Monitor-Well Installation and Investigation for Volatile Organics Contamination in Multilayered Coastal Plain Aquifer System of Virginia

Winfield G. Wright, John D. Powell, and David L. Nelms

Volatile organics are present in the multilayered aquifer system underlying the Defense General Supply Center (DGSC), Richmond, Virginia, located on the western edge of the Coastal Plain physiographic province. Trichloroethylene, tetrachloroethylene, chlorobenzene, 1,2-dichloroethane, 1,2-dichloroethylene in concentrations as high as 3,200 parts per billion have been detected. The sources of the contamination are assumed to be a former landfill and solvent storage and disposal areas used during the 1950’s. Contaminant plumes are migrating from these areas which are located within a 10 to 15-foot-thick unconfined, sand aquifer. Underlying this unconfined aquifer is a confining unit 8 to 12 feet thick composed of clay and silt having a hydraulic conductivity of approximately 8.6 x 10^-5 meters per day. Below the confining unit and overlying granitic bedrock, is a confined aquifer 20 to 30 feet thick composed of sand and gravel. The confined aquifer has been contaminated by downward migration of contaminants from the unconfined aquifer through the confining unit.

Clusters of monitoring wells constructed by the U.S. Geological Survey were used to define the extent of migration of contamination downgradient beyond the boundary of the DGSC. Ten clusters of wells are spaced in triangular patterns to define the areal extent of contamination migration. Monitoring wells within each cluster are finished with two-foot screens at the bottom, middle, and top of each aquifer to determine the vertical profile of contamination throughout the study area. The confined-aquifer wells were telescoped through the confining unit to prevent interaquifer contamination.

Results from sampling confined-aquifer wells show the presence of volatile-organic compounds with concentrations varying vertically within the aquifer. The higher concentrations of contaminants detected in the lower aquifer occur coincident with zones of more rapid ground-water flow.

Aquifer-test data show transmissivities differ within the top, middle, and bottom parts of the confined aquifer. These differences are significant when estimating the average-linear velocity of ground water and the horizontal extent of the contamination plume.

Wright, W.G., Powell, J.D., and Nelms, D.L., 1991, Monitor-well installation and investigation for volatile organics contamination in multilayered Coastal Plain aquifer system of Virginia [abs.], in Childress, C.J. Oblinger, and Vince, C.C., compilers, Abstracts from the technical sessions of the first U.S. Geological Survey water-quality workshop, Northeastern Region, Skyland, Virginia, March 31 - April 3, 1986: U.S. Geological Survey Open-File Report 91-225, p. 31-32

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