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Usgs groundwater age dating

The samples are then transported to the NRC-licensed laboratory for analysis. (1971) Atmospheric fluorine compounds as indicators of air movements.

In field sampling, a closed path is established between the well or pump through refrigeration-grade copper tubing in order to flush and cap a 125ml glass bottle with an aluminum foil lined cap.

Randall and Schultz (1976) also recognised the need for reconstruction of atmospheric mixing ratios of CFCs to improve dating capability, the possibility of detecting both CFC-11 and CFC-12 in the same analysis (at that time, analytical procedures were capable of detecting only CFC-11), and potential for dating using ratios of CFCs.

Thompson (1976), Hayes and Thompson (1977), and Thompson and Hayes (1979) refined field GC-ECD analytical procedures for CFC-11 and tested the feasibility of dating with CFC-11 in groundwater systems in parts of New Jersey, Arkansas, and Texas, where hydrologic conditions were well established.

Methods of collection and preservation of water samples prior to analysis have also evolved. (1992) Consumption of Freon CFC-11 and CFC-12 by anaerobic sediments and soils.

In the first groundwater studies (1970s), CFC analyses were performed in the field immediately after collection in glass syringes.

Groundwater dating with CFC-11, CFC-12 and CFC-113 is possible because (1) the atmospheric mixing ratios of these compounds are known and/or have been reconstructed from 1940 to the present, (2) the Henry's law solubilities in water are known, and (3) concentrations in air and young water are relatively high and can be measured. (1996) Sulfur hexafluoride - A powerful new atmospheric tracer. Chlorofluorocarbons(CFCs) are stable, synthetic, halogenated alkanes, developed in the early 1930s as safe alternatives to ammonia and sulphur dioxide in refrigeration. Production and release - World production and release of CCl (Fluorocarbons 11 and 12) through 1975. Production of CFC-12 (dichlorodifluoromethane, CF2Cl2) began in 1931 followed by CFC-11 (trichlorofluoromethane) in 1936. The feasibility of using CFCs as tracers of recent recharge and indicators of groundwater age was first recognized in the 1970s (Thompson et al., 1974; Schultz et al., 1976; Randall and Schultz, 1976; Thompson, 1976; Hayes and Thompson, 1977; Randall et al., 1977; Thompson and Hayes, 1979; Schultz, 1979). (1974) conducted tracer tests by injecting fluorescein dye, and later a solution containing 100 mg kg-1 of CFC-11 into an aquifer of poorly sorted sand and gravel. The fluorescein dye was not recovered whereas the CFC-11 arrived at a nearby monitoring well within the expected travel time.

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Because of Nuclear Regulatory Commission (NRC) licensing requirements for the radioactive 63Ni source in the ECD, it is no longer practical to measure CFC concentrations on site for most groundwater investigations.

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