Large Carbon Isotope Fractionation during Biodegradation of Chloroform by <italic>Dehalobacter</italic> Cultures

Compound specific isotope analysis (CSIA) has been applied to monitor bioremediation of groundwater contaminants and provide insight into mechanisms of transformation of chlorinated ethanes. To date there is little information on its applicability for chlorinated methanes. Moreover, published enrichment factors (ε) observed during the biotic and abiotic degradation of chlorinated alkanes, such as carbon tetrachloride (CT); 1,1,1-trichloroethane (1,1,1TCA); and 1,1-dichloroethane (1,1-DCA), range from −26.5‰ to −1.8‰ and illustrate a system where similar C−Cl bonds are cleaved but significantly different isotope enrichment factors are observed. In the current study, biotic degradation of chloroform (CF) to dichloromethane (DCM) was carried out by the Dehalobacter containing culture DHB-CF/MEL also shown to degrade 1,1,1-TCA and 1,1-DCA. The carbon isotope enrichment factor (ε) measured during biodegradation of CF was −27.5‰ ± 0.9‰, consistent with the theoretical maximum kinetic isotope effect for C−Cl bond cleavage. Unlike 1,1,1-TCA and 1,1-DCA, reductive dechlorination of CF by the Dehalobacter-containing culture shows no evidence of suppression of the intrinsic maximum kinetic isotope effect. Such a large fractionation effect, comparable to those published for cis-1,2dichloroethene (cDCE) and vinyl chloride (VC) suggests CSIA has significant potential to identify and monitor biodegradation of CF, as well as important implications for recent efforts to fingerprint natural versus anthropogenic sources of CF in soils and groundwater.