Use of Chlorofluorocarbons to Age Date Ground Water in Prince William County, Virginia

By David L. Nelms and Richard J. Ahlin

A procedure developed by the National Research program of the U.S. Geological Survey that uses chlorofluorocarbons (CFC’s) as an age-dating tool was applied to water samples collected from fractured-rock and Coastal Plain aquifers in Prince William County, Virginia.  Atmospheric partial pressures are determined by Henry’s Law from the recharge temperature and measured concentrations of trichlorofluoromethane (CCl₃F, F-11) and dichlorodifluoromethane (CCl₂F₂, F-12) in the ground water.  These calculated partial pressures are compared with the modeled atmospheric growth of CFC’s to determine the CFC-model recharge age.  This age equates to the time at which the water was isolated from air in the unsaturated zone.

A recharge temperature of 9 degrees Celsius, which was used in the calculations of CFC’s partial pressures, was determined on the bases of argon-nitrogen ratios in dissolved gas samples collected throughout the county.  Preliminary results indicate that young waters (post-1945) are present at depths greater than 200 feet.  For the entire county, median CFC-model recharge ages determined from F-11 and F-12 are 1972 and 1973, respectively; however, the median ages of water from the fractured-rock aquifers generally are younger than ages of water from Coastal Plain aquifers.  Water samples from urban areas were commonly contaminated with CFC’s; therefore, precise age dating in these areas was not possible.  The presence of CFC’s in urban areas, however, indicates that at least some ground water is younger than 1945.

Estimates from this study indicate a minimum age for the ground-water system; however, other data indicate that water in parts of the ground-water system is much older than the dates indicated.  These findings will have dramatic effects on the conceptualization of hydrogeologic systems and wellhead protection strategies in the Piedmont and Blue Ridge physiographic provinces.

Nelms, D.L., and Ahlin, R.J., 1993, Use of chlorofluorocarbons to age date ground water in Prince William County, Virginia [abs.], in Collected abstracts of the Virginia Water Resources Conference, Richmond, Va., 1993, p. 67.

Contact: GS-W-VArmd_webmaster@usgs.gov
U.S. Department of Interior, U.S. Geological Survey
Privacy Statement
Disclaimer
URL://va.water.usgs.gov/GLOBAL/Abst/nelms_pwc_cfc93.htm
 Last modified: 07/19/04 06:14:33 AM