Transport of Atmospheric Trace Gases to the Water Table: Implications for Groundwater Dating with Chlorofluorocarbons and Krypton 85

Chlorofluorocarbons and krypton 85 are trace gases whose atmospheric concentrations have been increasing over the past few decades. As they are soluble in water, they have been used as groundwater age indicators over timescales ranging from a few years to a few decades. In this paper we show that the time lag for transport of these atmospheric trace gases through the unsaturated zone is an important consideration when dating groundwaters that are recharged through thick unsaturated zones. The apparent time lag is largely dependent on the gas solubility, the gas diffusion coefficient, and the soil water content. In coarse‐grained soils the lag time will typically range between 1 and 2 years for a water table depth of 10 m to between 8 and 15 years for a water table depth of 30 m. For the shallower water tables (<10 m) the effect is not significant, as it is within the error of the dating methods. However, for deeper water tables (>10 m), if this effect is not considered, the use of these gaseous tracers will overestimate the age of the groundwater. In very fine‐grained soils where the soil water content in the unsaturated zone may be close to saturation, the effect will be much more pronounced.