Assessing in situ mineralization of recalcitrant organic compounds in vadose zone sediments using delta(13)C and (14)C measurements

Few techniques exist to measure the biodegradation of recalcitrant organic compounds such as chlorinated hydrocarbons (CHC) in situ, yet predictions of biodegradation rates are needed for assessing monitored natural attenuation. Traditional techniques measuring O(2), CO(2), or chemical concentrations (in situ respiration, metabolite and soil air monitoring) may not be sufficiently sensitive to estimate biodegradation rates for these compounds. This study combined isotopic measurements ((14)C and delta(13)C Of CO(2) and delta(13)C of CHCs) in conjunction with traditional methods to assess in situ biodegradation of perchloroethylene (PCE) and its metabolites in PCE-contaminated vadose zone sediments. CHC, ethene, ethane, methane, O(2), and CO(2) concentrations were measured over 56 days using gas chromatography (GC). 613C of PCE, trichloroethylene (TCE) and cis- 1,2-dichloroethylene (DCE), delta(13)C and (14)C of vadose zone CO(2) and sediment organic matter, and delta(13)C, (14)C, and deltaD of methane were measured using a GC-isotope ratio mass spectrometer or accelerator mass spectrometer. PCE metabolites accounted for 0.2% to 18% of CHC concentration suggesting limited reductive dechlorination. Metabolites TCE and DCE were significantly enriched in (13)C with respect to PCE indicating metabolite biodegradation. Average delta(13)C-CO(2) in source area wells (-23.5parts per thousand) was significantly lower compared to background wells (- 18.4%o) indicating CHC mineralization. Calculated CHC mineralization rates were 0.003 to 0.01 mg DCE/kg soil/day based on lower (14)C values Of CO(2) in the contaminated wells (63% to 107% modem carbon (pMC) relative to the control well (117 pMC). Approximately 74% of the methane was calculated to be derived from in situ CHC biodegradation based on the (14)C measurement of methane (29 pMC). (14)C-CO(2) analyses was a sensitive measurement for quantifying in situ recalcitrant organic compound mineralization in vadose zone sediments for which limited methodological tools exist. (C) 2004 Elsevier B.V All rights reserved.