Résultat de la recherche
Modelling of C/Cl isotopic behaviour during chloroethene biotic reductive dechlorination: Capabilities and limitations of simplified and comprehensive models
Predicting the fate of chloroethenes in groundwater is essential when evaluating remediation strategies. Such predictions are expected to be more accurate when incorporating isotopic parameters. Although secondary chlorine isotope effects have been observed during reductive dechlorination of chloroethenes, development of modelling frameworks and simulation has thus far been limited. We have developed a novel mathematical framework to simulate the C/Cl isotopic fractionation during reductive dechlorination of chloroethenes. This framework differs from the existing state of the art by incorporating secondary isotopic effects and considering both C and Cl isotopes simultaneously. A comprehensive general model (GM), which is expected to be the closest representation of reality thus far investigated, was implemented. A less computationally intensive simplified model (SM), with the potential for use in modelling of complex reactive transport scenarios, was subsequently validated based on its comparison to GM. The approach of GM considers all isotopocules (i.e. molecules differing in number and position of heavy and light isotopes) of each chloroethene as individual species, of which each is degraded at a different rate. Both models GM and SM simulated plausible C/Cl isotopic compositions of tetrachloroethene (PCE), trichloroethene (TCE) and cis-1,2-dichloroethene (cDCE) during sequential dechlorination when using experimentally relevant kinetic and isotopic parameters. The only major difference occurred in the case where different secondary isotopic effects occur at the different non-reacting positions when PCE is dechlorinated down to cDCE. This observed discrepancy stems from the unequal Cl isotope distribution in TCE that arises due to the occurrence of differential secondary Cl isotopic effects during transformation of PCE to TCE. Additionally, these models are shown to accurately reproduce experimental data obtained during reductive dechlorination by bacterial enrichments harbouring Sulfurospirillum spp. where secondary isotope effects are known to have occurred. These findings underscore a promising future for the development of reactive transport models that incorporate isotopic parameters.
Perchloroethene source delineation using carbon-chlorine isotopic analysis: field investigations of isotopic signature variability / Perchlorethen-Quellendifferenzierung mittels Kohlenstoff-Chlor-
When dealing with contaminated sites, identifying the source of contamination is critical for regulatory purposes. For chlorinated ethenes, previous studies have shown that dual carbon-chlorine (C-Cl) stable isotope analysis could be a key to address this issue as isotopic signatures vary between manufacturers and therefore, supposedly between sources. A successful application of this method relies on the assumption that different sources in the field will also show different signatures. Since the solvents used in the past are no longer available, this study aimed at investigating the extent of applicability of C-Cl stable isotope measurements for source identification based on field investigations. Ten sites which covered all of Switzerland and various sectors employing perchloroethene (PCE) were chosen. Differences were observed between some sites, suggesting that this method could be successfully applied. Other sites showed very similar isotopic signatures, indicating that this method applicability is site-specific. Additionally, the isotopic signature variability between sites was less significant than between the values previously reported for solvents from various manufacturers from North America. It was also confirmed that PCE reductive dechlorination should be considered when applying C-Cl isotope analysis for source identification. © 2015, Springer-Verlag Berlin Heidelberg.
Identification of abiotic and biotic reductive dechlorination in a chlorinated ethene plume after thermal source remediation by means of isotopic and molecular biology tools
Thermal tetrachloroethene (PCE) remediation by steam injection in a sandy aquifer led to the release of dissolved organic carbon (DOC) from aquifer sediments resulting in more reduced redox conditions, accelerated PCE biodegradation, and changes in microbial populations. These changes were documented by comparing data collected prior to the remediation event and eight years later. Based on the premise that dual C-Cl isotope slopes reflect ongoing degradation pathways, the slopes associated with PCE and TCE suggest the predominance of biotic reductive dechlorination near the source area. PCE was the predominant chlorinated ethene near the source area prior to thermal treatment. After thermal treatment, cDCE became predominant. The biotic contribution to these changes was supported by the presence of Dehalococcoides sp. DNA (Dhc) and Dhc targeted rRNA close to the source area. In contrast, dual C-Cl isotope analysis together with the almost absent VC 13C depletion in comparison to cDCE 13C depletion suggested that cDCE was subject to abiotic degradation due to the presence of pyrite, possible surface-bound iron (II) or reduced iron sulphides in the downgradient part of the plume. This interpretation is supported by the relative lack of Dhc in the downgradient part of the plume. The results of this study show that thermal remediation can enhance the biodegradation of chlorinated ethenes, and that this effect can be traced to the mobilisation of DOC due to steam injection. This, in turn, results in more reduced redox conditions which favor active reductive dechlorination and/or may lead to a series of redox reactions which may consecutively trigger biotically induced abiotic degradation. Finally, this study illustrates the valuable complementary application of compound-specific isotopic analysis combined with molecular biology tools to evaluate which biogeochemical processes are taking place in an aquifer contaminated with chlorinated ethenes.
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.