Uranium contamination of sediment and groundwater is a problem at many former uranium ore-processing sites (39
) where residual radionuclides have leached into the subsurface and pose a serious threat to human health and the natural environment, both locally and through off-site transport of soluble U(VI). Natural attenuation rates of decades, the extent of contamination, and the inefficiency and cost of pump and treat methods have spurred efforts to define low-cost methods for assessment and remediation of the many uranium-contaminated sites where U(VI) in groundwater remains above applicable standards (3
). Stimulating microbial reduction of soluble U(VI) to insoluble U(IV) has shown substantial promise as a strategy for in situ bioremediation of uranium-contaminated subsurface environments. For example, the addition of acetate to either aquifer (3
) or groundwater and sediment samples incubated in the laboratory (10
) promoted the growth and activity of dissimilatory Fe(III)-reducing microorganisms and resulted in removal of soluble U(VI) from contaminated groundwater. In these studies, uranium immobilization was associated with the growth of microorganisms in the family Geobacteraceae
and the concomitant stimulation of Fe(III) and U(VI) reduction (3
The Old Rifle site is located at a former uranium ore-processing facility in Rifle, Colo., which is now part of the Uranium Mill Tailings Remedial Action (UMTRA) program of the U.S. Department of Energy (39
). Continued leaching from spent mill tailings at this site has resulted in residual contamination of both groundwater and sediment within the local aquifer. In a previous study at this site, acetate injection into the aquifer was found to stimulate growth of Geobacteraceae
, Fe(III) reduction, and a decline in the U(VI) content of groundwater down-gradient of the injection site. Continued acetate addition resulted in a shift in the dominant terminal electron accepting process from Fe(III) reduction to sulfate reduction, as well as complete degradation of the injected acetate under sulfate-reducing conditions and an apparent decrease in the rate of removal of soluble U(VI) from groundwater (3
). These results indicated that the maintenance of Fe(III)-reducing conditions was critical for sustaining reductive precipitation of U(IV), and highlighted the need to correlate U(VI) removal from groundwater with alterations in microbial community composition in the subsurface.
A key requirement for understanding in situ bioremediation processes is an ability to predict the in situ activity and distribution of metal-reducing microorganisms relevant to bioremediation (14
). Current technology limits routine monitoring of microbial populations during in situ uranium bioremediation to groundwater sampling. Sediment sampling is expensive, labor-intensive, and limited to relatively few cores throughout the course of an in situ experiment due to cost and the potential to disrupt subsurface groundwater flow path and monitoring. However, it is unclear to what extent community analyses of groundwater samples reflect subsurface microbial communities and terminal electron-accepting processes present in sediment.
Here we report on an analysis of the microbial community and related geochemistry in both the groundwater and sediments of the Old Rifle site. The results demonstrated that microbial activity and distribution were highly impacted by the manner in which injected acetate moved through the site, which is highly heterogeneous along the groundwater flow path leading from the injection site. Our findings strongly emphasize the need for close interval sampling in future studies for understanding microbial partitioning and modeling the processes influencing in situ uranium bioremediation.