COUPLED-PROCESS MODELING OF IN SITU URANIUM BIOREMEDIATION NEAR RIFLE, COLORADO

A fully 3-D reactive transport model integrating abiotic and microbially mediated reactions has been developed to simulate biostimulation field experiments at a former Uranium Mill Tailings Remedial Action (UMTRA) site near Rifle, Colorado. Indicator simulations of lithofacies were combined with property transfer models to generate realizations of facies-specific hydrologic parameters (e.g., hydraulic conductivity, porosity, surface area) based on lithologic logs, slug tests, electromagnetic borehole flowmeter measurements, particle size distributions, and measurements of surface area. The 3-D reactive transport model includes variably saturated flow and a comprehensive reaction network (102 biogeochemical species and 7 minerals) whose evaluation required use of massively-parallel computational facilities. Modeling of a recent acetate biostimulation field experiment produced a good match to observations without specific model calibration. These results provide a dynamic picture of evolving uranium behavior during biostimulation, showing the effect of a falling water table and the transition from dominance by iron reducers to dominance by sulfate reducers. This outcome provides a rationale for using our current reactive transport model to plan upcoming field experiments at the site.