Carbon 14 dating of groundwater in confined aquifers: Implications of aquitard diffusion

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Abstract
The influence of diffusive losses into fine‐grained aquitards on the 14C age interpretation of groundwater in long, thin confined aquifers is examined. An analytical model is developed by coupling two one‐dimensional steady state differential equations, one representing advective‐dispersive transport in the aquifer and the other representing diffusion into the aquitards. The results indicate that aquitard diffusion is a mechanism that can cause a significant reduction in 14C concentrations in a confined aquifer. Groundwater velocities derived from 14C measurements that are unadjusted for diffusion would therefore be underestimated. The error that ensues is highly sensitive to the aquifer thickness and the magnitude of the diffusion coefficient. In contrast, the steady‐state distribution of 14C is shown to be relatively insensitive to the magnitude of the longitudinal dispersivity value of the aquifer. Dispersivities representative of the upper range of values that have been reported in the literature produced a negligible difference in steady state 14C concentration compared to results obtained using small dispersivities. An equation is presented which permits the adjustment of 14C data to account for diffusion, providing that the aquifer‐aquitard system is not stratigraphically complex and heterogeneous. The analysis can be extended to include age‐dating isotopes that have half‐lives different from that of 14C. The results of this study also apply to age dating in fractured porous media.