Faculté des sciences

Application of carbon-chlorine isotopic analysis to determine the origin and fate of chlorinated ethenes in groundwater

Badin, Alice ; Hunkeler, Daniel (Dir.) ; Schirmer, Mario (Codir.)

Thèse de doctorat : Université de Neuchâtel, 2015.

Chlorinated ethenes are ubiquitous groundwater contaminants posing a threat to one of our main drinking water sources. Despite their spill history dating back to more than 40 years ago, these contaminants are still found in groundwater in numerous industrially developed countries due to their persistence, difficult characterisation in the subsurface and resulting challenging remediation. When... Plus

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    Summary
    Chlorinated ethenes are ubiquitous groundwater contaminants posing a threat to one of our main drinking water sources. Despite their spill history dating back to more than 40 years ago, these contaminants are still found in groundwater in numerous industrially developed countries due to their persistence, difficult characterisation in the subsurface and resulting challenging remediation. When adequate redox conditions, microbial communities and/or minerals are present, these compounds are known to undergo natural degradation. Applying natural attenuation as a management strategy is thus being increasingly considered as it constitutes a cost-effective environmental friendly approach. Tools enabling to differentiate degradation pathways, predict the fate of contaminants as well as understand the mechanisms underlying their degradation thus constitute the key to a better management of chlorinated ethenes contaminated sites. Methods allowing for contaminant source tracking are also of interest in a legal context where the contamination precursor is searched for. Among the various tools applied to address such questions, compound specific isotopic analysis (CSIA) – which consists in measuring the ratio between light and heavy stable isotopes of one element (i.e. isotopic composition) of a compound – is being increasingly applied.
    This thesis was aimed at exploring the benefits and limits of applying CSIA to substantiate the origin and fate of chlorinated ethenes in groundwater. For this purpose, a first field study aimed at investigating the performance of dual Carbon-Chlorine (C-Cl) isotopic analysis for contaminant source discrimination was carried out. Laboratory experiments were then performed in view of getting more insight in the reaction mechanisms underlying tetrachloroethylene (PCE) reductive dechlorination and to explore the potential of dual C-Cl isotopic analysis to differentiate degradation pathways. A mathematical model was further developed to comprehensively simulate chlorinated ethenes C and Cl isotopic evolution during sequential dechlorination. Simulations were compared to experimental data in order to evaluate this model in its ability to reproduce and thus predict real data. Finally, the contribution of C and Cl isotopic analysis to identify changes in redox processes further affecting chlorinated ethenes in groundwater was challenged when assessing the effect of source thermal remediation by steam injection on a chlorinated ethenes plume.
    For regulatory reasons, determining the contamination perpetrator is often of interest. As the isotopic signature of solvents produced from different manufacturers showed a large variability, CSIA was suggested as a method to discriminate the origin of contamination between different suspected sources by comparing their isotopic signatures. Such application however relies on the assumption that isotopic signatures will also differ in the field. Our first goal was thus to determine the source isotopic variability of PCE at a country scale. For this purpose, the C and Cl isotopic composition of PCE found in groundwater underlying 10 contaminated sites located in Switzerland was compared to the so far reported isotopic signatures of PCE produced by different manufacturers. It was shown that such variability was less important between the 10 sites than between PCE from different manufacturers (i.e. -26.0 to -23.7 ‰ for C and -0.5 to 0.6 ‰ for Cl in Switzerland and -37.4 to -23.2 ‰ for C and -4.4 to 1.2 ‰ for Cl in PCE from manufacturers). Additionally, some sites could be differentiated based on their isotopic signatures while others could not. The successful application of CSIA therefore largely depends on cases.
    Once chlorinated ethenes have been detected in groundwater, it may be of interest to determine whether they are being naturally degraded or not, as this will influence the site management choice (e.g. application of monitored natural attenuation). Chlorinated ethenes are typically known to undergo sequential biotic reductive dechlorination in strictly anoxic conditions (i.e. PCE → trichloroethylene (TCE) → cis-dichloroethylene (cDCE) → vinyl chloride (VC) → ethene). However, the exact reaction mechanism underlying each step of reductive dechlorination remains at the stage of hypothesis where three different reaction mechanisms have so far been proposed.
    As molecules containing light isotopes are generally degraded faster than molecules containing heavy isotopes due to energetic reasons, the isotopic composition of chlorinated ethenes is bound to vary during their sequential degradation. CSIA has thus naturally been proposed as a tool to track the biochemical reactions affecting chlorinated ethenes during their degradation as different processes differently affect their isotopic composition. More specifically, rate-limiting steps control the extent of isotopic enrichment during the course of biotransformation. Rate-limiting steps occurring during substrate-enzyme interactions are yet expected to equally affect both elements since such interactions are not bond-specific contrary to the purely chemical degradation reaction which involves a bond breakage. It was hence suggested that simultaneously considering the isotopic composition of two elements of a compound undergoing degradation via the dual C-Cl isotope slope associated with this compound strictly reflected the chemical reaction underlying this compound degradation contrary to single element isotopic data.
    In view of getting more insight into the reaction mechanisms underlying reductive dechlorination of chlorinated ethenes, we studied the C and Cl isotopic evolution of PCE and TCE during their reductive dechlorination by two bacterial consortia (SL2-PCEc and SL2-PCEb) harbouring members of Sulfurospirillum spp. These consortia were specific in that they showed a different dechlorination pattern: SL2-PCEb was able to dechlorinate PCE or TCE until cDCE whereas SL2-PCEc only dechlorinated until TCE. Contrary to what was expected, two significantly different dual C-Cl isotope slopes of 2.7 ± 0.3 and 0.7 ± 0.2 associated with PCE reductive dechlorination were determined depending on the bacterial consortia. Such variability was attributed to the existence of two different reaction mechanisms underlying this reaction, under the assumption that dual C-Cl isotope slopes strictly reflect the chemical reaction. Two dual C-Cl isotope slopes associated with PCE reductive dechlorination in two field sites where each slope corresponded to one experimentally determined slope were also determined. This further supported the existence of two unique slopes and constituted another argument in favour of their corresponding to two different reaction mechanisms. It was moreover demonstrated that phylogenetically close bacteria could yield different C-Cl isotope slopes. The apparent kinetic isotope effect (AKIE) reflects the difference in reaction rates involving molecules containing light versus heavy isotopes of one element after correcting for non-reacting positions. Primary isotopic effects affect atoms located in reacting position as opposed to secondary isotopic effects which affect atoms located in non-reacting positions. Based on AKIEs calculations where secondary Cl isotopic effects were neglected, we furthermore suggested that one consortium (SL2-PCEc) more likely involved an electron-transfer or nucleophilic substitution as a first step of reaction mechanism than a nucleophilic addition. Comparing calculated AKIEs to the maximum theoretical kinetic isotope effects (or “semiclassical Streitwieser limits”) associated with C-Cl bond breakage suggested that either the primary Cl isotope effect was larger than the kinetic isotope effect given by the Streitwieser limit, or that a secondary Cl isotope effect occurred.
    The Cl isotopic composition of TCE produced by PCE reductive dechlorination was further studied in order to explore in more details the possibility that secondary Cl isotope effects occur. A 1.4 ± 0.2 ‰ to 3.1 ± 0.6 ‰ lighter TCE than PCE at the beginning of the reaction indicated the presence of an inverse secondary effect or at least a difference of -10.6 ± 1 ‰ to -15.9 ± 2.8 ‰ between primary and secondary Cl isotopic effects.
    In order reliably predict a chlorinated ethenes plume fate based on a modelling approach considering isotopic data, isotopic effects should be incorporated in a more comprehensive way than in the models so far proposed. A mathematical model aimed at simulating the evolution of C and Cl isotopic composition during sequential reductive dechlorination was thus developed where secondary isotopic effects were taken into account. So that the model reflects effectively occurring processes, Monod kinetics instead of first order kinetics were additionally considered. The rationale behind the approach consisted in considering all isotopocules (i.e. molecules differing in number and position of heavy and light isotopes) of each chlorinated ethene as individual species which were each degraded at different speed depending on the number and position of heavy and light isotopes in the isotopocule. Such difference in degradation rate between isotopocules was described by a matrix containing kinetic isotopocule fractionation factors. The definition of the latter is similar to that of the commonly used kinetic fractionation factor α which corresponds to the ratio between the degradation rate of heavy and light isotopes of a compound. More specifically, one comprehensive model (GM) considering C and Cl isotopes simultaneously was distinguished from a simplified model (SM) where C and Cl were considered separately. Both models almost identically simulated realistic C and Cl isotopic compositions of PCE, TCE and cDCE during sequential dechlorination when using experimentally plausible kinetic and isotopic parameters. They could additionally accurately reproduce our experimental data, leaving a promising future for the development of an integrative reactive transport model incorporating isotopic parameters. It also documented the slight impact of having different Cl secondary isotopic effects as well as the small effect induced by an unequal Cl isotopes distribution between positions of an asymmetric molecule on the produced compound Cl isotopic composition.
    Finally, field investigations were performed at a site located in Denmark which was explored in a previous work, prior to source thermal remediation. C and Cl isotopic analysis of chlorinated ethenes from groundwater samples taken along the plume centreline were used to verify and improve the interpretation of redox and chlorinated ethenes concentration data. Dual C-Cl isotope slopes associated with PCE and TCE in the first part of the plume were similar to experimentally determined slopes during biotic reductive dechlorination. Based on the assumption that dual C-Cl isotope slopes directly reflect degradation pathways, it was suggested that steam injection enhanced PCE and TCE biotic reductive dechlorination in the first part of the plume. This was in agreement with the occurrence of more reducing conditions resulting from the release of organic matter likely triggered by the thermal remediation. On the other hand, we suggested based on the dual isotope slope approach that cDCE was probably primarily abiotically degraded by pyrite in the downstream part of the plume before and after the remediation event. This differed from the original postulation which suggested the occurrence of either cDCE anaerobic oxidation or complete reductive dechlorination prior to remediation. Such different conclusion could be drawn based on newly reported dual isotope slopes associated with cDCE abiotic degradation which were not available at the time of the study preceding source remediation. In the middle of the plume, a cDCE C isotopic composition lighter than the estimated initial one for PCE C documented the occurrence of further cDCE degradation despite the very low VC concentrations. On the contrary, a cDCE C isotopic composition equal to that of the initially released PCE indicated the absence of or only little further cDCE degradation at the plume front. Such conclusion was in agreement with the observed plume expansion documented by larger concentration contours in the second campaign than in the first.
    To sum up, this thesis reveals that dual C-Cl isotopic analysis should be applied with caution for pathway and source differentiation in the field. It yet demonstrates that such analysis constitutes a valuable complementary tool to explore biochemical processes affecting chlorinated ethenes in groundwater, provided that it is applied at sites where the hydrogeological and biogeochemical contexts are well characterised. The performed studies additionally put more light on C and Cl isotopic effects occurring during PCE and TCE biotic reductive dechlorination even though the specific kinetic processes controlling isotopic behaviours remain unclear. Finally, this work proposes a mathematical model which opens the door to a better incorporation of isotope data when evaluating a plume fate based on a modelling approach.