The Virginia Aquifer Susceptibility Study: Dating of Ground Water for Source Water Assessment

By David L. Nelms and George E. Harlow, Jr., U.S. Geological Survey, 1730 East Parham Road, Richmond, VA 23228

The U.S. Geological Survey (USGS), in cooperation with the Virginia Department of Health (VDH), is conducting the Virginia Aquifer Susceptibility (VAS) study to determine the susceptibility to near-surface contamination of regional aquifers that serve as sources for public water supplies. The VAS study is part of the State Source Water Assessment program, which is coordinated by the VDH, Office of Water Programs. The results of this study will be used to categorize public ground-water supplies and identify those that require detailed source-water assessments. The fundamental premise of the VAS study is that ground-water age determinations can be used as a guide for classifying aquifers in terms of susceptibility. The age of ground water is defined as the time since the water was recharged and isolated from the atmosphere. Various environmental isotopes and tracers will be used to determine the apparent age of ground-water samples. Chlorofluorocarbons (CFC's, for example, Freon), tritium (a product of above-ground thermonuclear weapons testing), and tritium/helium techniques can be used to date ground water that is less than 50 years old. Carbon-14 can be used to date ground water that is older than 1,000 years. The VAS study is designed to sample public supply wells that are representative of Virginia's regional aquifers. Approximately 150 sites will be sampled beginning in 1998 through the year 2000. Susceptibility designations will be assigned to the regional aquifers on the basis of statistical and geospatial analysis of the age determinations from samples collected as part of the VAS and previous studies. Major production and usage of regulated chemicals and substances have occurred in the years following World War II. Regional aquifers containing ground water recharged during the past 50 years, therefore, will be considered susceptible to contamination from near-surface sources. A single regional aquifer may be assigned multiple susceptibility designations on the basis of the evaluation of the relation between ground-water age and such factors as hydrogeologic setting or well-construction practices.

Nelms, D.L., and Harlow, Jr., G.E., 2000, The Virginia aquifer susceptibility study:  Dating of ground water for source water assessment [abs.], in Poff, J.A., and Shabman, Leonard, (eds.) 1999 Virginia Water Research Symposium:  Integrating science into development and implementation of effective water resource policies Virginia Water Resources Research Center, p. 149.

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