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Research Approach

• Mechanisms of uranium (U) bioreduction illuminated by gene and protein expression.
• Interaction between biotic processes and abiotic processes (e.g.U bioreduction and U sorption).
• Minimally invasive monitoring of subsurface redox status associated with bioreduction.
• Comprehensive biogeochemical reactive transport modeling of subsurface systems undergoing bioreduction.
• Provide a "comprehensive and mechanistic understanding of the microbial factors and associated geochemistry controlling uranium mobility in the subsurface."

• Extending microbial iron (Fe) reduction in the subsurface: In the presence of mM sulfate concentrations in groundwater, the transition from Fe(III) to sulfate reduction during acetate amendment will occur when the readily bioavailable Fe(III) is depleted. Iron reduction (and concomitant U(VI) reduction) can be extended in time through 1) the addition of nanoparticulate or soluble Fe(III) to the subsurface, and 2) introduction of acetate at concentrations sufficient to support iron-reduction but not sulfate-reduction.
• Impact of reducing conditions on U(VI) sorption: The sorption of U(VI) under reduced conditions is decreased overall in comparison to more oxic conditions, but is still large enough to retard U(VI) transport in the Rifle aquifer relative to groundwater flow.
• Long-term post-biostimulation removal of U(VI): Long-term post-biostimulation removal of U(VI) is dependent on ferrous sulfide minerals precipitated during sulfate reduction. After cessation of acetate amendment, these minerals become electron donors for a post-biostimulation microbial community capable of using low ambient concentrations of oxygen and nitrate as terminal electron acceptors. U(VI) is sorbed on to 1) biopolymers specific to the post-biostimulation microbial consortia, and 2) the freshly oxidized Fe(III) mineral surfaces.
• Rates of natural bioreduction: Slow, naturally occurring rates of microbially mediated U(VI) reduction can be estimated (low, medium, high) using molecular biomarkers in Rifle samples by comparing the lowest acetate amendment in Hypothesis 1 with samples from other Rifle locations with no electron donor amendment.
• Each of these hypotheses will be addressed by a combination of laboratory and field experiments or field sampling and will use the approaches and techniques listed in the adjacent box.

• Protein expression in column and field experiments.
• ¹³C labeling and fieldable DNA/RNA chip arrays.
• Detailed analysis of mineralogic changes during in situ biostimulation.
• Lab and field sorption experiments under reducing conditions.
• Geophysical monitoring of subsurface TEAPs (especially ERT, complex resistivity).
• Detailed real-time monitoring of hydraulic conductivity before, during, and after biostimulation.
• Cellularly adsorptive tracers (CATs).
• Integration of diverse data-sets via joint inversion for spatial distribution of key properties.
• Biogeochemical reactive transport modeling, and multiphase, density, and mechanistic processes.