Isotopic and Geochemical Assessment of in Situ Biodegradation of Chlorinated Hydrocarbons

Currently there is no in situ method to detect and quantify complete mineralization of chlorinated hydrocarbons (CHCs) to CO₂. Combined isotopic measurements in conjunction with traditional chemical techniques were used to assess in situ biodegradation of trichloroethylene (TCE) and carbon tetrachloride (CT). Vadose zone CHC, ethene, ethane, methane, O₂, and CO₂ concentrations were analyzed using gas chromatography over 114 days at the Savannah River Site. δ¹³C of CHC and δ¹³C and ¹⁴C of vadose zone CO₂, sediment organic matter, and groundwater dissolved inorganic carbon (DIC) were measured. Intermediate metabolites of TCE and CT accounted for ≤10% of total CHCs. δ¹³C of <i>cis</i>-1,2-dichloroethylene (DCE) was always heavier than TCE indicating substantial DCE biodegradation. ¹⁴C-CO₂ values ranged from 84 to 128 percent modern carbon (pMC), suggesting that plant root-respired CO₂ was dominant. ¹⁴C-CO₂ values decreased over time (up to 12 pMC), and contaminated groundwater ¹⁴C-DIC (76 pMC) was substantially depleted relative to the control (121 pMC). ¹⁴C provided a direct measure of complete CHC mineralization in vadose zone and groundwater in situ and may improve remediation strategies.