INVESTIGATING URANIUM MOBILITY USING STABLE ISOTOPE PARTITIONING OF 238U/235U AND A REACTIVE TRANSPORT MODEL

In order to better understand the subsurface transport of the widespread contaminant uranium (U), we report a numerical reactive transport model which explicitly incorporates the effectively stable isotopes of U and the factors that influence their partitioning. Bioremediation, or the microbial reductive immobilization of aqueous U(VI) to solid U(IV) has been proposed as a U remediation technique since the reporting of U reducing microbes by Lovely et al. (1991). Both laboratory and field experiments have demonstrated that biogenic reduction of U(VI) alters the stable isotope ratio ²³⁸U/²³⁵U, producing an isotopically heavy solid U(IV) product. It has also been shown that other major U reactive transport processes do not fractionate isotopes to a consistently measurable level. This suggests the potential to quantify the extent of bioreduction occurring in groundwater containing U using ²³⁸U/²³⁵U ratios—a compliment to the current practice of using solely U concentration measurements. A recent study of a U bioremediation experiment at a DOE site in Rifle, Colorado, applied Rayleigh distillation models to quantify U stable isotope fractionation observed during biostimulation via acetate amendment. These simplified models were fit to the observations only by invoking a “memory-effect,” or a constant source of low-concentration, unfractionated U(VI). To more accurately interpret the measured U isotope ratios, we present a multi-component reactive transport model using CrunchTope, capable of reproducing observed trends in geochemistry and ²³⁸U/²³⁵U ratios from the field experiment. Model results suggest that the rate-limited transport properties of U in the Rifle aquifer are governed by the presence of low-permeability regions in the modeling domain and that these zones are responsible for the suggested “memory” effect observed in previous U isotope studies at this site—as well as potentially giving insight into the mechanism of bioreduction and U(IV) remobilization.