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

Abstract

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 occurring at greater depths and closer to the point of acetate injection. Clone libraries of 16S rRNA genes derived from sediments and groundwater indicated an enrichment of sulfate-reducing bacteria in the order Desulfobacterales in sediment and groundwater samples. These samples were 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 were observed in the sediment. Further down-gradient, metal-reducing conditions were present as indicated by intermediate Fe(II)/Fe(total) ratios, lower acid volatile sulfide values, and increased abundance of 16S rRNA gene 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 rRNA gene 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.

Phase Preference by Active, Acetate-Utilizing Bacteria at the Rifle, CO Integrated Field Research Challenge Site

“…Overall, our Winchester findings replicated the earlier results of Geobacter-like sequences being enriched in the groundwater phase while uranium reduction occurred (1,2). However, different…”

“…These Geobacter-like clones were found to diminish as the system approached sulfate reduction during continued acetate amendment (1). Likewise, a similar injection experiment in 2003, replicated uranium reduction during biostimulation with enrichment of Geobacter-like clones in groundwater (approx 75%) and sediment samples (<10%), suggesting differences between microbial populations in groundwater and soil particles (2). These reports indicated that Geobacter-like microbes responded positively to acetate utilization and may be important in uranium reduction.…”

Geophysical Monitoring of Coupled Microbial and Geochemical Processes During Stimulated Subsurface Bioremediation

“…1). Samples from each core were collected and dried anaerobically and analyzed for both acid volatile sulfide (AVS) and Fe(II)/Fe(III) content, as described elsewhere (30).…”

Structural Similarities between Biogenic Uraninites Produced by Phylogenetically and Metabolically Diverse Bacteria

“…However, Shewanella has not been found to be a prevalent DMRB during biostimulation at contaminated sites (9,17,18). For example, subsurface ethanol amendments enriched populations of DMRB and DSRB at Oak Ridge, TN.…”