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? farm B?

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? farm B?

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? Why?

T.J. Bornhorst

October 2007

Revised 1996