title: Carbon and chlorine isotope ratios of chlorinated ethenes migrating through a thick unsaturated zone of a sandy aquifer.
abstract: Compound-specific isotope analysis (CSIA) can potentially be used to relate vapor phase contamination by volatile organic compounds (VOCs) to their subsurface sources. This field and modeling study investigated how isotope ratios evolve during migration of gaseous chlorinated ethenes across a 18 m thick unsaturated zone of a sandy coastal plain aquifer. At the site, high concentrations of tetrachloroethene (PCE up to 380 μg/L), trichloroethene (TCE up to 31,600 μg/L), and cis-1,2-dichloroethene (cDCE up to 680 μg/L) were detected in groundwater. Chlorinated ethene concentrations were highest at the water table and steadily decreased upward toward the land surface and downward below the water table. Although isotopologues have different diffusion coefficients, constant carbon and chlorine isotope ratios were observed throughout the unsaturated zone, which corresponded to the isotope ratios measured at the water table. In the saturated zone, TCE became increasingly depleted along a concentration gradient, possibly due to isotope fractionation associated with aqueous phase diffusion. These results indicate that carbon and chlorine isotopes can be used to link vapor phase contamination to their source even if extensive migration of the vapors occurs. However, the numerical model revealed that constant isotope ratios are only expected for systems close to steady state.
authors.affiliation: Centre for Hydrogeology and Geothermics (CHYN),  University of Neuchâtel , Rue Emile Argand 11, CH-2000 Neuchâtel, Switzerland.
authors.forename: Daniel
authors.initials: D
authors.lastname: Hunkeler
authors.forename: Ramon
authors.initials: R
authors.lastname: Aravena
authors.forename: Orfan
authors.initials: O
authors.lastname: Shouakar-Stash
authors.forename: Noam
authors.initials: N
authors.lastname: Weisbrod
authors.forename: Ahmed
authors.initials: A
authors.lastname: Nasser
authors.forename: Lior
authors.initials: L
authors.lastname: Netzer
authors.forename: Daniel
authors.initials: D
authors.lastname: Ronen
mesh_terms: D002244:Carbon / Q000032:analysis*; D002247:Carbon Isotopes; D002713:Chlorine / Q000032:analysis*; D003198:Computer Simulation; D004000:Dichloroethylenes / Q000032:analysis; D044707:Forensic Sciences; D060587:Groundwater / Q000737:chemistry*; D006843:Hydrocarbons, Chlorinated / Q000032:analysis*; D007553:Isotope Labeling; D007557:Israel; D007700:Kinetics; D009038:Motion; D012822:Silicon Dioxide / Q000737:chemistry*; D013750:Tetrachloroethylene / Q000032:analysis; D014241:Trichloroethylene / Q000032:analysis
chemical_list: D002247:Carbon Isotopes; D004000:Dichloroethylenes; D006843:Hydrocarbons, Chlorinated; D014241:Trichloroethylene; D002713:Chlorine; D002244:Carbon; D012822:Silicon Dioxide; D013750:Tetrachloroethylene; C035384:1,2-dichloroethylene