Cr(VI) is a widespread groundwater contaminant. To stimulate bioreduction of Cr(VI) in the groundwater at the Hanford 100H field site, 18 kg of HRC(R) was injected into the aquifer. HRC(R) consists of polylactate esterified to a glycerol backbone that slowly releases lactic acid providing a source of carbon. 10 g of 13C-labeled polylactate was added to the HRC(R) to give it a d13C value of ~40 o/oo (versus an unlabeled value of -15 o/oo). Phospholipid fatty acid (PLFA) extracts of biomass in the samples were analyzed to assess the shift in microbial community structure in the extracted groundwater collected from 4 depth intervals in the injection well and in an extraction well located 5 m downgradient from the injection well. Standard geochemical parameters (dissolved O2, Eh, anion chemistry, etc.) and the d13C values of dissolved inorganic carbon (DIC), dissolved organic carbon, cell counts, and 16s rDNA analyses are also being monitored. The d13C values of the primary PLFA peaks (common to many organisms) showed a 13C-enrichment, but reached values (>200 o/oo) much higher the bulk d13C of the labeled HRC(R), reflecting the faster dissolution rate for the 13C-labeled polylactate relative to the HRC(R). After the d13C of the PLFA peaked, it quickly returned to background values (-15 o/oo) indicating rapid turnover of biomass in the system. Several PLFA peaks specific to organisms known to metabolize glycerol (e.g., Flavobacteria) increased significantly but did not show the same increase in d13C. Biomarkers PLFA associated with Desulfobacter, 10Me16:0, were identified and demonstrated 13C-enrichment. These data are being used to quantify the changes in biological activity resulting from HRC(R) injection.