| Abstract
| - Currently there is no in situ method to detect and quantifycomplete mineralization of chlorinated hydrocarbons(CHCs) to CO2. Combined isotopic measurements inconjunction with traditional chemical techniques wereused to assess in situ biodegradation of trichloroethylene(TCE) and carbon tetrachloride (CT). Vadose zone CHC,ethene, ethane, methane, O2, and CO2 concentrations wereanalyzed using gas chromatography over 114 days at theSavannah River Site. δ13C of CHC and δ13C and 14C of vadosezone CO2, sediment organic matter, and groundwaterdissolved inorganic carbon (DIC) were measured. Intermediatemetabolites of TCE and CT accounted for ≤10% of totalCHCs. δ13C of cis-1,2-dichloroethylene (DCE) was alwaysheavier than TCE indicating substantial DCE biodegradation.14C-CO2 values ranged from 84 to 128 percent moderncarbon (pMC), suggesting that plant root-respired CO2 wasdominant. 14C-CO2 values decreased over time (up to 12pMC), and contaminated groundwater 14C-DIC (76 pMC) wassubstantially depleted relative to the control (121 pMC).14C provided a direct measure of complete CHC mineralizationin vadose zone and groundwater in situ and may improveremediation strategies.
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