Stratification of chlorinated ethenes natural attenuation in an alluvial aquifer assessed by hydrochemical and biomolecular tools

Bentonite sterilization methods in relation to geological disposal of radioactive waste: comparative efficiency of dry heat and gamma radiation

AIMS

This study evaluates the effectiveness of two standard sterilization methods on microorganisms in bentonite, which is proposed as a buffer around metal canisters containing long-lived radioactive waste. Bentonite, as a natural clay, contains microorganisms with enhanced resistance to harsh conditions and the ability to reactivate upon decompaction. Sterile controls are crucial in experiments estimating the impact of microorganisms on nuclear waste repositories. Yet, the effectiveness of common sterilization methods on bentonite microorganisms has not been fully evaluated.

METHODS AND RESULTS

Two methods were compared: dry heat (nine cycles at 121°C for 4 h) and gamma irradiation (10-140 kGy at 147 Gy·min-1). Molecular-genetic, microscopic, and cultivation techniques were used to assess sterilization. Heat sterilization did not eliminate heat-resistant microorganisms, such as Bacillus, Paenibacillus, and Terribacillus, from bentonite powder even after nine heat cycles. However, bentonite suspended in deionized water was sterile after four heat cycles. In contrast, gamma irradiation effectively reduced microbial survivability above a dose of 10 kGy, with the highest doses (100-140 kGy) potentially degrading DNA.

CONCLUSIONS

Gamma irradiation at 30 kGy effectively sterilized bentonite powder. The findings of our experiments emphasize the importance of using appropriate sterilization methods to maintain sterile controls in experiments that evaluate the microbial impacts in nuclear waste repositories. However, further assessment is needed to determine the effects of potential alterations induced by gamma radiation on bentonite properties.

Dramatic loss of microbial viability in bentonite exposed to heat and gamma radiation: implications for deep geological repository

Bentonite is an integral part of the engineered barrier system (EBS) in deep geological repositories (DGR) for nuclear waste, but its indigenous microorganisms may jeopardize long-term EBS integrity. To predict microbial activity in DGRs, it is essential to understand microbial reactions to the early hot phase of DGR evolution. Two bentonites (BCV and MX-80) with varied bentonite/water ratios and saturation levels (compacted to 1600 kg.m- 3 dry density/powder/suspension), were subjected to heat (90-150 °C) and irradiation (0.4 Gy.h- 1) in the long-term experiments (up to 18 months). Molecular-genetic, microscopic, and cultivation-based techniques assessed microbial survivability. Exposure to 90 °C and 150 °C notably diminished microbial viability, irrespective of bentonite form, with negligible impacts from irradiation or sample type compared to temperature. Bentonite powder samples exhibited microbial recovery after 90 °C heating for up to 6 months but not 12 months in most cases; exposure to 150 °C had an even stronger effect. Further long-term experiments at additional temperatures combined with the mathematical prediction of temperature evolution in DGR are recommended to validate the possible evolution and spatial distribution of microbially depleted zones in bentonite buffer around the waste canisters and refine predictions of microbial effects over time in the DGR.

Multidisciplinary monitoring of an in-situ remediation test of chlorinated solvents

Pollutions on and within the underground poses risks for groundwater contamination and is a widespread global problem. Common remediation methods based on digging and removal can be expensive and have limitations, while in-situ remediation is an attractive alternative. However, there is a need to develop tools to monitor the effectiveness both in terms of the successful injection of remediation fluids but also the effectiveness of the treatment, i.e., degree of degradation/removal of the pollutants and possible metabolites. This paper presents a methodology for monitoring the changes following an in-situ remediation treatment of a site contaminated with chlorinated solvents. The methodology consists of two different methods, where Direct Current resistivity and time-domain Induced Polarization (DCIP) was used to acquire daily data and geochemical analyses on water samples were collected approximately every three months. The geophysical results provide insights on how the injected fluids are spreading and assist in acquiring a better understanding of the geological and hydrogeological system. On the other hand, the geochemical sampling enhances our knowledge about the hydrochemistry of the system and the concentration of the pollutants. Our research highlights the challenges of monitoring in-situ bioremediation experiments in complex environments and in cases where pollutants are situated in low hydraulic conductivity formations. The joint interpretation of the data shows the importance of an interdisciplinary approach to understand complex systems.

Machine Learning Modeling Based on Microbial Community for Prediction of Natural Attenuation in Groundwater

Natural attenuation is widely adopted as a remediation strategy, and the attenuation potential is crucial to evaluate whether remediation goals can be achieved within the specified time. In this work, long-term monitoring of indigenous microbial communities as well as benzene, toluene, ethylbenzene, and xylene (BTEX) and chlorinated aliphatic hydrocarbons (CAHs) in groundwater was conducted at a historic pesticide manufacturing site. A machine learning approach for natural attenuation prediction was developed with random forest classification (RFC) followed by either random forest regression (RFR) or artificial neural networks (ANNs), utilizing microbiological information and contaminant attenuation rates for model training and cross-validation. Results showed that the RFC could accurately predict the feasibility of natural attenuation for both BTEX and CAHs, and it could successfully identify the key genera. The RFR model was sufficient for the BTEX natural attenuation rate prediction but unreliable for CAHs. The ANN model showed better performance in the prediction of the attenuation rates for both BTEX and CAHs. Based on the assessments, a composite modeling method of RFC and ANN was proposed, which could reduce the mean absolute percentage errors. This study reveals that the combined machine learning approach under the synergistic use of field microbial data has promising potential for predicting natural attenuation.

Survivability and proliferation of microorganisms in bentonite with implication to radioactive waste geological disposal: strong effect of temperature and negligible effect of pressure

As bentonite hosts a diverse spectrum of indigenous microorganisms with the potential to influence the long-term stability of deep geological repositories, it is essential to understand the factors influencing microbial activity under repository conditions. Here, we focus on two factors, i.e., temperature and swelling pressure, using a suspension of Cerny Vrch bentonite to boost microbial activity and evaluate microbial response. Suspensions were exposed either to different pressures (10, 12 and 15 MPa; to simulate the effect of swelling pressure) or elevated temperatures (60, 70, 80 and 90 °C; to simulate the effect of cannister heating) for four weeks. Each treatment was followed by a period of anaerobic incubation at atmospheric pressure/laboratory temperature to assess microbial recovery after treatment. Microbial load and community structure were then estimated using molecular-genetic methods, with presence of living cells confirmed through microscopic analysis. Our study demonstrated that discrete application of pressure did not influence on overall microbial activity or proliferation, implying that pressure evolution during bentonite swelling is not the critical factor responsible for microbial suppression in saturated bentonites. However, pressure treatment caused significant shifts in microbial community structure. We also demonstrated that microbial activity decreased with increasing temperature, and that heat treatment strongly influenced bentonite microbial community structure, with several thermophilic taxa identified. A temperature of 90 °C proved to be limiting for microbial activity and proliferation in all bentonite suspensions. Our study emphasizes the crucial role of a deep understanding of microbial activity under repository-relevant conditions in identifying possible strategies to mitigate the microbial potential within the deep geological repository and increase its long-term stability and safety.

Biodiversity in wetland+ system: a passive solution for HCH dump effluents

The hexachlorocyclohexane isomers (HCH) are long-banned pesticides. Even though their use has been prohibited for decades, their presence in the environment is still reported worldwide. Wetland + is a registered trademark of the remedial treatment technology consisting of an aerobic sedimentary tank, a permeable reactive barrier, a biosorption system, and an aerobic wetland. This proven method combines a reductive treatment known from PRBs with the natural wetland self-cleaning processes. The average efficiency of the system is 96.8% for chlorobenzenes (ClB) and 81.7% for HCH, during the first 12 months of the system operation. The presence of the genes encoding enzymes involved in the degradation of the HCH compounds indicates that the removal of HCH and ClB occurs not only by chemical removal but also through aerobic and anaerobic combining biodegradation. Changes in abundance and the composition of the diatom community were found to be suitable indicators of the water quality and of the impact of the Wetland + operation on the water ecosystem. The system's annual operation exhibited a markedly higher number of diatom species in the closing profiles of the Ostrovský Creek, the Wetland + effluent recipient.

Mobilization pilot test of PCE sources in the transition zone to aquitards by combining mZVI and biostimulation with lactic acid

The potential toxic and carcinogenic effects of chlorinated solvents in groundwater on human health and aquatic ecosystems require very effective remediation strategies of contaminated groundwater to achieve the low legal cleanup targets required. The transition zones between aquifers and bottom aquitards occur mainly in prograding alluvial fan geological contexts. Hence, they are very frequent from a hydrogeological point of view. The transition zone consists of numerous thin layers of fine to coarse-grained clastic fragments (e.g., medium sands and gravels), which alternate with fine-grained materials (clays and silts). When the transition zones are affected by DNAPL spills, free-phase pools accumulate on the less conductive layers. Owing to the low overall conductivity of this zone, the pools are very recalcitrant. Little field research has been done on transition zone remediation techniques. Injection of iron microparticles has the disadvantage of the limited accessibility of this reagent to reach the entire source of contamination. Biostimulation of indigenous microorganisms in the medium has the disadvantage that few of the microorganisms are capable of complete biodegradation to total mineralization of the parent contaminant and metabolites. A field pilot test was conducted at a site where a transition zone existed in which DNAPL pools of PCE had accumulated. In particular, the interface with the bottom aquitard was where PCE concentrations were the highest. In this pilot test, a combined strategy using ZVI in microparticles and biostimulation with lactate in the form of lactic acid was conducted. Throughout the test it was found that the interdependence of the coupled biotic and abiotic processes generated synergies between these processes. This resulted in a greater degradation of the PCE and its transformation products. With the combination of the two techniques, the mobilization of the contaminant source of PCE was extremely effective.

Prediction of Dichloroethene Concentration in the Groundwater of a Contaminated Site Using XGBoost and LSTM

Chlorinated aliphatic hydrocarbons (CAHs) are widely used in agriculture and industries and have become one of the most common groundwater contaminations. With the excellent performance of the deep learning method in predicting, LSTM and XGBoost were used to forecast dichloroethene (DCE) concentrations in a pesticide-contaminated site undergoing natural attenuation. The input variables included BTEX, vinyl chloride (VC), and five water quality indicators. In this study, the predictive performances of long short-term memory (LSTM) and extreme gradient boosting (XGBoost) were compared, and the influences of variables on models’ performances were evaluated. The results indicated XGBoost was more likely to capture DCE variation and was robust in high values, while the LSTM model presented better accuracy for all wells. The well with higher DCE concentrations would lower the model’s accuracy, and its influence was more evident in XGBoost than LSTM. The explanation of the SHapley Additive exPlanations (SHAP) value of each variable indicated high consistency with the rules of biodegradation in the real environment. LSTM and XGBoost could predict DCE concentrations through only using water quality variables, and LSTM performed better than XGBoost.

Field study of microbial community structure and dechlorination activity in a multi-solvents co-contaminated site undergoing natural attenuation

BTEX and chlorinated aliphatic hydrocarbons (CAHs) are the common pollutants found at contaminated sites, and natural attenuation (NA) of CAHs was widely observed where they coexist. In this work, the groundwater in a site co-contaminated with BTEX and CAHs was monitored for 1 year. The compositions and activities of the microfloras, especially dechlorinators and their relationships with the contaminants, geochemical properties, seasons and depth were evaluated. The results are consistent with the well-known NA conceptual model where CAHs are not able to stimulate the enrichment of dechlorinators alone, but BTEX does promote dechlorination. The higher temperature, rather than ORP in the deeper groundwater of the wet season became a key factor to promote the abundance of dechlorinators, but only when BTEX was available, indicating that the substrates from the BTEX biodegradation played an important role in the dechlorinator enrichment. The elevated ORP in the shallower groundwater exceeded the optimum conditions for reductive dechlorination and no significant seasonal variation of dechlorinators was found. The co-occurrence network revealed the cooperative interactions among the functional microfloras in which dechlorinators, BTEX degraders, and fermentative bacteria jointly promoted the dechlorination. These findings provided us a further understanding of the NA processes in a commingled plume.

Analysis of nitrifying bacteria growth on two new types of biomass carrier using respirometry and molecular genetic methods

This work addresses the testing of two newly produced biomass carriers (micro- and nanofibers) and one commercially available AnoxKaldnes™ K3 carrier in a laboratory post-nitrification reactor. The carriers were prepared under parameters suitable for high-quality biomass adhesion to their surface, and each was characterized by its specific structures. As part of the evaluation of the biofilms using respirometry and molecular genetic methods, the carriers were assessed in terms of their effectiveness and comparability. The rate of biofilm development was dependent on the structure and surface properties of the individual carriers. The results showed that the biofilm most strongly adhered to nanofiber carriers, where nitrating bacteria's slower but more abundant development occurred. Microfiber carriers were more stable, but a diverse internal structure may be unsuitable in a populated carrier's early stages. The AnoxKaldnes™ K3 carriers showed the slowest growth of biofilm, but the monitored nitrifying bacteria were abundant after an extended time. AOB representatives are likely to prefer an environment with a high amount of biomass and a large active area. Conversely, NOB representatives thrive better in a slowly forming biofilm. The methods used to monitor biofilm are challenging to compare directly, but they do complement each other, which aids in verifying the individual test results. Developing new types of biomass carriers with the potential for high-quality adhesion of microorganisms is a prerequisite for the expansion of highly efficient biotechnological processes, especially for wastewater treatment.

Miscanthus x giganteus role in phytodegradation and changes in bacterial community of soil contaminated by petroleum industry

The second generation energy crop Miscanthus x giganteus (Mxg) was cultivated in pots with mixtures of clean and petroleum industry contaminated soil affected by petroleum, Pb, Zn contamination and high salinity. The survival rate reached 100%, nevertheless the biomass parameters were negatively affected even in the lowest proportion of contaminated soil. In the lowest contamination, where the plant grew still quite successfully, C10-C40 degradation was significantly enhanced compared to the unplanted control with degradation of 58 ± 14%. The plant contribution to aliphatics degradation was significantly correlated with biomass, thus it was negligible in higher contamination. A similar pattern was documented in development of the soil bacterial community. The shift in community structure after Mxg cultivation was observed mainly in the soil with the lowest contaminant proportion, though an increase of bacterial diversity in the miscanthus rhizosphere was observed in all cases. Relative abundance of Actinobacteria was reduced on behalf of several less abundant phyla (Verrucomicrobia, Bacterioides, Acidobacteria). The majority of genera identified as potential petroleum degraders (Pseudomonas, Shinella, Altererythrobacter, Azospirillum, Mesorhizobium, Dyella) were more abundant in contaminated soil with miscanthus, suggesting that Mxg could be a promising crop for phytomanagement of petroleum contaminated soils but salt phytotoxicity needs to be mitigated first.

Application of nanofiber carriers for sampling of microbial biomass from contaminated groundwater

Sampling of microbial biomass is crucial for understanding and controlling remediation processes ongoing at contaminated sites in general, particularly when molecular genetic analyses are employed. In this study, fiber-based carriers with a nanofiber layer were developed and tested as a method to sample microbial biomass in groundwater for molecular genetic analysis. Nanofiber carriers, varying in the shape and the linear density of nanofibers, were examined throughout a 27-month monitoring period in groundwater contaminated with benzene, toluene, ethylbenzene and xylene isomers (BTEX), and chlorinated ethenes. The effect of carrier shape and nanofiber layer density on the microbial surface colonization and composition of the microbial biofilm was determined using real-time PCR and next-generation sequencing (NGS) analysis. Differences in microbial community composition between nanofiber carriers, groundwater, and soil samples were also analyzed to assess the applicability of carriers for biomass sampling at contaminated sites. The nanofiber carriers showed their applicability as a sampling tool, particularly because of their easy manipulation that facilitates DNA isolation. The majority of taxa (Proteobacteria, Firmicutes, and Bacteroidetes) present on the carrier surfaces were also detected in the groundwater. Moreover, the microbial community on all nanofiber carriers reflected the changes in the chemical composition of groundwater. Although the carrier characteristics (shape, nanofiber layer) did not substantially influence the microbial community on the carrier surface, the circular and planar carriers with a nanofiber layer displayed faster microbial surface colonization. However, the circular carrier was the most suitable for biomass sampling in groundwater because of its high contact area and because it does not require pre-treatment prior to DNA extraction.

A Multidisciplinary Approach to Evaluate the Effectiveness of Natural Attenuation at a Contaminated Site

This study evaluates the natural attenuation of chlorinated hydrocarbons as remediation action in a contaminated site downtown the city of Parma (Italy). To achieve this goal, a combination of new investigation methods (bio-molecular analysis, compound specific isotope analysis, phytoscreening) has been proposed. The approach (named circular multi step) allows to: fully understand the phenomena that occur at the study site, design new investigation activities, and manage best practices. Consequently, each step of the approach improves the conceptual and numerical models with new knowledge. The activities carried out at the study site allowed to detect a contamination of perchloroethylene in a large part of the city of Parma and, of main importance, underneath a kindergarten. The results of the study did not show significant natural attenuation of chlorinated hydrocarbons and that the detected contamination could refer to the same unknown contaminant source. Furthermore, the innovative phytoscreening technique was applied to assess the presence of chlorinated hydrocarbons at the ground level. The plume spread was estimated through numerical modeling starting from potential contaminant sources. This study enhances the knowledge of groundwater flow and contamination in Parma and allows authorities to design new investigation/reclamation activities through management actions.

The role of ecotones in the dehalogenation of chloroethenes in alluvial fan aquifers

The presence of ecotones in transition zones between geological strata (e.g. layers of gravel and sand interbedded with layers of silt in distal alluvial fan deposits) in aquifers plays a significant role in regulating the flux of matter and energy between compartments. Ecotones are characterised by steep physicochemical and biological gradients and considerable biological diversity. However, the link between organic pollutants and degradation potential in ecotones has scarcely been studied. The aim of this study is to relate the presence of ecotones with the dehalogenation of chloroethenes. A field site was selected where chloroethene contamination occurs in a granular aquifer with geological heterogeneities. The site is monitored by multilevel and conventional wells. Groundwater samples were analysed by chemical, isotopic, and molecular techniques. The main results were as follows: (1) two ecotones were characterised in the source area, one in the upper part of the aquifer and the second in the transition zone to the bottom aquitard, where the aged pool is located; (2) the ecotone located in the transition zone to the bottom aquitard has greater microbial diversity, due to higher geological heterogeneities; (3) both ecotones show the reductive dehalogenation of perchloroethylene and trichloroethylene; and (4) these ecotones are the main zones of the reductive dehalogenation of the pollutants, given the more reductive conditions at the centre of the plume. These findings suggest that ecotones are responsible for natural attenuation, where oxic conditions prevailed at the aquifer and bioremediation strategies could be applied more effectively in these zones to promote complete reductive dehalogenation.

Deciphering indigenous bacteria in compacted bentonite through a novel and efficient DNA extraction method: Insights into biogeochemical processes within the Deep Geological Disposal of nuclear waste concept

Compacted bentonites are one of the best sealing and backfilling clays considered for use in Deep Geological Repositories of radioactive wastes. However, an in-depth understanding of their behavior after placement in the repository is required, including if the activity of indigenous microorganisms affects safety conditions. Here we provide an optimized phenol:chloroform based protocol that facilitates higher DNA-yields when other methods failed. To demonstrate the efficiency of this method, DNA was extracted from acetate-treated bentonites compacted at 1.5 and 1.7 g/cm³ densities after 24 months anoxic incubation. Among the 16S rRNA gene sequences identified, those most similar to taxa mediating biogeochemical sulfur cycling included sulfur oxidizing (e.g., Thiobacillus, and Sulfurimonas) and sulfate reducing (e.g., Desulfuromonas and Desulfosporosinus) bacteria. In addition, iron-cycling populations included iron oxidizing (e.g., Thiobacillus and Rhodobacter) plus reducing taxa (e.g., Geobacillus). Genera described for their capacity to utilize acetate as a carbon source were also detected such as Delftia and Stenotrophomonas. Lastly, microscopic analyses revealed pores and cracks that could host nanobacteria or spores. This study highlights the potential role of microbial driven biogeochemical processes in compacted bentonites and the effect of high compaction on microbial diversity in Deep Geological Repositories.

Diversity of bacteria and archaea in the groundwater contaminated by chlorinated solvents undergoing natural attenuation

Chlorinated solvents (CS)-contaminated groundwater poses serious risks to the environment and public health. Microorganisms play a vital role in efficient remediation of CS. In this study, the microbial community (bacterial and archaeal) composition of three CS-contaminated groundwater wells located at an abandoned chemical factory which covers three orders of magnitude in concentration (0.02-16.15 mg/L) were investigated via 16S rRNA gene high-throughput sequencing. The results indicated that Proteobacteria and Thaumarchaeota were the most abundant bacterial and archaeal groups at the phylum level in groundwater, respectively. The major bacterial genera (Flavobacterium sp., Mycobacterium sp. and unclassified Parcubacteria taxa, etc.) and archaeal genera (Thaumarchaeota Group C3, Miscellaneous Crenarchaeotic Group and Miscellaneous Euryarchaeotic Group, etc.) might be involved in the dechlorination processes. In addition, Pearson's correlation analyses showed that alpha diversity of the bacterial community was not significantly correlated with CS concentration, while alpha diversity of archaeal community greatly decreased with the increased contamination of CS. Moreover, partial Mantel test indicated that oxidation-reduction potential, dissolved oxygen, temperature and methane concentration were major drivers of bacterial and archaeal community composition, whereas CS concentration had no significant impact, indicating that both indigenous bacterial and archaeal community compositions are capable of withstanding elevated CS contamination. This study improves our understanding of how the natural microbial community responds to high CS-contaminated groundwater.

Hydrochemical Conditions for Aerobic/Anaerobic Biodegradation of Chlorinated Ethenes—A Multi-Site Assessment

A stall of cis-1,2-DCE and vinyl chloride (VC) is frequently observed during bioremediation of groundwater chloroethenes via reductive dechlorination. These chloroethenes may be oxidised by aerobic methanotrophs or ethenotrophs co-metabolically and/or metabolically. We assessed the potential for such oxidation at 12 sites (49 groundwater samples) using hydrochemical and molecular biological tools. Both ethenotroph (etnC and etnE) and methanotroph (mmoX and pmoA) functional genes were identified in 90% of samples, while reductive dehalogenase functional genes (vcrA and bvcA) were identified in 82%. All functional genes were simultaneously detected in 78% of samples, in actively biostimulated sites in 88% of samples. Correlation analysis revealed that cis-1,2-DCE concentration was positively correlated with vcrA, etnC and etnE, while VC concentration was correlated with etnC, etnE, vcrA and bvcA. However, feature selection based on random forest classification indicated a significant relationship for the vcrA in relation to cis-1,2-DCE, and vcrA, bvcA and etnE for VC and no prove of relationship between cis-1,2-DCE or VC and the methanotroph functional genes. Analysis of hydrochemical parameters indicated that aerobic oxidation of chloroethenes by ethenotrophs may take place under a range of redox conditions of aquifers and coincide with high ethene and VC concentrations.

Engineered in situ biogeochemical transformation as a secondary treatment following ISCO - A field test

ISCO using activated sodium persulphate is a widely used technology for treating chlorinated solvent source zones. In sensitive areas, however, high groundwater sulphate concentrations following treatment may be a drawback. In situ biogeochemical transformation, a technology that degrades contaminants via reduced iron minerals formed by microbial activity, offers a potential solution for such sites, the bioreduction of sulphate and production of iron sulphides that abiotically degrade chlorinated ethenes acting as a secondary technology following ISCO. This study assesses this approach in the field using hydrochemical and molecular tools, solid phase analysis and geochemical modelling. Following a neutralisation and bioaugmentation, favourable conditions for iron- and sulphate-reducers were created, resulting in a remarkable increase in their relative abundance. The abundance of dechlorinating bacteria (Dehalococcoides mccartyi, Dehalobacter sp. and Desulfitobacterium spp.) remained low throughout this process. The activity of iron- and sulphate-reducers was further stimulated through application of magnetite plus starch and microiron plus starch, resulting in an increase in ferrous iron concentration (from <LOQ to 337 mg/l), a decrease in sulphate concentration by 74-95% and production of hydrogen sulphide (from <LOQ to 25.9 mg/l). At the same time, a gradual revival of dechlorinators and an increase in ethene concentration was also observed. Tetrachloroethene and trichloroethene concentrations decreased by 98.5-99.98% and 75.4-98.5%, respectively. A decline in chlorine number indicated that biological dechlorination contributed to CVOC removal. This study brings new insights into biogeochemical processes that, when properly engineered, could provide a viable solution for secondary treatment.

Dual carbon - chlorine isotope fractionation during dichloroelimination of 1,1,2-trichloroethane by an enrichment culture containing Dehalogenimonas sp

Chlorinated ethanes are frequent groundwater contaminants but compound specific isotope analysis (CSIA) has been scarcely applied to investigate their degradation pathways. In this study, dual carbon and chlorine isotope fractionation was used to investigate for the first time the anoxic biodegradation of 1,1,2-trichloroethane (1,1,2-TCA) using a Dehalogenimonas-containing culture. The isotopic fractionation values obtained for the biodegradation of 1,1,2-TCA were ɛ_C = -6.9 ± 0.4‰ and ɛ_Cl = -2.7 ± 0.3‰. The detection of vinyl chloride (VC) as unique byproduct and a closed carbon isotopic mass balance corroborated that dichloroelimination was the degradation pathway used by this strain. Combining the values of δ¹³C and δ³⁷Cl resulted in a dual element C-Cl isotope slope of Λ = 2.5 ± 0.2‰. Investigation of the apparent kinetic isotope effects (AKIEs) expected for cleavage of a CCl bond showed an important masking of the intrinsic isotope fractionation. Theoretical calculation of Λ suggested that dichloroelimination of 1,1,2-TCA was taking place via simultaneous cleavage of two CCl bonds (concerted reaction mechanism). The isotope data obtained in this study can be useful to monitor natural attenuation of 1,1,2-TCA via dichloroelimination and provide insights into the source and fate of VC in contaminated groundwaters.

Abundance of Chlorinated Solvent and 1,4-Dioxane Degrading Microorganisms at Five Chlorinated Solvent Contaminated Sites Determined via Shotgun Sequencing

Shotgun sequencing was used for the quantification of taxonomic and functional biomarkers associated with chlorinated solvent bioremediation in 20 groundwater samples (five sites), following bioaugmentation with SDC-9. The analysis determined the abundance of (1) genera associated with chlorinated solvent degradation, (2) reductive dehalogenase (RDases) genes, (3) genes associated with 1,4-dioxane removal, (4) genes associated with aerobic chlorinated solvent degradation, and (5) D. mccartyi genes associated with hydrogen and corrinoid metabolism. The taxonomic analysis revealed numerous genera previously linked to chlorinated solvent degradation, including Dehalococcoides, Desulfitobacterium, and Dehalogenimonas. The functional gene analysis indicated vcrA and tceA from D. mccartyi were the RDases with the highest relative abundance. Reads aligning with both aerobic and anaerobic biomarkers were observed across all sites. Aerobic solvent degradation genes, etnC or etnE, were detected in at least one sample from each site, as were pmoA and mmoX. The most abundant 1,4-dioxane biomarker detected was Methylosinus trichosporium OB3b mmoX. Reads aligning to thmA or Pseudonocardia were not found. The work illustrates the importance of shotgun sequencing to provide a more complete picture of the functional abilities of microbial communities. The approach is advantageous over current methods because an unlimited number of functional genes can be quantified.

Thermally enhanced in situ bioremediation of groundwater contaminated with chlorinated solvents - A field test

In situ bioremediation (ISB) using reductive dechlorination is a widely accepted but relatively slow approach compared to other technologies for the treatment of groundwater contaminated by chlorinated ethenes (CVOCs). Due to the known positive kinetic effect on microbial metabolism, thermal enhancement may be a viable means of accelerating ISB. We tested thermally enhanced ISB in aquifers situated in sandy saprolite and underlying fractured granite. The system comprised pumping, heating and subsequent injection of contaminated groundwater aiming at an aquifer temperature of 20-30°C. A fermentable substrate (whey) was injected in separate batches. The test was monitored using hydrochemical and molecular tools (qPCR and NGS). The addition of the substrate and increase in temperature resulted in a rapid increase in the abundance of reductive dechlorinators (e.g., Dehalococcoides mccartyi, Dehalobacter sp. and functional genes vcrA and bvcA) and a strong increase in CVOC degradation. On day 34, the CVOC concentrations decreased by 87% to 96% in groundwater from the wells most affected by the heating and substrate. On day 103, the CVOC concentrations were below the LOQ resulting in degradation half-lives of 5 to 6days. Neither an increase in biomarkers nor a distinct decrease in the CVOC concentrations was observed in a deep well affected by the heating but not by the substrate. NGS analysis detected Chloroflexi dechlorinating genera (Dehalogenimonas and GIF9 and MSBL5 clades) and other genera capable of anaerobic metabolic degradation of CVOCs. Of these, bacteria of the genera Acetobacterium, Desulfomonile, Geobacter, Sulfurospirillum, Methanosarcina and Methanobacterium were stimulated by the substrate and heating. In contrast, groundwater from the deep well (affected by heating only) hosted representatives of aerobic metabolic and aerobic cometabolic CVOC degraders. The test results document that heating of the treated aquifer significantly accelerated the treatment process but only in the case of an abundant substrate.