Biotransformation involved in sustained reductive removal of uranium in contaminant aquifers

This report summarizes progress made from August 2004 to July 2005. During this period research focused primarily on obtaining a better understanding of the factors controlling the reduction of U(VI) during in situ uranium bioremediation as well as investigating the potential for using electrodes as an alternative electron donor to promote in situ uranium reduction. Analysis of the 2003 experiment at the field study site in Rifle, CO was completed. The results demonstrated the substantial heterogeneity of the zone undergoing bioremediation, both in terms of geochemistry and microbiology. The lack of U(VI) reduction under sulfate-reducing conditions was clearly documented. The need for more detailed sampling both with time and with depth in the aquifer was demonstrated. For the first time a comparison between the composition of the microbial community in the sediments and the microbes in the corresponding groundwater was attempted. The findings from this study are important not only in further demonstrating the potential for in situ uranium bioremediation, but also for indicating how methods and sampling approaches should be improved in the future. A manuscript summarizing these findings has been accepted for publication in Applied and Environmental Microbiology. In summer of 2004 a new field experiment was conducted at the Rifle site. A novel feature of this study was much more intensive sampling in order to better define the progression of microbial processes during in situ uranium bioremediation. The results demonstrated that stimulation of in situ uranium bioremediation with added acetate was a repeatable phenomenon and that U(VI) reduction was clearly linked to the presence and activity of microorganisms in the family Geobacteraceae. A manuscript summarizing these results is in preparation. A surprising result of the field studies at the Rifle site was that although Geobacter species actively reduced U(VI) in the groundwater, removing it from solution, a high percentage of the uranium in sediments was recovered as U(VI). In order to evaluate this further, studies were conducted in which sediments from the Rifle site were incubated under strict anaerobic conditions. This permitted accurate monitoring of the uranium speciation in the sediments as well as in the groundwater over time. When acetate was added to simulate the in situ uranium bioremediation strategy, U(VI) in the groundwater was reduced with a corresponding increase in U(IV) in the sediments. However, the U(VI) in the sediments was not reduced, even after long-term incubations. The resistance of U(VI) adsorbed to sediments to microbial reduction was not previously suspected and has important implications for in situ uranium bioremediation