Porosity
6. Click Reset and repeat steps 1 through 5 for each of the 13 soil types.
7. Complete the Journal questions.
Conserving Resources: When is water safe to drink?
Data Table
Complete the Journal questions.
a. What determines the texture of soil?
b. What is meant by the permeability of a soil? What is porosity and how is it related to
permeability?
c. Based on your experiment, which type of soil is the most permeable? Which is the least
permeable? How do you know?
d. Explain why surface runoff, or rain not absorbed by the soil, occurs much more often in areas
with soils with a high clay content.
e. Soil type and texture are very important to farmers. Too little water in the soil may cause plants
to wilt and die. Too much water in the soil can disrupt a plant's ability to take in oxygen. Based
on your data, which type of soil do you think is best for agriculture? Explain your answer.
f.
Farmers often have problems with soil erosion when they clear their fields after the growing
season. Soil type is one factor that influences erosion. Some soil types are more prone to
erosion than others. Based on the data you gathered in this experiment, which soil types do you
think are the most easily eroded, soils with a high sand content or soils with high clay content?
Explain your answer.