USGS scientists (Busenberg and Plummer, 1992) adapted analytical procedures developed by the oceanographic scientific community for ground-water studies and designed sampling equipment and procedures for collection and preservation of water samples in the field.Water samples for CFC analysis are now routinely collected from domestic, irrigation, monitoring, and municipal wells, and from springs. Nuclear Regulatory Commission-licensed USGS laboratory for analysis of CFC content by gas chromatography to a detection limit of about 0.3 picograms per kilogram (0.3 pg/kg) of water, which is equivalent to 0.3x10 Ground-water dating with CFC-11, CFC-12 and CFC-113 is possible because (1) their amounts in the atmosphere over the past 50 years have been reconstructed, (2) their solubilities in water are known, and (3) concentrations in air and young water are high enough that they can be measured.A closed path is established between the well or pump to a valve system that is used to fill glass ampoules with water, creating a headspace with CFC-free, ultra-pure nitrogen gas. Age is determined from CFCs by relating their measured concentrations in ground water back to known historical atmospheric concentrations and/or to calculated concentrations expected in water in equilibrium with air.

Measurements of dissolved methane are useful in recognizing environments where all three CFCs can be degraded.

Measurements of dissolved nitrogen and argon can be used to help determine recharge temperature and excess air and to recognize environments undergoing denitrification.

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Information about ground-water age can be used to determine recharge rates and refine hydrologic models of ground-water systems (Reilly and others, 1994; Szabo and others, 1996) and thus to predict the contamination potential and estimate the time needed to flush contaminants through a ground-water system.

The 0- to 50-year time scale is particularly relevant to environmentally sensitive shallow ground-water systems.

The detection of chlorofluorocarbons and tritium in ground water provides valuable information that can be used for dating and tracing young ground water—techniques that help water-resources managers develop management strategies for shallow ground-water systems.

Young ground water in shallow ground-water systems Young ground water is typically found at depths from 0 to 100 feet in unconsolidated sediments and at depths up to 1000 feet in fractured-rock systems.

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