Correcting for Diffusion in Carbon‐14 Dating of Ground Water

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Abstract
It has generally been recognized that molecular diffusion can be a significant process affecting the transport of carbon‐14 in the subsurface when occurring either from a permeable aquifer into a confining layer or from a fracture into a rock matrix. An analytical solution that is valid for steady‐state radionuclide transport through fractured rock is shown to be applicable to many multilayered aquifer systems. By plotting the ratio of the rate of diffusion to the rate of decay of carbon‐14 over the length scales representative of several common hydrogeologic settings, it is demonstrated that diffusion of carbon‐14 should often be not only a significant process, but a dominant one relative to decay. An age‐correction formula is developed and applied to the Bangkok Basin of Thailand, where a mean carbon‐14‐based age of 21,000 years was adjusted to 11,000 years to account for diffusion. This formula and its graphical representation should prove useful for many studies, for they can be used first to estimate the potential role of diffusion and then to make a simple first‐order age correction if necessary.

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