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INTRODUCTION:
In almost any town, a large variety of chemicals and
wastes are used or disposed of in day-to-day life.
We are now learning that if things like gasoline, road
salt, pesticides or sewage are not used or discarded
wisely, they can contaminate a town's water supply.
We are also learning that some sources of water are
easier to contaminate than other sources. Whether or
not your town's supply is vulnerable to contamination
depends on many different factors. These factors may
add together to protect the supply, or to leave it very
vulnerable to contamination.
To estimate the vulnerability of the ground water
flowing under an area of land, a hydrogeologist
measures several factors which affect how quickly
rain water moves through the ground in that area.
Pollutants will usually move in the same way as rain
water.
Once you know something about each of these
factors, you will be able to decide what must be
done to be sure your drinking water will always be
safe.
O5JLCTIVD
In this activity, you will use a simple mathematical
model of ground water vulnerability to estimate the
vulnerability of a small town's water supply.
Table 1
Estimated Value of Five Factors Affecting
Groundwater Vulnerability
FACTOR
1. Yearly Rainfall
(Total amount of rain
that falls in one year)
2. Depth to Water
(Vertical depth from
surface to aquifer)
3. Aquifer Type
(Type of soil/rock
aquifer passes
through)
4. Soil Type
(Main type of soil and
rock above aquifer)
5. Lay of the Land
(The general slope of
surface of the land)
VALUE
3
2
1
3
2
1
3
2
1
4
3
2
1
3
2
1
..if more than 40 in.
..if from 15 to 40 in.
...if less than 15 in.
...if less than 10ft.
...if from 10 to 75 ft.
if greater than 75 ft.
...if sand or gravel
...if limestone
...if bedrock
...if sand or gravel
...if limestone
...if loam or silt
...if clay or shale
...if flat
.if gently rolling hills
...if steep
hills/mountains
Bedrock
Sand/Gravel
Shale
Clay
Limestone Figure 1. Aquifers fonn where water-carrying layers of earth, like
sand, cover layers of shale or clay which do not allow water to pass
Loam through. If the layer form ing the aquifer is "confined" above by shale
or clay, the aquifer is less vulnerable to contamination.
Confined Aquifer
Ur>con(ined aquifers
Confining layers ol shale
' r'i'iT'1
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MODLL OF GROUND WATLK
VULNLRA5ILITY:
There are many factors affecting the vulnerability of a water
supply, but we will only look at the five factors described in
Table 1. A value of 1 means it is harder for rain water (and
pollutants) to reach the supply, while a value of 3 means it
is easier. It may be easy to see that the greater the depth to
water, the longer it will take rain water to reach the supply.
But how does a steep slope make the area less
vulnerable? Figure 1 shows how some of these factors
affect the vulnerability of various aquifers.
DIRECTIONS:
Use Table 1 to find out how many points should be given
for each of the five factors.
For example, Table 1 tells you that if the depth to water is
less than 15 ft, you should give 3 points for this factor in
Quadrant 1. Values from Table 1 may be averaged.
Fill in the rest of the blanks for each factor, then add them
up to find the vulnerability of each quadrant.
Quadrant 1
Depth to Water
Yearly Rainfall
Aquifer Type
Soil Type
Lay of the Land
Table 2
12ft
45"
Sand/Gravel
Loam/Sand
Flat
VULNERABILITY SCORE
Quadrant 2
Depth to Water 40ft
Yearly Rainfall
Aquifer Type
Soil Type
45"
Limestone
Limestone/Loam
Lay of the Land Gentle Slope
VULNERABILITY SCORE
Quadrant 3
Depth to Water
Yearly Rainfall
Aquifer Type
Soil Type
60ft
38"
Limestone
Limestone/Clay
Lay of the Land Rolling Hills
VULNERABILITY SCORE
Quadrant 4
Depth to Water 100ft
Yearly Rainfall
Aquifer Type
Soil Type
Lay of the Land
34"
Sand/Gravel
Shale/Clay
Steep Hills
VULNERABILITY SCORE
MOW TOUSLTHrLMODLb
You can get a rough idea of the vulnerability of the
underlying aquifer in each of Priceford's four
quadrants. By using these five factors to give each
quadrant a "score" on how easy it would be for a
pollutant to pass through the ground to contaminate the
aquifer.
Follow the instructions for filling out Table 2, then use
your results along with the map of the Priceford area to
answer the questions at the bottom of the page. Give
your reasons for each answer!
QUESTIONS:
1. Discuss how factors 2-5 described in Table 1 affect
the vulnerability of water supplies at Points B, C and D
in Figure 1. If three towns get their water supplies at
Points B, C and D, which supply would be the most
vulnerable? The least vulnerable?
2. Use Table 3 below to interpret the vulnerability
scores you calculated in Table 2. Which town's water
supply would be most likely to be contaminated if a
larger tanker truck full of a toxic chemical spilled its
contents during a traffic accident on the nearest road?
3. Compare the vulnerability values you calculated in
the four towns in the above map to Points A, B, C and
D in Figure 1. Which of these towns is most likely to be
located at which of these Points?
4. How would one town's pollutants affect the other
town's supplies? If a wood preserving chemical is
found in Smalltown's water, but not in Riverville's,
where is the most likely area where the source of
contamination might be found?
5
very low
3.
Vulnerability Score
7.5 10 12,5 15
1 | L__J
•••" r t • •
moderate
Relative Vulnerability
i
very high
// Office of Water (4606M) • EPA 816F08024 • 06 / 2008 • www.epa.gov/safewater
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