Microbiological and Geochemical Heterogeneity in an In Situ Uranium Bioremediation Field Site

The geochemistry and microbiology of a uranium-contaminated subsurface environment that had undergone two seasons of acetate addition to stimulate microbial U(VI) reduction was examined. There were distinct horizontal and vertical geochemical gradients that could be attributed in large part to the manner in which acetate was distributed in the aquifer with more reduction of Fe(III) and sulfate at deeper depths and closer to the point of acetate injection. 16S rDNA clone libraries derived from sediments and groundwater indicated that the there was an enrichment of sulfate-reducing bacteria in the order Desulfobacterales in sediment and groundwater samples collected nearest the injection gallery where microbially reducible Fe(III) oxides were highly depleted, groundwater sulfate concentrations were low, and increases in acid volatile sulfide (A VS) in the sediment were observed. Further down-gradient, metal reducing conditions were present as indicated by intermediate Fe(II):Fe(total) ratios, lower A VS values, and increased abundance of 16S rDNA sequences belonging to the dissimilatory Fe(III)- and U(VI)-reducing family, Geobacteraceae. Maximal Fe(III) and U(VI) reduction correlated with maximal recovery of Geobacteraceae 16S rDNA sequences in both groundwater and sediment, however the sites at which these maxima occurred were spatially separated within the aquifer. The substantial microbial and geochemical heterogeneity at this site demonstrates that attempts should be made to deliver acetate in a more uniform manner and that closely spaced sampling intervals, horizontally and vertically, in both sediment and groundwater, are necessary in order to obtain a more in-depth understanding of microbial processes and the relative contribution of attached and planktonic populations to in situ uranium bioremediation