Effects of physical and geochemical heterogeneities on mineral transformation and biomass accumulation during uranium bioremediation at Rifle, Colorado

Effects of physical and geochemical heterogeneities on mineral transformation and biomass accumulation during uranium bioremediation at Rifle, Colorado Li Li, Carl I. Steefel, Michael B. Kowalsky, Andreas Englert*, Susan S. Hubbard Lawrence Berkeley National Laboratory, Earth Sciences Division, 1 Cyclotron Road, MS 90- 1116, Berkeley, CA 94720 *Now at Ruhr University Bochum, Applied Geology Department, Universitaetsstr. 150, 44801 Bochum, Germany Abstract Electron donor amendment for bioremediation often results in precipitation of secondary minerals and the growth of biomass, which can potentially change flow paths and the efficacy of bioremediation. Quantitative estimation of precipitate and biomass distribution has remained challenging, partly due to the intrinsic heterogeneities of natural porous media and the scarcity of field data. In this work, we examine the effects of physical and geochemical heterogeneities on the spatial distributions of mineral precipitates and biomass accumulated during a uranium bioremediation experiment near Rifle, Colorado. Field bromide breakthrough data were used to infer a heterogeneous distribution of hydraulic conductivity through inverse transport modeling, while the solid phase Fe(III) content was determined by assuming a negative correlation with hydraulic conductivity. Validated by field aqueous geochemical data, reactive transport modeling was used to explicitly keep track of the microbial community dynamics and estimate the spatial distribution of precipitates and biomass. The results show that the maximum mineral precipitation and biomass accumulation occurs in the vicinity of the injection wells, occupying up to 5.4% volume of the pore space, and is dominated by reaction products of sulfate reduction. Accumulation near the injection wells is not strongly affected by heterogeneities present in the system due to the ubiquitous presence of sulfate in the groundwater. However, accumulation in the down-gradient regions is dominated by the iron-reducing reaction products, whose spatial patterns are strongly controlled by both physical and geochemical heterogeneities. Heterogeneities can lead to localized large accumulation of mineral precipitates and biomass, increasing the possibility of pore clogging. Although ignoring the heterogeneities of the system can lead to adequate prediction of the average behavior of sulfate-reducing related products, it can also lead to an overestimation of the overall accumulation of iron-reducing bacteria, as well as the rate and extent of iron reduction. These findings have important implications not only for uranium bioremediation at the Rifle site and for bioremediation of other redox sensitive contaminants at sites with similar characteristics, but also for development of optimal amendment delivery strategies. Keywords: bioremediation, heterogeneity, mineral transformation, biomass accumulation, reactive transport modeling, biogeochemical process, amendment delivery 1. Introduction Contamination by redox-sensitive chemicals, including radionuclides, fuel and chlorinated hydrocarbons, organic acids, and complexing agents, is a growing environmental concern.