MICROBIOLOGICAL, GEOCHEMICAL AND HYDROLOGIC PROCESSES CONTROLLING URANIUM MOBILITY: AN INTEGRATED FIELD-SCALE SUBSURFACE RESEARCH CHALLENGE SITE AT RIFLE, COLORADO

The U.S. Department of Energy faces the challenge of cleaning up and/or monitoring large, dilute plumes contaminated by metals, such as U and Cr, whose mobility and solubility change with redox status. At the Uranium Mill Tailings Site in Rifle, CO, field-scale experiments with acetate as the electron donor have stimulated metal reducing bacteria to effectively remove uranium [U(VI)] from groundwater. The shallow depth to groundwater (3-4 m), thin saturated zone (~2.5 m), and well-defined groundwater flow system at the Rifle site facilitated the monitoring of microbial and geochemical processes which led to two important findings: decreased U(VI) bioreduction after the transition from iron reduction to sulfate reduction, and continued U(VI) removal from groundwater after acetate amendment was terminated, with removal increasing during the following 18 months. The objective of the research planned for the Rifle site is to gain a comprehensive and mechanistic understanding of the microbial factors and associated geochemistry controlling uranium mobility so that DOE can confidently remediate uranium plumes as well as support long-term stewardship of uranium-contaminated sites. Four hypotheses that address knowledge gaps in the following areas will be tested: 1) geochemical and microbial controls on stimulated U(VI) bioreduction by iron-reducers, 2) U(VI) sorption under Fe-reducing conditions, 3) post-biostimulation U(VI) stability and removal, and 4) rates of natural bioreduction of U(VI). The approach targets new knowledge that can be translated into scientifically defensible flow and reactive transport process models of microbially mediated and abiotic reactions. Hypotheses will be tested with a focused set of field and lab experiments that use recently developed sciences of proteogenomics and stable isotope probing to track microbial metabolic status during acetate amendment. This information will be linked to changes in Fe redox status and sulfide minerals, with field-scale changes detected by non-invasive hydrogeophysics, including 3-D resistivity tomography. This linkage will enable optimization of controllable factors such as electron donor concentration and identification of nutrient limitations, which, if relieved, could further enhance bioremediation of redox-sensitive metals.