Preliminary cholorofluorocarbon ages for groundwater samples from production wells in the Lawrenceville, Georgia, area
Williams, Lester J.
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Water samples were collected by the U.S. Geological Survey (USGS) from six production wells in 2001 in the Lawrenceville area, Georgia, for analysis of chlorofluorocarbons (CFCs) to estimate the apparent age of groundwater in a crystalline-rock aquifer (Fig. 1). Apparent CFC ages were calculated by comparing CFCs in groundwater samples to atmospheric CFCs-mixing ratios measured in Niwot Ridge, Colorado, after adjust-ment for recharge elevation (Plummer and Busenberg, 1999; U.S. Geological Survey, 2004). The apparent CFC age is the time since the recharge water was isolated from air in the unsaturated zone and may provide an estimate of groundwater residence time. The apparent CFC ages obtained from the samples ranged from mid-1950s to modern (post 1995) (Table 1). The oldest groundwater ages were from samples collected from well 13FF21, which derives most of its yield from a single high-yield (120 gallons per minute) water-bearing zone that shows no interconnection with the regolith. The youngest groundwater age was from a sample collected from well 14FF55, located about 4,000 feet from an operating municipal production well; well 14FF55 taps multiple water-bearing zones ranging in depth from about 65 feet to about 420 feet. Some of the samples are inter-preted, using the ratios of the CFCs, to represent a binary mixture of young (less than 60 years) and old (greater than 60 years) waters (Plummer and Busenberg, 1999). Using a simple binary mixing model for CFC-113/CFC-12, most of the samples are interpreted to be mixtures of young and old waters, probably derived from different water-bearing zones in each well. Plots of CFC-113 versus CFC-12 indicate that about one-third of the samples lie on or near the piston flow line, indicating groundwater flow can be described in terms of a simple piston-flow system (Fig. 2)