Investigating uranium mobility using groundwater 238U/235U data and a reactive transport model

uranium;bioremediation;reactive transport;stable isotopes;CrunchTope;rate-limited desorption

Bizjack, Matthew Thomas ; Johnson ; Thomas M. ; Druhan ; Jennifer L

关键词: uranium;    bioremediation;    reactive transport;    stable isotopes;    CrunchTope;    rate-limited desorption;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/90833/BIZJACK-THESIS-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In order to better understand chemical reactions affecting the subsurface transport of the widespread contaminant uranium (U), we present a numerical reactive transport model which explicitly incorporates variations in the 238U/235U isotope ratio. Bioremediation, the microbial reductive immobilization of aqueous U(VI) to solid U(IV), has been proposed as a U remediation technique. Both laboratory and field experiments have demonstrated that microbial reduction of U(VI) alters 238U/235U, producing a 238U-enriched solid U(IV) product. Other major U reactive transport processes do not fractionate isotopes significantly. This suggests the potential to quantify the extent of bioreduction occurring in groundwater containing U using 238U/235U as a compliment to the information gained through U concentration measurements. A recent study of a U bioremediation experiment at a contaminated DOE site in Rifle, Colorado, applied Rayleigh distillation models to quantify U stable isotope fractionation observed during biostimulation via acetate amendment. These simplified models have known inaccuracies and do not incorporate the complex hydrologic and geochemical aspects of the site. To more accurately interpret these measured U isotope ratios, we present a multi-component reactive transport model 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. Accurate modeling of observed U isotope ratios is crucial to their use as a method of tracking bioremediation, and this study serves to advance the quantitative application of isotope systems in reactive transport.