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| - Evaluation of Isotopic EnrichmentFactors for the Biodegradation ofChlorinated Ethenes Using aParameter Estimation Model: Toward an Improved Quantificationof Biodegradation
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| - A model was developed to predict the concentrations ofchlorinated ethenes and ethene during sequential reductivedechlorination of tetrachloroethene (PCE) from stablecarbon isotope values using Rayleigh model principlesand specified isotopic enrichment factors for each step ofdechlorination. The model was tested using three separatedatasets of concentration and isotope values measured duringthree experiments involving the degradation of PCE tovinyl chloride (VC), trichloroethene (TCE) to ethene, and cis-1,2-dichloroethene (cDCE) to ethene. The model wasthen coupled to a parameter estimation method to estimatevalues for the isotopic enrichment factors of TCE, cDCE,and VC when they are intermediates in the dechlorinationto ethene. The enrichment factors estimated for TCEand cDCE when they were intermediates in biodegradationexperiments were close to or within the published rangeof enrichment factors determined from experiments whereTCE or cDCE were the initial substrates. In contrast, theenrichment factors determined by parameter estimation forexperiments in which VC was an intermediate inbiodegradation experiments were consistently morenegative (by ∼10‰) than the most negative publishedenrichment factor determined from experiments where VCwas the initial substrate. This finding suggests that therange of enrichment factors for VC dechlorination may notbe as narrow as previously suggested (−21.5‰ to−26.6‰) and that fractionation during VC dechlorinationwhen VC is an intermediate compound may be significantlylarger than when VC is the initial substrate. Thesefindings have important implications both for the currentpractice of extrapolating laboratory-derived isotopicenrichment factors to quantify biodegradation of chlorinatedethenes in the field and for understanding the details ofenzymatic reductive dechlorination.
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