Several GeoChip-related studies have examined microbial communities from U-contaminated groundwater at the U.S. Department of Energy (DOE) Oak Ridge Integrated Field Research Challenge (OR-IFRC) site. Groundwater samples covering a range of contamination levels and an uncontaminated background sample were compared using GeoChip 1.0 (Wu et al., 2006a) . Samples from the uncontaminated site and those with lower levels of contaminants had higher functional gene diversity and gene numbers. In addition, as expected based on the contaminants present at this site, genes for denitrification, organic contaminant degradation, metal resistance, and sulfite reduction (dsr) were detected. A similar sample set using the same array was examined in greater detail in a later study (Waldron et al., 2009) . In this study, sulfate, pH, U, and Tc were found to be the most important drivers in determining the microbial community structure, with pH and the combination of U and Tc explaining ∼21% of the variance observed or 29-40% when all four variables were included.