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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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