The importance of considering diffusion when using carbon-14 to estimate groundwater recharge to an unconfined aquifer

A number of studies have used ¹⁴C to estimate ages of unconfined ground water and to infer recharge rates. Some of these studies have been in arid or semi-arid regions, and the inferred recharge rates have been very low. The implicit assumption has been that the vertical movement of ¹⁴C within the unconfined ground water is entirely by convective processes. In this paper we extend the ground water ¹⁴C model of previous workers to include diffusion within the aquifer, and show that for recharge rates less than approximately 1 mm year^-1, diffusion can be important in defining the ¹⁴C distribution. For this reason it is not possible to use ¹⁴C profiles to estimate recharge in these cases. We also show how neglecting diffusion can lead to serious underestimates of groundwater ages in unconfined aquifers where recharge rates are similarly low.