Microbiological, Geochemical and Hydrologic Processes Controlling Redox Status in an Alluvial Aquifer: Insights From Field Scale Experiments.

Field-scale experiments and biogeochemical sampling in an alluvial aquifer involving electron donor and bicarbonate amendment to groundwater have provided insight into the coupling of microbiology, biogeochemistry, and hydrogeology in the subsurface, elucidating controls on redox status of metals, metalloids, and sulfur. Research at the Integrated Field Research Challenge site (IFRC) at Rifle, Colorado, USA, initially focused on testing the concept that Fe-reducing bacterial such as Geobacter sp. could enzymatically reduce soluble U(VI) to insoluble U(IV) during electron donor amendment (acetate) and that this could be used as a bioremediation strategy. Initial experiments correlated an increase in Geobacter sp. with a decrease in U(VI) concentration, but also resulted in dominance of sulfate reduction 20 to 30 days after starting electron donor amendment. Subsequent experiments directly linked gene expression in Geobacter sp. to U(VI) concentration. More recently it has been possible to sample and analyze proteomes, metagenomes, and single cells (cryo-TEM/STXM), leading to an understanding of the metabolic capability of the subsurface microbial community and its active metabolic processes.

Current field-scale research at the Rifle site is focused on natural organic carbon and metal cycling (e.g. Fe, V, and U) in the context of the metabolic potential of the entire subsurface microbial community. Organic carbon in the alluvial aquifer is concentrated in naturally reduced zones (NRZ’s). Redox gradients between these zones and surrounding sediments appear to exert important controls on metal mobility and carbon fluxes across the alluvial aquifer. Initial results from metagenomic sequencing show that assembly of nearly complete genomes for microbes present at the 0.5% level is possible. Coupled with proteomics, transcriptomics, and selected physiological studies, prediction of microbially-mediated C, Fe, U, and V redox cycling should be possible for typical oligotrophic conditions in both naturally reduced and more oxidized parts of alluvial aquifers.