Groundwater age dating with chlorofluorocarbons
However, this is not always the case, particularly if the unsaturated zone is thick (Weeks et al., 1982; Severinghaus et al., 1994; Cook and Solomon, 1995). A time lag associated with gas diffusion through the unsaturated zone is strongly dependent on the soil water content and CFC solubility, and to a lesser extent on the recharge rate. Because the atmospheric concentration curve is approximately linear with time, dispersion has a minimal effect on concentration profiles. Hence, allowing for analytical precision and uncertainty in atmospheric concentrations, the accuracy of apparent CFC ages in a purely advective flow system is better than ± 4 years, provided that excess air can be estimated to within 1 cm, and recharge temperature to within 1 degree C. One of the assumptions of groundwater dating with CFCs is that concentrations in the soil gas immediately above the water table are in equilibrium with the atmosphere. However as a result of various environmental regulations limiting the use of CFCs, current production estimates are less than half of the peak values of the late 1980s. CFC-11 (CFCl) have relatively long residence times in the atmosphere (44, 180 and 85 years, respectively; Cunnold et al., 1994; Ko and Jackman, 1994), where they undergo equilibration with surface waters as a function of temperature.
CFCs have certain advantages over tritium because CFCs are detectable in lower concentrations than tritium, and are, therefore, more sensitive indicators of modern water where modern and old water mix.
These concentrations are compared with the atmospheric concentration curve to obtain an apparent CFC age.