In this same remediation system, the sediment microbial community was examined with GeoChip 2.0 after 2 years of operation (Xu et al., 2010) . Sediment samples were collected from 11 wells, 5 from the outer loop and 6 from the inner loop. Results revealed significant differences between the microbial communities in the inner and outer loops. The inner loop communities had higher gene numbers and greater diversity than those in the outer loop and inner and outer loop samples were grouped separately based on hierarchical clustering and principle component analysis, indicating that the ethanol injections stimulated the microbial communities in the inner loop. In addition, genes important for U(VI) reduction such as cytochrome c, dsr, and denitrification as well as genes involved in metal resistance and organic contaminant degradation were enriched in the inner loop where electron donor was added. This study demonstrated the importance of U(VI)reducing populations for the maintenance of reducing U(IV) in this bioremediation system.