Geohydrology and the occurrence of volatile organic compounds in ground water, Culpeper basin of Prince William County, Virginia

Abstract

The Culpeper basin of Prince William County comprises an interbedded sequence of sedimentary and basaltic rocks of Late Triassic and Early Jurassic age. This sequence is intersected by diabase intrusives and thermally metamorphosed rocks. The rocks of the Culpeper basin are highly fractured and overlain by a thin cover of overburden. The sedimentary rocks are the most productive aquifers, whereas the igneous and metamorphic rocks generally have poor water-bearing potential. Ground water in the Culpeper basin generally flows from the uplands (recharge areas) along lineaments (linear-surface expressions of fracture sets) to the lowlands or valleys (discharge areas). The flow system generally is under water-table (unconfined) conditions with the potentiometric surface following the topography. Two cones of depression are present in the Manassas-Manassas Park area in response to pumpage. Volatile organic compounds have been detected in ground water in 5 areas of the Culpeper basin in the county. Contaminant concentrations and compositions, and possible contaminant migration varies because of rock type, presence of lineaments, and regional flow direction. The dominant volatile organic compounds are tetrachloroethylene (PCE), trichloroethylene (TCE), and 1,1,1-trichloroethane. Concentrations of the volatile organic compounds range from 0.1 to 5,300.0 micrograms per liter. Isolated areas of minor volatile organic contamination also have been identified. Septic systems and domestic and commercial waste-disposal practices may be the means of groundwater contamination in these areas. The results of previous studies of the geohydrology and groundwater resources of the Culpeper basin in Prince William County were reviewed. Groundwater samples were analyzed to identify any additional volatile organic contamination. Geologic and hydrologic data were collected, compiled, and entered into a geographic information system (GIS). The CIS was used (1) to store and manage a comprehensive groundwater data base for the Culpeper basin of Prince William County and (2) to spatially manipulate and display various layers of information. Digital data compiled for this study are available from the U.S. Geological Survey for further analysis and future investigations concerning the county's groundwater resources. Location of Study Area Prince William County is located in northern Virginia, about 25 miles southwest of Washington, D.C. It is bordered by Loudoun and Fairfax Counties to the north, Stafford County to the south, Fauquier County to the west, and the State of Maryland to the east (fig. 1). The study area covers approximately two-thirds of the county and is part of the Culpeper basin, one of nine sedimentary basins of early Mesozoic age in the Piedmont physiographic province of Virginia (Froelich and Olsen, 1985). The term Triassic basin commonly is used locally to describe the section of the Culpeper basin within Prince William County. The city of Manassas (population 24,777), a major industrial and residential center in the county, i...