GE2000 TAKE HOME EXERCISE
GROUNDWATER POLLUTION
The direction and velocity of groundwater movement are important issues with regards to pollution. The
direction of groundwater flow (flow lines) is perpendicular to contoured elevations of the water table or
potentiometric surface. Flow lines cannot cross each other but can converge or diverge.
The Smith family is considering the purchase of a small farm in Catfish Creek area. A hydrogeological study
of the area is available from the state geological survey. It is an area of recharge. Well A is to a depth of 80
feet and taps a sand aquifer. Well A contains a clay-rich sand from the surface down to a depth of 40 feet
below the surface, silt-rich sand with a few clean sand lenses (the sand lenses are local aquifers) occur from
40 to 90 feet depth below the surface and a clay layer occurs from 90 to 95 feet below the surface.
Groundwater movement in the upper 40 feet is approximately vertical. In the silt-rich sands, water
movement is both vertical and horizontal. The better local aquifers tend to be located just above the lower
clay-rich layer. Assume average vertical and horizontal velocity is 0.01 ft/day.
In this area there are three significant possible sources of contamination. The largest possible source for
pollution of the groundwater is a recently closed landfill, which accepted "standard" household and business
garbage. The base of the landfill was 20 feet below the original surface topography. It was capped with clay
to limit rainwater seepage downward through the garbage which limits groundwater and surface water
runoff contamination. The second possible source is a cluster of gasoline storage tanks. A berm around the
tanks limits escape to surface water but this also means that any gasoline leaking from the tank is likely to
move downward. The third possible source is a septic/leach field system near each house.
Use the attached map to consider the following questions in order from 1 to 6 and ultimately recommend
the more favorable farm for the Smith's to purchase. The contour lines are depth to the water table. Draw
groundwater flow lines to help answer questions.
1.
Is the landfill a source of concern for farm A?
farm B?
2.
Is the gasoline storage area a source of concern for farm A?
3.
Assume that organic contamination from gasoline moves at the same velocity as the groundwater.
Estimate the time it would take contamination to be detected by a monitoring well, 80 ft. deep at
well A.
4.
Is the septic/leach field system a source of concern for farm A?
5.
Assume that organic contamination from the septic/leach field system is naturally decomposed
in 20 years. Will contamination reach the 80 ft deep aquifer?
6.
Which farm would you recommend be purchased?
farm B?
farm B?
Why?
T.J. Bornhorst
October 2007
Revised 1996