6. Chlorofluorocarbon Dating Method
Chlorofluorocarbons (CFCs) are compounds that were first synthesized in the late 1920s as refrigerants to replace more toxic compounds such as ammonia and methyl chloride. The term CFCs is used for a range of compounds containing carbon, chlorine, and fluorine. The most‑used CFCs for groundwater dating have been dichlorodifluoromethane ([latex]{CF}_{2}{Cl}_{2}[/latex], known as CFC‑12), trichlorofluoromethane ([latex]{CFCl}_{3}[/latex], known as CFC‑11), and trichlorotrifluoroethane ([latex]{C_{2}F_{3}Cl}_{3}[/latex], known as CFC‑113).
Atmospheric concentrations of CFCs began rising in the 1940s and received considerable attention in the 1970s as agents of ozone depletion (Molina & Rowland, 1974). Beginning in 1987, many industrial countries agreed to phase out the use of CFCs as part of the Montreal Protocol on Substances that Deplete the Ozone Layer (United Nations Environmental Programme, 1987). As a result, the atmospheric concentrations of CFC‑11, CFC‑12, and CFC‑113 peaked in 1994, 2001, and 1996, respectively (Montzka et al., 1999; Plummer & Busenberg, 2006).
The rise in atmospheric CFC concentrations through the second half of the twentieth century created an opportunity for groundwater dating (Busenberg & Plummer, 1992). Although the decline in atmospheric concentrations in the twenty‑first century has created a non‑uniqueness in relationship between age versus concentration, CFCs continue to be a useful dating tool for waters that recharged between about 1940 to 1990.
A comprehensive guide book on the use of CFCs for groundwater dating⭧ is available from the IAEA. In addition to covering the basic concepts of CFC groundwater dating (Plummer, Busenberg, & Cook, 2006), this guidebook covers effects and processes that can modify CFC tracer ages (Cook, Plummer, et al., 2006), binary mixtures of young and old waters (Plummer, Busenberg, & Han, 2006), models of groundwater age and residence times (Solomon, Cook, & Plummer, 2006), applications of CFCs in groundwater studies (Solomon, Plummer, et al., 2006), case studies (Plummer, Busenberg, Cook, Oster, et al., 2006), comparison with other dating methods (Han et al., 2006), and sampling and analytical methods (Busenberg et al., 2006). Because of the existence and availability of this guidebook, only a summary of the basic concepts, sample collection/analysis, and interpretations is included in this section. Readers who intend to use CFCs in their research are strongly advised to review the guidebook.