Deuterium and oxygen-18 diffusion in a confined aquifer: a numerical model of stable isotope diffusion across aquitard-aquifer boundaries

Abstract:

Deuterium and oxygen-18, combined with noble gases and radioisotopes (e.g., ³H, ¹⁴C, ³⁶Cl), are routinely used to infer climate during recharge and groundwater age. However, along flow paths on the order of 10 – 10³ km long, groundwater velocities may be low enough to allow diffusion of ²H and ¹⁸O between a confined aquifer and bounding aquitards, which could alter isotope concentrations and the inferred temperature of recharge. While the need to account for ¹⁴C diffusion between aquifer waters and confining layers has been suggested by a prior model (Sudicky and Frind, 1981), a literature review revealed no similar study of stable water isotopes. Based on the geologic and hydraulic properties of the confined Wilcox aquifer in the middle Mississippi Valley, we are constructing a numerical model to determine whether, and to what degree, diffusion can influence ²H and ¹⁸O concentrations in regional aquifers with residence times on the order of 10⁴ – 10⁵ y. This model combines solutions for a 1D forward-in-time, finite-difference groundwater flow equation and a combined explicit-implicit, advection-diffusion Crank-Nicholson algorithm to solve for flow velocity and isotope concentration.