The first criterion for designing a properly run system is to have a good understanding of the hydrogeology of the site and the areal and vertical extent of the contaminants. Usually the contamination originates at one or only a few source areas. After the source area(s) is identified, the shape and direction of the plume can be used as a ground water tracer to design the recovery system, which should incorporate characteristics of the natural flow system to facilitate the cleanup. In other words, the natural flow direction should be used to locate the recovery wells in areas to intercept the flow rather than “pull” the ground water in a direction that it would not normally flow.
The objectives of a ground water recovery system usually are to contain the plume by preventing further movement and to reduce the mass (volume) of contamination in the ground water, ideally to below regulatory standards or non-detectable levels.
Once the plume(s) has been sufficiently characterized, the contaminants in the source area(s) should be removed (excavated) to stop the contribution of contaminants to ground water, otherwise, ground water recovery will continue much longer than if the sources were removed.
The fewer the number of ground water recovery wells, generally the better. Location of the recovery wells should be just inside the leading edge of the plume (roughly 25 percent of the distance upgradient of the leading edge, towards the source). The pumping rate of the well should be set at the effective capture width* of the well at steady state, plus a small safety factor (capture width plus 10 percent to 20 percent). If the capture width can be obtained with a single well, do it! Multiple pumping wells will have dead zones or no-flow zones between pumping wells where the plume will barely move. If the single well cannot effectively stop migration of the leading edge of the plume, a smaller (volume) recovery well may be needed at the leading edge to stop further migration.
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