Chlorofluorocarbons (CFCs) are stable synthetic organic compounds that were first manufactured in the 1930's and used as refrigerants, aerosol propellants, cleaning agents, solvents, and blowing agents in the production of foam rubber and plastics (Busenberg and Plummer, 1992). CFCs are eventually released to the atmosphere and hydrosphere and depletion of the Earth's ozone layer has been attributed to the rapid atmospheric accumulation of these compounds (Molina and Rowland, 1974). The method developed by Busenberg and Plummer (1992) uses CFCs as dating tools--trichlorofluoromethane (CCl₃F, CFC-11, Freon 11), dichlorodifluoromethane (CCl₂F₂, CFC-12, Freon 12), and trichlorotrifluoroethane (C₂Cl₃F₃, CFC-113). CFC-11 and CFC-12 make up 77 percent of total global production of CFCs (Derra, 1990).

Atmospheric partial pressures of CFCs are determined by Henry's law from the recharge temperature estimated from dissolved gases in the sample (N₂/Ar ratio) and measured concentrations of CFCs in the ground-water samples. These calculated partial pressures are compared with the atmospheric mixing ratios of CFCs to determine the CFC-modeled recharge date, which equates to the time that the water was isolated from air in the unsaturated zone.

Plummer and Busenberg (1999) state that this "clock" is set between the seasonal high and low water table. The apparent age is simply the difference between the date of sample collection and the estimated date the tracer entered the ground-water system.

Busenberg, Eurybiades, and Plummer, L.N., 1992, Use of chlorofluorocarbons (CCl₃F and CCl₂F₂) as hydrologic tracers and age-dating tools: The alluvium and terrace system of central Oklahoma: Water Resources Research, v.28, no. 9, p. 2257-2283.

Derra, S., 1990, CFCs--No easy solutions: Research and Development, v. 32, p. 54-66.

Molina, M., and Rowland, F.S., 1974, Stratospheric sink for chlorofluoromethanes: Chlorine atom catalyzed destruction of ozone: Nature, no. 249, p. 810-812.

Plummer, L.N., and Busenberg, Eurybiades, 2000, Chlorofluorocarbons, in Cook, P.G., and Herczeg, Andrew, eds., Environmental tracers in subsurface hydrology: Kluwer Academic Publishers, p. 441-478.