INTERDISCIPLINARY COOPERATIVE GEOLOGIC MAPPING AND GROUND-WATER RESOURCE ANALYSIS OF FREDERICK COUNTY, VIRGINIA BY THE U.S. GEOLOGICAL SURVEY

As a rapidly expanding “bedroom community” for the Washington, D.C. metropolitan area, the northern Shenandoah Valley of Virginia is facing increased human impacts on ground-water resources. Expansion of industrial and housing development combined with low water tables experienced during recent drought years have prompted local governments to support scientific studies of the karst aquifers of the northern Shenandoah Valley. The aquifer comprises Lower Paleozoic carbonate rocks that have been extensively folded, faulted, and fractured. In many places the fractures have been enlarged by solution into caves and conduits for ground water. In cooperation with the Frederick Co. Sanitation Authority, U.S. Geological Survey (USGS) geologists and hydrologists are studying the geology and ground-water resource in the area around Winchester, VA. The effort, supported by the National Cooperative Geologic Mapping Program (NCGMP), includes new 1:24,000 scale geologic mapping in quadrangles containing the karst aquifer. Special emphasis is put on identifying faults and other structures that may affect the partitioning and movement of ground water on a regional scale. Fracture measurement and analysis are being performed in an attempt to characterize the anisotropy of the bedrock with respect to ground-water flow at local and regional scales. Preliminary results indicate that ground water moves preferentially along strike in the folded and faulted rocks of this area, and many conduits develop along the intersections of joints and bedding planes. The effort also includes installation and monitoring of stream gauges for base-flow data as well as an extensive well monitoring and aquifer tests. Data collected will be incorporated into a numeric ground-water flow model being developed for the entire Shenandoah Valley watershed.