6.7 Summary

CFCs are relatively stable gases and atmospheric concentrations increased monotonically from the 1940s to the late 1990s. During this period, it was possible to relate measured concentrations in groundwater samples to the atmospheric concentration each year and hence estimate a recharge year.

Although waters that recharged after the late 1990s—when CFC atmospheric concentrations started declining—cannot be uniquely dated, it is still possible to date older waters with this technique. Measurements of CFCs, while still somewhat specialized, are less complicated than tracers such as [latex]_{}^{3}H[/latex] and noble gases. The process of calculating a tracer age involves measuring an aqueous concentration, calculating the atmospheric (gas-phase) concentration that would be in equilibrium (after corrections for excess air) with the aqueous concentration, and comparing the atmospheric-equivalent value with the history of CFCs in the atmosphere.

In addition to the non‑uniqueness for young groundwaters, the primary limitations to CFC dating are the existence of non‑atmospheric sources of CFCs and non‑conservative transport behavior under certain biogeochemical conditions in the subsurface. CFC‑11 can microbially degrade under reducing conditions, and CFC‑113 can sorb onto aquifer solids. Despite these limitations, excellent agreement between CFC ages (especially CFC‑12) and other tracers has been reported (Cook et al., 1995; Ekwurzel et al., 1994; Plummer et al., 1993). Into the future, it may be possible to date young waters using the declining limb of the atmospheric curve along with some other tracer and/or hydrogeologic information that identifies the water as being recharged after about 2000.