Usgs groundwater age dating

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.

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Total production of CFC-11 and CFC-12 in 1987 was 7.0 and 9.4 million metric tons, respectively, and that of CFC-113 in 1989 was 1.7 million metric tons. By measuring concentrations of CFC-12, CFC-11 and CFC-113, it is possible to identify groundwater recharged since approximately 1941, 1947, and 1955, respectively. 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. Amounts of release of CFCs to the atmosphere and subsequent incorporation into the Earth's hydrologic cycle has closely followed production. For example, it has been estimated that CFC-11 and CFC-12 produced for aerosol propellants were released, on average, within 6 months of sale (Gamlen et al., 1986), and emissions of CFCs used as blowing agents in open-cell foams and extruded foams took place within less than 1 year of manufacture of the chemicals (Midgley and Fisher, 1993).

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