Use of dual carbon-chlorine isotope analysis to assess the degradation pathways of 1,1,1-trichloroethane in groundwater

Compound-specific isotope analysis (CSIA) is a powerful tool to track contaminant fate in groundwater.However, the application of CSIA to chlorinated ethanes has received little attention so far. Thesecompounds are toxic and prevalent groundwater contaminants of environmental concern. The highsusceptibility of chlorinated ethanes like 1,1,1-trichloroethane (1,1,1-TCA) to be transformed via differentcompeting pathways (biotic and abiotic) complicates the assessment of their fate in the subsurface. Inthis study, the use of a dual CeCl isotope approach to identify the active degradation pathways of 1,1,1-TCA is evaluated for thefirst time in an aerobic aquifer impacted by 1,1,1-TCA and trichloroethylene (TCE)with concentrations of up to 20 mg/L and 3.4 mg/L, respectively. The reaction-specific dual carbonechlorine (CeCl) isotope trends determined in a recent laboratory study illustrated the potential of adual isotope approach to identify contaminant degradation pathways of 1,1,1-TCA. Compared to the dualisotope slopes (Dd13C/Dd37Cl) previously determined in the laboratory for dehydrohalogenation/hydro-lysis (DH/HY, 0.33±0.04) and oxidation by persulfate (∞), the slope determined fromfield samples(0.6±0.2, r2¼0.75) is closer to the one observed for DH/HY, pointing to DH/HY as the predominantdegradation pathway of 1,1,1-TCA in the aquifer. The observed deviation could be explained by a minorcontribution of additional degradation processes. This result, along with the little degradation of TCEdetermined from isotope measurements, confirmed that 1,1,1-TCA is the main source of the 1,1-dichlorethylene (1,1-DCE) detected in the aquifer with concentrations of up to 10 mg/L. This studydemonstrates that a dual CeCl isotope approach can strongly improve the qualitative and quantitativeassessment of 1,1,1-TCA degradation processes in the field.