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Ground Water
Advances in Age Dating

Being able to date ground-water is important because the dates help scientists and resource managers to determine recharge rates, better define the direction and velocity of ground-water flow, and predict contamination potential and the time needed to flush contaminants through a ground-water system. Ground water recharged during the past 50 years generally contains atmospheric contaminants; if the atmospheric concentrations of theseCFC and SF6 in air over North America during last 50 years contaminants change over time, they can be used in tools to date ground water. By reconstructing the amount of chlorofluorocarbons (CFCs or Freon
TM) in the atmosphere, USGS had developed a method that was able to use CFC concentrations to date recent ground water, but a ban on their use in industrialized countries has led to their decrease in the atmosphere. USGS has now developed a dating method that is based on determining the concentration of sulfur hexafluoride (SF6) in water. SF6 began to be produced industrially in 1953 for use in electrical switches and has rapidly increased in the atmosphere.

This method has been used successfully to date shallow ground water (e.g., the Atlantic coastal plain sand aquifers and springs near the top of the Blue Ridge Mountains of Virginia). It is applicable to other environments, except where significant sources of natural, igneous SF6 are found and complicate dating. For additional background information, including an extensive list of references, and links to recent research studies, see the Reston Chlorofluorocarbon Laboratory site or contact:

L. Niel Plummer or Eurybiades Busenberg, U.S. Geological Survey, 432 National Center, 12201 Sunrise Valley Drive, Reston, VA 22092

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Department of the Interior, US Geological Survey
National Research Program || Last Updated: 12/1/2006
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