Alice Badin

Multiple dual C-Cl isotope patterns associated with reductive dechlorination of tetrachloroethene.

Dual isotope slopes are increasingly used to identify transformation pathways of contaminants. We investigated if reductive dechlorination of tetrachloroethene (PCE) by consortia containing bacteria with different reductive dehalogenases (rdhA) genes can lead to variable dual C-Cl isotope slopes and if different slopes also occur in the field. Two bacterial enrichments harboring Sulfurospirillum spp. but different rdhA genes yielded two distinct δ(13)C to δ(37)Cl slopes of 2.7 ± 0.3 and 0.7 ± 0.2 despite a high similarity in gene sequences. This suggests that PCE reductive dechlorination could be catalyzed according to at least two distinct reaction mechanisms or that rate-limiting steps might vary. At two field sites, two distinct dual isotope slopes of 0.7 ± 0.3 and 3.5 ± 1.6 were obtained, each of which fits one of the laboratory slopes within the range of uncertainty. This study hence provides additional insight into multiple reaction mechanisms underlying PCE reductive dechlorination. It also demonstrates that caution is necessary if a dual isotope approach is used to differentiate between transformation pathways of chlorinated ethenes.

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 (13)C depletion in comparison to cDCE (13)C 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.

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.

Perchlorethen-Quellendifferenzierung mittels Kohlenstoff-Chlor-Isotopenanalyse: Felduntersuchungen zur Beurteilung der Variabilität der Isotopensignatur

ZusammenfassungBei der Untersuchung von belasteten Standorten stellt sich häufig die Frage, ob unterschiedliche Schadstoffquellen zu einer Grundwasserbelastung beitragen. Chlorierte Kohlenwasserstoffe (CKWs) von verschiedenen Produzenten können unterschiedliche Kohlenstoff- und Chlor-Isotopensignaturen (C-Cl-Isotopensignaturen) aufweisen, was eine Differenzierung von CKW-Quellen ermöglicht. Dazu muss aber die Variabilität der Isotopensignatur bekannt sein. Da die ursprünglich verwendeten Lösungsmittel meist nicht mehr zugänglich sind, werden in dieser Studie die Perchlorethen (PCE) C-Cl-Isotopensignaturen an belasteten Standorten in der Schweiz untersucht. Zehn Standorte wurden ausgewählt, die verschiedene Branchen und Landesregionen abdecken. Eine Variabilität der C-Cl-Isotopensignaturen zwischen einigen Standorten bestätigt die Anwendbarkeit dieser Methode zur Quellendifferenzierung. Gewisse Standorte zeigen jedoch ähnliche Isotopensignaturen. Deswegen ist der Erfolg dieser Identifikationsmethode standortspezifisch. Außerdem ist die Variabilität geringer als publizierte Isotopensignaturen von nordamerikanischen Herstellern. Es hat sich außerdem bestätigt, dass der biologische Abbau von PCE durch reduktive Dechlorierung bei der Identifikation der Kontaminationsquellen berücksichtigt werden muss.AbstractWhen 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.