A Numerical Model of Deuterium and Oxygen-18 Diffusion in the Confined Lower Wilcox Aquifer of the Lower Mississippi Valley (USA)

Abstract:

Deuterium and oxygen-18 are routinely used with noble gases and radioisotopes (e.g., ²H, ¹⁴C, ³⁶Cl) to infer climate during groundwater recharge. However, diffusion of ²H and ¹⁸O between a confined aquifer and bounding aquitards could alter total isotope concentrations and the inferred temperature during recharge if groundwater flow is sufficiently slow. Hendry and Schwartz (WRR 24(10), 1988) explained anomalous ²H and ¹⁸O enrichment in the Milk River aquifer of Alberta by analytically modeling isotope diffusion between the lower bounding aquitard and the aquifer. Haile (PhD dissertation, U. Kentucky, 2011) inferred the same mechanism to explain ²H and ¹⁸O enrichment along a flowpath in the confined Lower Wilcox aquifer of the northern Gulf Coastal Plain in Missouri and Arkansas. Based on the geologic and hydraulic properties of the Lower Wilcox aquifer, a numerical model has been constructed to determine how diffusion may influence ²H and ¹⁸O concentrations in regional aquifers with residence times on the order of 10⁴ to 10⁵ years. The model combines solutions for a 1D forward-in-time, finite-difference groundwater flow equation with an explicit-implicit Crank-Nicholson algorithm for advection and diffusion to solve for flow velocity and isotope concentration. Initial results are consistent with the analytical solution of Hendry and Schwartz (1988), indicating diffusion as a means of isotopic enrichment along regional groundwater flowpaths.