Monitoring Microbial Dechlorination of Tetrachloroethene (PCE) in Groundwater Using Compound-Specific Stable Carbon Isotope Ratios: Microcosm and Field Studies

The determination of compound-specific stable isotope ratios is a promising new tool to assess biodegradation of organic compounds in groundwater. In this study, the occurrence of carbon isotope fractionation during dechlorination of tetrachloroethene (PCE) to ethene was evaluated in a PCE-contaminated aquifer and in a microcosm that was based on aquifer material from the site. In the microcosm, all dechlorination steps were accompanied by carbon isotope fractionation. The largest fractionation occurred during dechlorination of <i>cis</i>-1,2-dichloroethene (cDCE) and vinyl chloride (VC), resulting in a large enrichment of ¹³C in the remaining cDCE and VC. Stable carbon isotope ratios (δ¹³C) of cDCE and VC increased from -25.7 to -1.5‰ and -37.0 to -2.5‰, respectively. The δ¹³C of ethene was initially -60.2‰ and approached the δ¹³C of the added PCE (-27.3‰) as dechlorination came to completion. A similar carbon isotope pattern was observed for PCE dechlorination at the field site. Strong enrichment of ¹³C in cDCE and VC during microbial dechlorination may serve as a powerful tool to monitor the last two dechlorination steps, which frequently determine the rate of complete dechlorination of chlorinated ethenes at field sites undergoing intrinsic bioremediation.