Susceptibility of Fractured-Rock Aquifers in Virginia Based on Ground-Water Dating Techniques

Nelms, D.L., U.S. Geological Survey, 1730 E. Parham Rd., Richmond, VA 23228, (804) 261-2630, (804) 261-2659 (Fax), dlnelms@usgs.gov, Harlow, G.E., Jr., U.S. Geological Survey, Richmond, VA, Plummer, L. Niel, U.S. Geological Survey, Reston, VA, and Busenberg, Eurybiades, U.S. Geological Survey, Reston, VA.


The U.S. Geological Survey, in cooperation with the Virginia Department of Health, sampled water from more than 100 wells and springs in the fractured-rock aquifers of Virginia between 1999 and 2000 as part of the Virginia Aquifer Susceptibility study.  The fundamental premise of the study is that estimates of ground-water age can be used to determine the susceptibility to contamination of aquifers used for drinking-water supplies in the Commonwealth.  Environmental tracers, including chlorofluorocarbons (CFCs), tritium/helium-3 (3H/3He), tritium (3H), and sulfur hexafluoride (SF6), were used to estimate the apparent age of ground water.  A concentration of CFCs greater than 5 pg/kg (picograms per kilogram) was used as a threshold to indicate that parts of the aquifer sampled have a component of young water (younger than 50 years) and, therefore, are susceptible to near-surface contamination.  More than 95 percent of the samples from the fractured-rock aquifers contained concentrations of CFCs above this threshold.  No relation between the presence of CFCs and depth of the wells (119 to 950 ft) or water-bearing zones (40 to 950 ft) was evident.  Several samples contained concentrations of CFCs in excess of the air-water equilibrium, which indicates that sources other than the atmosphere have introduced CFCs into the ground water.  Other volatile halocarbon compounds were qualitatively identified in water samples during the CFC analysis by purge and trap gas chromatography with electron-capture detector; the presence of these compounds also indicates the susceptibility of the fractured-rock aquifers to contamination.  In some cases, the ratio of two CFCs was used to estimate the percentage of the young fraction and young fraction age by assuming a binary mixing model.  The percentage of young water ranged from 15 to 85 percent in these samples, and the apparent CFC ages of the young fraction ranged from 12 to 50 years.  The apparent 3H/3He ages of young fractions ranged from 1.5 to 30 years.  By comparison, water samples from 31 wells in the confined unconsolidated aquifers of the Coastal Plain of Virginia having water-bearing zones of depths (92 to 1130 ft), similar to those in the fractured-rock aquifers, did not contain CFCs and have adjusted radiocarbon dates older than 10,000 years.  Results from this study indicate that ground-water dating techniques offer an important tool for classifying aquifers in terms of susceptibility to contamination and that wells and springs in the fractured-rock aquifers of Virginia are generally susceptible to contamination.


Nelms, D.L., Harlow, G.E., Jr., Plummer, L. Niel, and Busenberg, Eurybiades, Susceptibility of fractured-rock aquifers in Virginia based on ground-water dating techniques [abs.]: National Ground Water Association Fractured-Rock Aquifers 2002, Denver, Colo., p. 51.


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