Currently there is no in situ method to detect and quantify complete mineralization of chlorinated hydrocarbons (CHCs) to CO 2 . 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, O 2 , and CO 2 concentrations were analyzed using gas chromatography over 114 days at the Savannah River Site. δ 13 C of CHC and δ 13 C and 14 C of vadose zone CO 2 , sediment organic matter, and groundwater dissolved inorganic carbon (DIC) were measured. Intermediate metabolites of TCE and CT accounted for e10% of total CHCs. δ 13 C of cis-1,2-dichloroethylene (DCE) was always heavier than TCE indicating substantial DCE biodegradation. 14 C-CO 2 values ranged from 84 to 128 percent modern carbon (pMC), suggesting that plant root-respired CO 2 was dominant. 14 C-CO 2 values decreased over time (up to 12 pMC), and contaminated groundwater 14 C-DIC (76 pMC) was substantially depleted relative to the control (121 pMC). 14 C provided a direct measure of complete CHC mineralization in vadose zone and groundwater in situ and may improve remediation strategies.

Radiocarbon ( 14 C) measurement from vadose zone air and groundwater is an alternative technique to quantitatively estimate aerobic microbial CO 2 production from petroleum mineralization. The objectives of this study were (1) to demonstrate the effectiveness of radiocarbon in providing direct quantified evidence of in situ aerobic petroleum hydrocarbon biodegradation from vadose zone CO 2 and groundwater DIC at a gasoline-contaminated site being remediated by an air sparging/soil vapor extraction (AS/ SVE) system and (2) to quantify the microbial contribution to vadose zone CO 2 and DIC production. Vadose zone CO 2 and groundwater DIC were collected from soil vapor probes, SVE wells, AS/SVE stack exhaust, and groundwater monitoring wells. Analogous gas and groundwater samples were collected from a geologically similar uncontaminated site. Vadose zone CO 2 and DIC extracted from groundwater were concentrated using NaOH traps and sent to analytical laboratories for conventional 14 C analysis using β counters. 14 C values ranged from 15.9 to 47.7 percent modern carbon (PMC) and demonstrated isotopic depletion from aerobic microbial mineralization of the petroleum. Aerobic biodegradation was calculated to account for 59-87% of the CO 2 produced. 14 C analysis clearly illustrated biodegradation from one time point and even in samples of low CO 2 content after CO 2 was concentrated.