Résultat de la recherche
Assessment of Degradation Pathways in an Aquifer with Mixed Chlorinated Hydrocarbon Contamination Using Stable Isotope Analysis
The demonstration of monitored natural attenuation (MNA) of chlorinated hydrocarbons in groundwater is typically conducted through the evaluation of concentration trends and parent−daughter product relationships along prevailing groundwater flow paths. Unfortunately, at sites contaminated by mixtures of chlorinated ethenes, ethanes, and methanes, the evaluation of MNA by using solely concentration data and parent−daughter relationships can result in erroneous conclusions regarding the degradation mechanisms that are truly active at the site, since many of the daughter products can be derived from multiple parent compounds. Stable carbon isotope analysis was used, in conjunction with concentration data, to clarify and confirm the active degradation pathways at a former waste solvent disposal site where at least 14 different chlorinated hydrocarbons have been detected in the groundwater. The isotope data indicate that TCE, initially believed to be present as a disposed product and/or a PCE dechlorination intermediate, is attributable to dehydrochlorination of 1,1,2,2-PCA. The isotope data further support that vinyl chloride and ethene in the site groundwater result from dichloroelimination of 1,1,2-trichlorethane and 1,2-dichloroethane, respectively, rather than from reductive dechlorination of the chlorinated ethenes PCE, TCE, or 1,2-DCE. The isotope data confirm that the chlorinated ethanes and chlorinated methanes are undergoing significant intrinsic degradation, whereas degradation of the chlorinated ethenes may be limited. In addition to the classical trend of enriched isotope values of the parent compounds with increasing distance associated to biodegradation, shifts of isotope ratios of degradation byproduct in the opposite direction due to mixing of isotopically light byproducts of biodegradation with compounds from the source are shown to be of high diagnostic value. These data underline the value of stable isotope analysis in confirming transformation processes at sites with complex mixtures of chlorinated compounds.
Carbon isotopes as a tool to evaluate the origin and fate of vinyl chloride: Laboratory experiments and modeling of isotope evolution
Accumulation of vinyl chloride (VC) is often a main concern at sites contaminated with chlorinated ethenes and ethanes due to its high toxicity. Since there can be several possible sources of VC and ethene at such sites, assessing the origin and fate of VC can be complicated. Aim of this study was to evaluate carbon isotope fractionation associated with various anaerobic processes that lead to the production of VC and ethene in view of using isotopes to evaluate the origin and fate of these compounds in groundwater. Microcosms were constructed using sediments and groundwater from a contaminated site and amended with potential precursors for VC and ethene production. In the microcosms with dichloroethene isomers, sequential reductive dechlorination was observed, and isotopic enrichment factors of -19.9+/-1.5%,for cis- 1,2-dichloroethene -30.3+/-1.9%o for trans-1,2-dichloroethene, and -7.3+/-0.4%o for 1,1,1-dichloroethene were obtained. In microcosms with chlorinated ethanes, 1,2-dichloroethane (1,2-DCA) and 1,1,2-trichloroethane (1,1,2-TCA) were predominantly transformed by dichloroelimination to ethene and VC, respectively, and enrichment factors of -32.1+/-1.1%o for 1,2-DCA and -2.0+/-0.2%o for 1,1,2-TCA were observed. Except for 1,1,2-TCA, a strong C-13 enrichment in each of the potential precursor of VC was observed, which opens the possibility to trace the origin of VC based on the isotope ratio of potential precursors, Furthermore, it was possible to model the isotope evolution of VC present as substrate or intermediate product as a function of time. The study demonstrates that carbon isotope ratios can potentially be used for qualitative and possibly quantitative evaluation of the origin and fate of VC at sites with complex contaminant mixtures.
Carbon Isotopes as a Tool To Evaluate the Origin and Fate of Vinyl Chloride: Laboratory Experiments and Modeling of Isotope Evolution
Accumulation of vinyl chloride (VC) is often a main concern at sites contaminated with chlorinated ethenes and ethanes due to its high toxicity. Since there can be several possible sources of VC and ethene at such sites, assessing the origin and fate of VC can be complicated. Aim of this study was to evaluate carbon isotope fractionation associated with various anaerobic processes that lead to the production of VC and ethene in view of using isotopes to evaluate the origin and fate of these compounds in groundwater. Microcosms were constructed using sediments and groundwater from a contaminated site and amended with potential precursors for VC and ethene production. In the microcosms with dichloroethene isomers, sequential reductive dechlorination was observed, and isotopic enrichment factors of −19.9 ± 1.5‰ for cis-1,2-dichloroethene, −30.3 ± 1.9‰ for trans-1,2-dichloroethene, and −7.3 ± 0.4‰ for 1,1-dichloroethene were obtained. In microcosms with chlorinated ethanes, 1,2-dichloroethane (1,2-DCA) and 1,1,2-trichloroethane (1,1,2-TCA) were predominantly transformed by dichloroelimination to ethene and VC, respectively, and enrichment factors of −32.1 ± 1.1‰ for 1,2-DCA and −2.0 ± 0.2‰ for 1,1,2-TCA were observed. Except for 1,1,2-TCA, a strong 13C enrichment in each of the potential precursor of VC was observed, which opens the possibility to trace the origin of VC based on the isotope ratio of potential precursors. Furthermore, it was possible to model the isotope evolution of VC present as substrate or intermediate product as a function of time. The study demonstrates that carbon isotope ratios can potentially be used for qualitative and possibly quantitative evaluation of the origin and fate of VC at sites with complex contaminant mixtures.