DEUTERIUM AND OXYGEN-18 DIFFUSION IN A CONFINED AQUIFER: A NUMERICAL MODEL OF STABLE ISOTOPE DIFFUSION ACROSS AQUITARD-AQUIFER BOUNDARIES

Abstract

OF THESIS DEUTERIUM AND OXYGEN-18 DIFFUSION IN A CONFINED AQUIFER: A NUMERICAL MODEL OF STABLE ISOTOPE DIFFUSION ACROSS AQUITARDAQUIFER BOUNDARIES The stable isotopes H and O, combined with noble gases and radioisotopes (e.g., H, C, Cl), are used to infer groundwater age and climate during recharge. Flow regimes within low-velocity flowpaths and long residence times could allow an aquitard-aquifer diffusive flux to alter isotope abundance. Consequently, the diffusion of isotopes (e.g., C, H and O) between aquifers and confining layers needs to be considered in such conditions. In this study, COMSOL Multiphysics was used to determine if diffusion of O (and H by proxy) from a bounding aquitard could explain observed downgradient enrichment of H and O within a regional aquifer. Using the geologic and hydraulic properties of the lower Wilcox aquifer of the Mississippi Embayment aquifer system in Missouri and Arkansas, the advection-dispersion equation was solved along a 1-D groundwater flow domain, coupled with a cross-contact aquitard-aquifer diffusive flux. Although the observed signal within the lower Wilcox was not matched, a sensitivity analysis indicated the importance of the isotope composition gradient between the aquifer and aquitard. Furthermore, groundwater velocity was suggested as a controlling influence on aquitard-aquifer exchange that could alter aquifer isotope composition.