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

Wednesday, 17 December 2014
Benjamin James Currens, Audrey H Sawyer and Alan E Fryar, University of Kentucky, Lexington, KY, United States
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.