Groundwater age dating with chlorofluorocarbons
The sensitivity of the CFC dating method depends on the rate of change of the atmospheric CFC concentration with time, and thus the ability to date very young water will diminish with time.
However, the ability to date groundwater that entered the saturated zone prior to the Year 2000 will not change for several decades.
Comparison of Chlorofluorocarbon-Age Dating with Paticle-Tracking Results of a Regional Ground-water Flow Model of the Portland Basin, Oregon and Washington.
This time lag is negligible if the unsaturated zone thickness is less than 5 m, and varies between 0.5-3 years for a unsaturated zone thickness of 10 m and between 5-20 years for a 25 to 30 m thickness (Johnston, 1994; Cook and Solomon, 1995). Modeling results show that only waters older than 20 years are significantly affected.
The effect of dispersion on apparent CFC concentrations and ages has been discussed by Busenberg and Plummer (1992), Plummer et al. For dispersivities of less than 0.5 m, the age error will be less than 3 years for groundwaters recharged since 1955 (Plummer et al., 1993; Ekwurzel et al., 1994).
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.
These concentrations are compared with the atmospheric concentration curve to obtain an apparent CFC age.