North-Central Section - 50th Annual Meeting - 2016

Paper No. 23-9
Presentation Time: 11:00 AM


BIZJACK, Matthew1, DRUHAN, Jennifer L.2, JOHNSON, Thomas M.1 and SHIEL, Alyssa E.3, (1)Geology, University of Illinois, 156 Computing Applications Building, 605 E. Springfield Ave, Champaign, IL 61820, (2)Geology, University of Illinois at Urbana-Champaign, 156 Computing Applications Building, 605 E. Springfield Ave, Champaign, IL 61820, (3)College of Earth, Ocean and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Bldg., Corvallis, OR 97331,

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.