title: Use of dual carbon-chlorine isotope analysis to assess the degradation pathways of 1,1,1-trichloroethane in groundwater.
abstract: 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. These compounds are toxic and prevalent groundwater contaminants of environmental concern. The high susceptibility of chlorinated ethanes like 1,1,1-trichloroethane (1,1,1-TCA) to be transformed via different competing pathways (biotic and abiotic) complicates the assessment of their fate in the subsurface. In this study, the use of a dual C-Cl isotope approach to identify the active degradation pathways of 1,1,1-TCA is evaluated for the first 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 carbon-chlorine (C-Cl) isotope trends determined in a recent laboratory study illustrated the potential of a dual isotope approach to identify contaminant degradation pathways of 1,1,1-TCA. Compared to the dual isotope slopes (Δδ(13)C/Δδ(37)Cl) previously determined in the laboratory for dehydrohalogenation/hydrolysis (DH/HY, 0.33 ± 0.04) and oxidation by persulfate (∞), the slope determined from field samples (0.6 ± 0.2, r(2) = 0.75) is closer to the one observed for DH/HY, pointing to DH/HY as the predominant degradation pathway of 1,1,1-TCA in the aquifer. The observed deviation could be explained by a minor contribution of additional degradation processes. This result, along with the little degradation of TCE determined 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 study demonstrates that a dual C-Cl isotope approach can strongly improve the qualitative and quantitative assessment of 1,1,1-TCA degradation processes in the field.
authors.affiliation: University of Neuchâtel, Centre for Hydrogeology and Geothermics (CHYN), Building UniMail, Rue Emile-Argand 11, 2000 Neuchâtel, Switzerland; Institute of Environmental Assessment and Water Research (IDAEA), CSIC, Jordi Girona 18-26, Barcelona 08034, Spain. Electronic address: jordi.palau@ub.edu.
authors.forename: Jordi
authors.initials: J
authors.lastname: Palau
authors.affiliation: University of Liège, Fac. Applied Sciences, Dpt ArGEnCo, Geo³-Hydrogeology and Environmental Geology, Building B52, 4000 Sart-Tilman, Belgium.
authors.forename: Pierre
authors.initials: P
authors.lastname: Jamin
authors.affiliation: University of Neuchâtel, Centre for Hydrogeology and Geothermics (CHYN), Building UniMail, Rue Emile-Argand 11, 2000 Neuchâtel, Switzerland.
authors.forename: Alice
authors.initials: A
authors.lastname: Badin
authors.affiliation: AECOM, Maria-Theresiastraat 34A, 3000 Leuven, Belgium.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Vanhecke
authors.affiliation: AECOM, Maria-Theresiastraat 34A, 3000 Leuven, Belgium.
authors.forename: Bruno
authors.initials: B
authors.lastname: Haerens
authors.affiliation: University of Liège, Fac. Applied Sciences, Dpt ArGEnCo, Geo³-Hydrogeology and Environmental Geology, Building B52, 4000 Sart-Tilman, Belgium.
authors.forename: Serge
authors.initials: S
authors.lastname: Brouyère
authors.affiliation: University of Neuchâtel, Centre for Hydrogeology and Geothermics (CHYN), Building UniMail, Rue Emile-Argand 11, 2000 Neuchâtel, Switzerland.
authors.forename: Daniel
authors.initials: D
authors.lastname: Hunkeler
mesh_terms: D002247:Carbon Isotopes; D002713:Chlorine / Q000737:chemistry*; D060587:Groundwater / Q000737:chemistry*; D054879:Halogenation; D006838:Hydrocarbons / Q000032:analysis; D007553:Isotope Labeling / Q000379:methods*; D014240:Trichloroethanes / Q000737:chemistry*; D014241:Trichloroethylene / Q000032:analysis; D014874:Water Pollutants, Chemical / Q000032:analysis
chemical_list: D002247:Carbon Isotopes; D006838:Hydrocarbons; D014240:Trichloroethanes; D014874:Water Pollutants, Chemical; C024566:1,1,1-trichloroethane; D014241:Trichloroethylene; D002713:Chlorine
keywords: Abiotic degradation; Chlorinated solvents; Compound-specific isotope analysis; Contaminant fate; Groundwater remediation; Volatile organic contaminants