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Due to its adverse health and environmental impacts, groundwater contamination by toxic organic compounds such as chlorinated solvents is a global concern. The slow-release permanganate gel (SRP-G) is a mixture of potassium permanganate (KMnO 4 ) and colloidal silica solution. The SRP-G is designed to radially spread after injection via wells, gelate in situ to form gel barriers containing permanganate (MnO 4 - to treat plumes of chlorinated solvents in groundwater. This study aimed to characterize the effects of temperature on the dynamics of SRP-G in saturated porous media. In gelation batch tests, the viscosity of ambient-temperature (24 °C) SRP-G with 30 g/L-KMnO 4 - to treat plumes of chlorinated solvents in groundwater. This study aimed to characterize the effects of temperature on the dynamics of SRP-G in saturated porous media. In gelation batch tests, the viscosity of ambient-temperature (24 °C) SRP-G with 30 g/L-KMnO 4 was 21 cP at 70 min, 134 cP at 176 min, and peaked at 946 cP to solidification at 229 min. The viscosity of low-temperature (4 °C) SRP-G with 30 g/L-KMnO 4 was 71 cP at 273 min, 402 cP at 392 min, and peaked at 818 cP to solidification at 485 min. A similar pattern, e.g., increased gelation lag time with low-temperature SRP-G, was observed for SRP-Gs with 40 g/L, 50 g/L, and 60 g/L KMnO 4 gel barriers can be increased at low temperatures. The low-temperature SRP-G scheme can be useful for treating large or dilute dissolved plumes of chlorinated solvents or other pollutants in groundwater.4 (aq), 0.2 mL/min, 2 mm/min, and 38 h for ambient-temperature SRP-G, and 0.4 mL/min, 16 mm/min, and 115 h for low-temperature SRP-G, respectively. These results indicated that the injectability, injection rate, and gelation lag time of SRP-G and the size, release rate, and release duration of MnO 4 - gel barriers can be increased at low temperatures. The low-temperature SRP-G scheme can be useful for treating large or dilute dissolved plumes of chlorinated solvents or other pollutants in groundwater., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Biodegradation and biotransformation of contaminants in groundwater commonly occurs naturally. However, natural biodegradation rates can be slow leading to elongated contaminant plumes and prolonged risks that demand greater remedial intervention. Enhancement of the biodegradation of contaminants in groundwater can be induced by the addition of amendments to change the geochemical conditions to those that are more favorable for indigenous or added biota. Enhancing biodegradation requires collocation of the contaminant of concern with the 'right' microbial communities under the 'right' geochemical conditions, so that the microbiota thrive and bio-transform, degrade or lock up the contaminant of interest. This is most easily achievable at laboratory or bench scale where mixing is easily performed, and mass transfer limitations are minimized. However, inducing such changes at field scale in aquifers is non-trivial - amendments do not easily mix into groundwater because it is a laminar (non-turbulent) and low-energy flow environment. Bioaugmentation of cultured or genetically modified organisms have also been considered to add to groundwater to enhance contaminant degradation rates. Here we provide an overview of research studies over approximately 40 years that highlight the progression of understanding from natural biodegradation of plumes in groundwater to active bioremediation efforts that have been variably successful at field scale. Investigated contaminants providing insights include petroleum hydrocarbons, chlorinated and brominated hydrocarbons, ammonium, metals, munition compounds, atrazine and per- and polyfluorinated alkyl substances. The redox and electron acceptor/donor conditions that are inducive to biodegradation for a range of contaminants are highlighted. Biodegradation is challenged by the availability of electron donors/acceptors in the core of plumes and on plume fringes. Cases for bioaugmentation are identified. A long history of investigations provides examples of the importance of amendment delivery mechanisms, scale-up from laboratory to field, and field-scale demonstration of the effectiveness of groundwater bioremediation technologies. Advantages and disadvantages of remedial approaches are tabulated. The value and contributions of integrative modelling advances are identified. The literature review and example cases provide a deep understanding of what scale of bioremediation might be achievable for groundwater plumes. Limitations to bioremediation strategies outlined here will help direct future efforts. Addressing the sources of groundwater plumes as well as bioremediation of the plume itself will achieve more effective outcomes. Twelve 'lessons learnt' are synthesized from the review., Competing Interests: The author declares no conflict of interest., (© 2023 The Author(s). Published by IMR Press.)

In this work, chloride ions were used as conservative tracers and supplemented with conservative amounts of chloroethenes (PCE, TCE, Cis-DCE, 1,1-DCE), chloroethanes (1,1,1-TCA, 1,1-DCA), and the carbon isotope ratios of certain compounds, the most representative on the sites studied, which is a novelty compared to the optimization methods developed in the scientific literature so far. A location of the potential missing sources is then proposed in view of the balances of the calculated mixing fractions. A test of the influence of measurement errors on the results shows that the uncertainties in the calculation of the mixture fractions are less than 11%, indicating that the source identification method developed is a robust tool for identifying sources of chlorinated solvents in groundwater., (© 2023. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

This study examined soil, sediment and groundwater microbial communities for a set of key functional genes important for contaminant biodegradation. This involved PICRUSt2 (Phylogenetic Investigation of Communities by Reconstruction of Unobserved States) predictions based on 16S rRNA gene amplicon datasets from three separate studies with different inocula and incubation conditions, as follows: aerobic soils, oxygen-limited microcosms containing sediments and groundwater, as well as methanogenic microcosms with different inocula. PICRUSt2 predicts functional profiles of microbial communities based on marker gene (16S rRNA gene) data. The relative abundances of genera previously associated with the biodegradation of chlorinated solvents/metabolites and/or 1,4-dioxane were also determined. Predicted values for each functional gene varied between the three datasets. In all, values were high for propane monooxygenase and low for soluble methane monooxygenase. Common phylotypes associated with propane monooxygenase in two of the three datasets included Mycobacterium, Rhodococcus and Pseudonocardia. Toluene monooxygenase predicted values were greater in the oxygen-limited microcosms compared to the other two datasets. The methanogenic microcosms exhibited the highest predicted values for particulate methane/ammonia monooxygenase. The most common genera detected, previously reported as chlorinated solvents/metabolites and/or 1,4-dioxane degraders, included Pseudomonas, Sphingomonas, Rhodococcus and Rhodanobacter. Eighteen of the queried genera were not detected. As expected, more potential anaerobic degrading genera were found in the oxygen-limited and methanogenic microcosms compared to the aerobic soils. The results provide key insights as to which genes and genera may be important for biodegradation over a range of inocula and redox conditions., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare no conflict of interest., (Copyright © 2022 Elsevier B.V. All rights reserved.)

This study aims to precisely determine the effective diffusion coefficients of chlorohydrocarbons in low permeable units under in-situ field conditions. To this end, two controlled release field experiments using TCE and PCE as dense non-aqueous phase liquids (DNAPLs) were conducted in two natural clayey deposits. Several months to years after the controlled DNAPL release, highly resolved concentration profiles were determined for the chlorohydrocarbons that had diffused into the clayey deposits. Effective diffusion coefficients for TCE and PCE were then determined by calibrating a 3D numerical and 1D analytical model, respectively, to the measured high-resolution concentration profiles. The simulations revealed that the effective diffusion coefficients vary by as much as a factor of four within the same low permeability unit being consistent with observed small-scale heterogeneities. The determined chlorohydrocarbon effective diffusion coefficients were further used to determine the equivalent thickness of DNAPL that would completely dissolve in an idealized, parallel-plate fracture by diffusion transport into clayey deposits for the time periods of the controlled release field experiments. The equivalent TCE and PCE DNAPL film thicknesses ranged between 36 and 581 μm, respectively, comparable and exceeding fracture apertures measured in naturally fractured clay rich deposits. Hence, films of DNAPL initially contained within fractures in clay-rich deposits can completely dissolve away within a few months to a few years due to diffusion. This stored contaminant mass poses a risk to adjacent aquifers if it is re-released due to diffusion out of the matrix after source depletion or remediation., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

Chlorinated ethene (CE) groundwater contamination is commonly treated through anaerobic biodegradation (i.e., reductive dechlorination) either as part of an engineered system or through natural attenuation. Aerobic biodegradation has also been recognized as a potentially significant pathway for the removal of the lower CEs cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC). However, the role of aerobic biodegradation under low oxygen conditions typical of contaminated groundwater is unclear. Bacteria capable of aerobic VC biodegradation appear to be common in the environment, while aerobic biodegradation of cDCE is less common and little is known regarding the organisms responsible. In this study, we investigate the role of aerobic cDCE and VC biodegradation in a mixed contaminant plume (including CEs, BTEX, and ketones) at Naval Air Station North Island, Installation Restoration Site 9. Sediment and groundwater collected from the plume source area, mid-plume, and shoreline were used to prepare microcosms under fully aerobic (8 mg/L dissolved oxygen (DO)) and suboxic (< 1 mg/L DO) conditions. In the shoreline microcosms, VC and cDCE were rapidly degraded under suboxic conditions (100% and 77% removal in < 62 days). In the suboxic VC microcosms, biodegradation was associated with a > 5 order of magnitude increase in the abundance of functional gene etnE, part of the aerobic VC utilization pathway. VC and cDCE were degraded more slowly under fully aerobic conditions (74% and 30% removal) in 110 days. High-throughput 16S rRNA and etnE sequencing suggest the presence of novel VC- and cDCE-degrading bacteria. These results suggest that natural aerobic biodegradation of cDCE and VC is occurring at the site and provide new evidence that low (< 1 mg/L) DO levels play a significant role in natural attenuation of cDCE and VC., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

This study contrasts the use of high-resolution passive sampling and traditional groundwater monitoring wells (GWMW) to characterize a chlorinated solvent site and assess the effectiveness of a biowall (mulch, compost and sand) that was installed to remediate trichloroethene (TCE), the primary contaminant of concern. High-resolution passive profilers (HRPPs) were direct driven hydraulically upgradient, within, and hydraulically downgradient of the biowall and in close proximity to existing GWMWs. Compared with hydraulically upgradient locations, the biowall was highly reducing, there were higher densities of bacteria/genes capable of reductive dechlorination, and TCE was being reductively transformed, but not completely, as cis-1,2-dichloroethene (cis-DCE) was detected within and hydraulically downgradient of the biowall. However, based on the high-resolution data, there were a number of important findings which were not discoverable using data from GWMWs alone. Data from the HRPPs indicate that the biowall was completely transforming TCE to ethene (C 2 H 4 ) except within a high velocity interval, where the concentrations were reduced, but breakthrough of cis-DCE was apparent. Hydraulically upgradient of the biowall, concentrations of TCE increased with depth where a very low permeability zone exists that will likely remain as a long-term source. In addition, although low concentrations of cis-DCE were present downgradient of the biowall, surfacing into a downgradient stream was not detected. This study demonstrates the advantages of high-resolution passive sampling of aquifers to assess the performance of remediation techniques compared to traditional methods such as GWMWs., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

Monitored Natural Attenuation (MNA) is a preferred remedy for sites contaminated with 1,4-dioxane due to its low cost and limited environmental impacts compared to active remediation. Having a robust estimate of the rate at which biodegradation occurs is an essential component of assessing MNA. In this study, an assay was developed using 14 C-labeled 1,4-dioxane to measure rate constants for biodegradation based on accumulation of 14 C products. Purification of the 14 C-1,4-dioxane stock solution lowered the level of 14 C impurities to below 1% of the total 14 C activity. This enabled determination of rate constants in groundwater as low as 0.0021 yr -1 , equating to a half-life greater than 300 years. Of the 54 groundwater samples collected from 10 sites in the US, statistically significant rate constants were determined with the 14 C assay for 24. The median rate constant was 0.0138 yr -1 (half-life = 50 yr); the maximum rate constant was 0.367 yr -1 (half-life = 1.9 yr). The results confirmed that biodegradation of 1,4-dioxane is occurring at 9 of the 10 sites sampled, albeit with considerable variability in the level of activity. The specificity of the assay was confirmed using acetylene and the absence of oxygen to inhibit monooxygenases., (Copyright © 2021 Elsevier B.V. All rights reserved.)

There is a growing need to assess long-term impacts of active remediation strategies on treated aquifers. A variety of biogeochemical alterations can result from interactions of the amendment with the aquifer, conceivably leading to a geophysical signal associated with the long-term alteration of an aquifer. This concept of post-remediation geophysical assessment was investigated in a shallow, chlorinated solvent-contaminated aquifer six to eight years after amendment delivery. Surface resistivity imaging and cross-borehole resistivity and induced polarization (IP) imaging were performed on a transect that spanned treated and untreated zones of the aquifer. Established relationships between IP parameters and surface electrical conductivity were used to predict vertical profiles of electrolytic conductivity and surface conductivity from the inverted cross-borehole images. Aqueous geochemistry data, along with natural gamma and magnetic susceptibility logs, were used to constrain the interpretation. The electrical conductivity structure determined from surface and borehole imaging was foremost controlled by the electrolytic conductivity of the interconnected pore space, being linearly related to fluid specific conductance. The electrolytic conductivity (and thus the conductivity images alone) did not discriminate between treated and untreated zones of the aquifer. In contrast, inverted phase angles and surface conductivities did discriminate between treated and untreated zones of the aquifer, with the treated zone being up to an order of magnitude more polarizable in places. Supporting aqueous chemistry and borehole logging datasets indicate that this geophysical signal from the long-term impact of the remediation on the aquifer is most likely associated with the formation of polarizable, dispersed iron sulfide minerals., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Bioremediation through the injection of electron donors and bacterial cultures is effective at treating chlorinated solvent contamination. However, it has had limited application in low permeability zones where amendments cannot be delivered successfully. This field-scale study investigated the application of electrokinetics to enhance the delivery of lactate at a clay site contaminated with chlorinated solvents. Groundwater and soil samples were collected before, during and for 1 year after the 71-day field test and analyzed for a wide suite of chemical and biological parameters. Lactate was successfully delivered to all monitoring locations. Lactate emplacement resulted in the stimulation of bacterial populations, specifically within the phylum Firmicutes, which contains fermenters and strict anaerobes. This likely led to biodegradation, as the field trial resulted in significant decreases in both soil and aqueous phase chlorinated solvent concentrations. Contaminant decreases were also partially attributable to dilution, given evidence of some advective lactate flux. This research provides evidence that electrokinetically-enhanced bioremediation has potential as a treatment strategy for contaminated low permeability strata., (Copyright © 2021. Published by Elsevier Ltd.)

A site in mid-western Sweden contaminated with chlorinated solvents originating from a previous dry cleaning facility, was investigated using conventional groundwater analysis combined with compound-specific isotope data of carbon, microbial DNA analysis, and geoelectrical tomography techniques. We show the value of this multidisciplinary approach, as the different results supported each interpretation, and show where natural degradation occurs at the site. The zone where natural degradation occurred was identified in the transition between two geological units, where the change in hydraulic conductivity may have facilitated biofilm formation and microbial activity. This observation was confirmed by all methods and the examination of the impact of geological conditions on the biotransformation process was facilitated by the unique combination of the applied methods. There is thus significant benefit from deploying an extended array of methods for these investigations, with the potential to reduce costs involved in remediation of contaminated sediment and groundwater.

In the densely urbanised Milan Metropolitan area (northern Italy), the long history of anthropogenic activities still exerts a significant pressure on groundwater resource. One of the most serious threats to the water quality of urban aquifers is attributed to diffuse contamination, which is caused by a series of unknown small sources (i.e., multiple point sources) distributed over large areas. In the study area and in many industrialised regions of the world, tetrachloroethylene [PCE], trichloroethylene [TCE] and hexavalent chromium [Cr(VI)] represent the common example of long-standing and persistent pollution in groundwater. In the Milan Metropolitan area, high levels of PCE + TCE and Cr(VI) were detected in the shallow aquifer as well as in the deep aquifer. To assess and map the shallow and deep aquifers susceptibility to PCE + TCE and Cr(VI) contamination at a regional scale, the Weights of Evidence modelling technique has been applied. This method has been used to objectively evaluate the spatial correlation between the high presence of these pollutants in each aquifer and hydrogeological and land use factors that can potentially influence the contamination. Moreover, the results allowed us to quantify on a large scale the effect that preferential flowpaths, due to both thickness variation in the aquitard and the areal density of multi aquifer wells, have in reducing the protection of the underlying deep aquifer. The end-products of the study constitute a key tool to be used by water-resource managers and decision-makers for the improvement of groundwater management and protection strategies., (Copyright © 2021 Elsevier B.V. All rights reserved.)

In situ chemical oxidation (ISCO) has demonstrated success in remediating soil and groundwater contaminated with chlorinated volatile organic compounds (CVOCs). However, its performance is often hindered in low-permeability or heterogeneous media due to an inability to effectively deliver the oxidants. This field-scale study investigated the novel approach of applying electrokinetics (EK) to enhance the delivery of persulfate in a low-permeability media and the ability of electrical resistance heating (ERH) to thermally activate the delivered persulfate. Results showed that 40% of the mass of total sulfur delivered was due to EK mechanisms, demonstrating that EK has the potential to enhance oxidant delivery. ERH may have activated some of the persulfate, but catalytic reactions with reduced forms of iron likely resulted in appreciable persulfate decomposition prior to ERH. Significant decreases (>80%) in the aqueous concentration of CVOCs was observed before and after ERH initiation, attributed to in situ transformation and physical processes (e.g., dilution). In situ transformation of CVOCs was assessed by compound-specific isotope analysis (CSIA) of 1,2-dichloroethane (1,2-DCA) samples collected after ERH application. Enrichment of 13 C was only measured in the well with appreciable persulfate breakthrough, confirming dechlorination of 1,2-DCA. Results from this field study demonstrate that EK and ERH can be used for persulfate delivery and activation for remediation of CVOCs in low-permeability media., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:, (Copyright © 2020 Elsevier Ltd. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relavtionships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Inc. All rights reserved.)

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

Vinyl chloride (VC) is a common groundwater contaminant and known human carcinogen. Three major bacterial guilds are known to participate in VC biodegradation: aerobic etheneotrophs and methanotrophs, and anaerobic organohalide-respiring VC-dechlorinators. We investigated the spatial relationships between functional genes representing these three groups of bacteria (as determined by qPCR) with chlorinated ethene concentrations in a surficial aquifer at a contaminated site. We used cryogenic soil coring to collect high-resolution aquifer sediment samples and to preserve sample geochemistry and nucleic acids under field conditions. All samples appeared to be anaerobic (i.e., contained little to no dissolved oxygen). VC biodegradation associated functional genes from etheneotrophs (etnC and/or etnE), methanotrophs (mmoX and/or pmoA), and anaerobic VC-dechlorinators (bvcA and/or vcrA) coexisted in 48% of the samples. Transcripts of etnC/etnE and bvcA/vcrA were quantified in contemporaneous groundwater samples, indicating co-located gene expression. Functional genes from etheneotrophs and anaerobic VC-dechlorinators were correlated to VC concentrations in the lower surficial aquifer (p < 0.05). Methanotroph functional genes were not correlated to VC concentrations. Cryogenic soil coring proved to be a powerful tool for capturing high-spatial resolution trends in geochemical and nucleic acid data in aquifer sediments. We conclude that both aerobic etheneotrophs and anaerobic VC-dechlorinators may play a significant role in VC biodegradation in aquifers that have little dissolved oxygen., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Dissolved phase contaminants, transported by diffusion into the low permeability matrix of fractured sedimentary rock, pose a challenge to groundwater cleanup efforts because this stored mass may persist even when the upgradient source zone is removed. In this context, if contaminant biodegradation takes place within the low permeability matrix, plume persistence may be substantially reduced. Therefore, it is important to characterize microbial communities within the low permeability, rock matrix pores, instead of only from groundwater samples, which represent biomass from fast flowing fractures. This research relies on depth-discrete data from both core and groundwater samples collected from two locations representing a mid-plume and plume front condition within an aged, mixed organic contaminant plume in a sedimentary rock aquifer. Results from multiple analyte measurements on rock and groundwater indicate that biodegradation in the lower permeability matrix of fractured sedimentary rocks and the microbial consortia is spatially variable due to differences in hydrochemistry, redox conditions, and contaminant concentrations. Dechlorinating microorganisms were detected in the sandstone matrix at both locations, but the detected microbial diversity calculated with PCR-DGGE was significantly higher in samples collected from the core located closer to the source zone, where contaminant concentrations are higher and contaminant compositions more diverse, compared to samples from the plume front location., (Copyright © 2018 Elsevier B.V. All rights reserved.)

A novel approach using foam to manage hazardous waste was successfully demonstrated under active site conditions. The purpose of the foam was to divert groundwater flow, that would normally enter the source zone area, to reduce dissolved contaminant release to the aquifer. During the demonstration, foam was pre generated and directly injected surrounding the chlorinated solvent source zone. Despite the constraints related to the industrial activities and non-optimal position of the injection points, the applicability and effectiveness of the approach have been highlighted using multiple metrics. A combination of measurements and modelling allowed definition of the foam extent surrounding each injection point, and this appears to be the critical metric to define the success of the foam injection approach. Information on the transport of chlorinated solvents in groundwater showed a decrease of contaminant flux by a factor of 4.4 downstream of the confined area. The effective permeability reduction was maintained over a period of three months. The successful containment provides evidence for consideration of the use of foam to improve traditional flushing techniques, by increasing the targeting of contaminants by remedial agents., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Application of a low-intensity electric field is known to influence oxidation-reduction (redox) potential in a saturated matrix. In this study, such redox manipulation was attempted in at a site with contaminated aquifer. At the experiment field site, electrodes connected to a direct current (DC) source provided an electric field with an intensity of 1.82 V m -1 . Redox potentials at locations 3.0 m and 7.9 m from the cathode decreased by 111 mV and 33 mV within a few hours, respectively, indicating that reducing conditions in the aquifer may be established within the electric field. Overall, it is possible to manipulate in situ redox potential in saturated subsurface matrices by applying low-intensity electric fields.

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., (Copyright © 2017. Published by Elsevier B.V.)

Chlorinated solvents in groundwater pose threats to human health and the environment due to their carcinogenesis and bioaccumulation. These problems are often more severe in developing countries such as China. Thus, methods for chlorinated solvent-contaminated groundwater remediation are urgently needed. This study presents a technique of in situ remediation via the direct-push amendment injection that enhances the reductive dechlorination of chlorinated solvents in groundwater in the low-permeability aquifer. A field-based pilot test and a following real-world, full-scale application were conducted at an active manufacturing facility in Shanghai, China. The chlorinated solvents found at the clay till site included 1,1,1-trichloroethane (1,1,1-TCA), 1,1-dichloroethane (1,1-DCA), 1,1-dichloroethylene (1,1-DCE), vinyl chloride (VC), and chloroethane (CA). A commercially available amendment (EHC ® , Peroxychem, Philadelphia, PA) combining zero-valent iron and organic carbon was used to treat the above pollutants. Pilot test results showed that direct-push EHC injection efficiently facilitated the in situ reductive remediation of groundwater contaminated with chlorinated solvents. The mean removal rates of 1,1,1-TCA, 1,1-DCA, and 1,1-DCE at 270 days post-injection were 99.6, 99.3, and 73.3%, respectively, which were obviously higher than those of VC and CA (42.3 and 37.1%, respectively). Clear decreases in oxidation-reduction potential and dissolved oxygen concentration, and increases in Fe 2+ and total organic carbon concentration, were also observed during the monitoring period. These indicate that EHC promotes the anaerobic degradation of chlorinated hydrocarbons primarily via long-term biological reductive dechlorination, with instant chemical reductive dechlorination acting as a secondary pathway. The optimal effective time of EHC injection was 0-90 days, and its radius of influence was 1.5 m. In full-scale application, the maximum concentrations of 1,1,1-TCA and 1,1-DCA in the contaminate plume fell below the relevant Dutch Intervention Values at 180 days post-injection. Moreover, the dynamics of the target pollutant concentrations mirrored those of the pilot test. Thus, we have demonstrated that the direct-push injection of EHC successfully leads to the remediation of chlorinated solvent-contaminated groundwater in a real-world scenario. The parameters determined by this study (e.g., effectiveness, injection amount, injection depth, injection pressures, and radius of influence) are applicable to other low-permeability contaminated sites where in situ remediation by enhanced reductive dechlorination is required.

Biomolecular and hydrochemical tools were used to evaluate natural attenuation of chlorinated ethenes in a Quaternary alluvial aquifer located close to a historical source of large-scale tetrachloroethylene (PCE) contamination. Distinct stratification of redox zones was observed, despite the aquifer's small thickness (2.8 m). The uppermost zone of the target aquifer was characterised by oxygen- and nitrate-reducing conditions, with mixed iron- to sulphate-reducing conditions dominant in the lower zone, along with indications of methanogenesis. Natural attenuation of PCE was strongly influenced by redox heterogeneity, while higher levels of PCE degradation coincided with iron- to sulphate reducing conditions. Next generation sequencing of the middle and/or lower zones identified anaerobic bacteria (Firmicutes, Chloroflexi, Actinobacteria and Bacteroidetes) associated with reductive dechlorination. The relative abundance of dechlorinators (Dehalococcoides mccartyi, Dehalobacter sp.) identified by real-time PCR in soil from the lower levels supports the hypothesis that there is a significant potential for reductive dechlorination of PCE. Local conditions were insufficiently reducing for rapid complete dechlorination of PCE to harmless ethene. For reliable assessment of natural attenuation, or when designing monitoring or remedial systems, vertical stratification of key biological and hydrochemical markers should be analysed as standard, even in shallow aquifers., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Chlorinated ethenes (CE) are among the most common volatile organic compounds (VOC) that contaminate groundwater, currently representing a major source of pollution worldwide. Phytoscreening has been developed and employed through different applications at numerous sites, where it was generally useful for detection of subsurface chlorinated solvents. We aimed at delineating subsurface CE contamination at a chlor-alkali facility using tree core data that we compared with soil data. For this investigation a total of 170 trees from experimental zones was sampled and analyzed for perchloroethene (PCE) and trichloroethene (TCE) concentrations, measured by solid phase microextraction gas chromatography coupled to mass spectrometry. Within the panel of tree genera sampled, Quercus and Ulmus appeared to be efficient biomonitors of subjacent TCE and PCE contamination, in addition to the well known and widely used Populus and Salix genera. Among the 28 trees located above the dense non-aqueous phase liquid (DNAPL) phase zone, 19 tree cores contained detectable amounts of CE, with concentrations ranging from 3 to 3000 μg L -1 . Our tree core dataset was found to be well related to soil gas sampling results, although the tree coring data were more informative. Our data further emphasized the need for choosing the relevant tree species and sampling periods, as well as taking into consideration the nature of the soil and its heterogeneity. Overall, this low-invasive screening method appeared useful to delineate contaminants at a small-scale site impacted by multiple sources of chlorinated solvents., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

A sequential reductive-oxidative treatment was developed in this study in a continuous-flow bioelectrochemical reactor to address bioremediation of groundwater contaminated by trichloroethene (TCE) and less-chlorinated but still harmful intermediates, such as vinyl chloride. In order to optimize the anodic compartment, whereby the oxygen-driven microbial oxidation of TCE-daughter products occurs, abiotic batch experiments were performed with various anode materials poised at +1.20 V vs. SHE (i.e., graphite rods and titanium mesh anode coated with mixed metal oxides (MMO)) and setups (i.e., electrodes embedded within a bed of silica beads or graphite granule). The MMO anode displayed higher efficiency (>90%) for oxygen generation compared to the graphite electrodes. Additionally, the graphite bed presence adversely affects oxygen generation, likely due to the oxygen scavenging. This effect was completely eliminated by replacing the graphite granules with silica beads. The anodic setups were thereafter verified in a mentioned reactor at an applied TCE loading rate of approximately 20 μM d -1 and a hydraulic retention time of 1.4 d in each compartment. The cathode consisted of a bed of graphite granules and was potentiostatically controlled at -0.65 V vs. SHE. The best reactor performance in terms of removal efficiency (i.e., >97%), removal rate (i.e., 121.8 ± 2.7 μeq L -1 d -1 ), and the residual concentration (i.e., 5.03 ± 0.63 μeq L -1 ) of chlorinated contaminants was achieved with the MMO anode placed in a silica bed. Ecotoxicity tests performed with algae confirmed these results by showing progressive toxicity reduction from inlet to cathodic and anodic effluent using this reactor configuration., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

In 2009, we identified a TCE plume at an abandoned landfill that was located in a low permeable silty-clay aquifer. To treat the TCE, we manufactured slow-release potassium permanganate cylinders (oxidant candles) that had diameters of either 5.1 or 7.6 cm and were 91.4 cm long. In 2010, we compared two methods of candle installation by inserting equal masses of the oxidant candles (7.6-cm vs 5.1-cm dia). The 5.1-cm dia candles were inserted with direct-push rods while the 7.6-cm candles were housed in screens and lowered into 10 permanent wells. Since installation, the 7.6-cm oxidant candles have been refurbished approximately once per year by gently scraping off surface oxides. In 2012, we reported initial results; in this paper, we provide a 5-yr performance review since installation. Temporal sampling shows oxidant candles placed in wells have steadily reduced migrating TCE concentrations. Moreover, these candles still maintain an inner core of oxidant that has yet to contribute to the dissolution front and should provide several more years of service. Oxidant candles inserted by direct-push have stopped reducing TCE concentrations because a MnO2 scale developed on the outside of the candles. To counteract oxide scaling, we fabricated a second generation of oxidant candles that contain sodium hexametaphosphate. Laboratory experiments (batch and flow-through) show that these second-generation permanganate candles have better release characteristics and are less prone to oxide scaling. This improvement should reduce the need to perform maintenance on candles placed in wells and provide greater longevity for candles inserted by direct-push., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

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. These compounds are toxic and prevalent groundwater contaminants of environmental concern. The high susceptibility of chlorinated ethanes like 1,1,1-trichloroethane (1,1,1-TCA) to be transformed via different competing pathways (biotic and abiotic) complicates the assessment of their fate in the subsurface. In this study, the use of a dual C-Cl isotope approach to identify the active degradation pathways of 1,1,1-TCA is evaluated for the first 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 carbon-chlorine (C-Cl) isotope trends determined in a recent laboratory study illustrated the potential of a dual isotope approach to identify contaminant degradation pathways of 1,1,1-TCA. Compared to the dual isotope slopes (Δδ(13)C/Δδ(37)Cl) previously determined in the laboratory for dehydrohalogenation/hydrolysis (DH/HY, 0.33 ± 0.04) and oxidation by persulfate (∞), the slope determined from field samples (0.6 ± 0.2, r(2) = 0.75) is closer to the one observed for DH/HY, pointing to DH/HY as the predominant degradation pathway of 1,1,1-TCA in the aquifer. The observed deviation could be explained by a minor contribution of additional degradation processes. This result, along with the little degradation of TCE determined 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 study demonstrates that a dual C-Cl isotope approach can strongly improve the qualitative and quantitative assessment of 1,1,1-TCA degradation processes in the field., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Direct injection of reactive nanoscale zerovalent iron particles (NZVI) is considered to be a promising approach for remediation of aquifers contaminated by chlorinated organic pollutants. In this study we show that the extent of sulfidation of NZVI enhances the rate of dechlorination of trichloroethylene (TCE) compared to that by unamended NZVI, and the enhancement depends on the Fe/S molar ratio. Experiments where TCE was reacted with NZVI sulfidated to different extents (Fe/S molar ratios 0.62-66) showed that the surface-area normalized first-order TCE degradation rate constant increased up to 40 folds compared to non-sulfidated NZVI. Fe/S ratios in the range of 12-25 provided the highest TCE dechlorination rates, and rates decreased at both higher and lower Fe/S. In contrast, sulfidated NZVI exposed to water in the absence of TCE showed significantly lower hydrogen evolution rate (2.75 μmol L(-1) h(-1)) compared to that by an unamended NZVI (6.92 μmol L(-1) h(-1)), indicating that sulfidation of NZVI suppressed corrosion reactions with water. Sulfide (HS(-)) ions reacted rapidly with NZVI and X-ray photoelectron spectroscopy analyses showed formation of a surface layer of FeS and FeS2. We propose that more electrons are preferentially conducted from sulfidated NZVI than from unamended NZVI to TCE, likely because of greater binding of TCE on the reactive sites of the iron sulfide outer layer. Resuspending sulfidated NZVI in sulfide-free or sulfide containing solutions altered the TCE degradation rate constants because of changes in the FeS layer thickness. Sulfidated NZVI maintained its high reactivity in the presence of multiple mono and divalent ions and with polyelectrolyte coatings. Thus, sulfide ions in groundwater can significantly alter NZVI reactivity., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

This paper investigated the reductive dechlorination (RD) of cis-dichloroethylene (cis-DCE) (average influent 14.2±0.7 μM) by a bioelectrochemical system (BES), in the presence of real contaminated groundwater containing high levels of nitrate and sulfate. The BES enhanced both the RD and competing reactions, such as nitrate and sulfate reductions, which occurred with neither an external organic carbon source nor any inoculum other than the indigenous microbial consortia in the real groundwater. In preliminary batch tests, RD and full nitrate removal occurred after a short lag phase, whereas sulfate reduction occurred slowly and alongside the RD. Under continuous flow conditions (hydraulic retention time, HRT, 1.4 d), the competition of different electron acceptors was strongly affected by the cathodic potential in the range -550 to -750 mV vs. standard hydrogen electrode (SHE). Nitrate reduction was driven to completion at all tested cathodic potentials, whereas sulfate reduction and the RD rate increased as the cathodic potential became more negative. At -750 mV vs. SHE, strong methanogenesis was also observed and became the most important sink of electrons. The overall coulombic efficiency decreased while the potential became more negative. The RD contribution was always less than 1%. Hence, greater energy consumption was required to obtain higher RD rate and better conversion. Anodic oxidation was only observed at -750 mV vs. SHE where almost 39% of residual vinyl chloride (VC) was oxidized and the sulfate was formed back from sulfide (further contributing to electric waste)., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

The possible approaches for in situ aerobic cometabolism of aquifers and vadose zones contaminated by chlorinated solvents are critically evaluated. Bioaugmentation of resting-cells previously grown in a fermenter and in-well addition of oxygen and growth substrate appear to be the most promising approaches for aquifer bioremediation. Other solutions involving the sparging of air lead to satisfactory pollutant removals, but must be integrated by the extraction and subsequent treatment of vapors to avoid the dispersion of volatile chlorinated solvents in the atmosphere. Cometabolic bioventing is the only possible approach for the aerobic cometabolic bioremediation of the vadose zone. The examined studies indicate that in situ aerobic cometabolism leads to the biodegradation of a wide range of chlorinated solvents within remediation times that vary between 1 and 17 months. Numerous studies include a simulation of the experimental field data. The modeling of the process attained a high reliability, and represents a crucial tool for the elaboration of field data obtained in pilot tests and for the design of the full-scale systems. Further research is needed to attain higher concentrations of chlorinated solvent degrading microbes and more reliable cost estimates. Lastly, a procedure for the design of full-scale in situ aerobic cometabolic bioremediation processes is proposed., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Phytoscreening has been demonstrated at a variety of sites over the past 15 years as a low-impact, sustainable tool in delineation of shallow groundwater contaminated with chlorinated solvents. Collection of tree cores is rapid and straightforward, but low concentrations in tree tissues requires sensitive analytics. Solid-phase microextraction (SPME) is amenable to the complex matrix while allowing for solvent-less extraction. Accurate quantification requires the absence of competitive sorption, examined here both in laboratory experiments and through comprehensive examination of field data. Analysis of approximately 2,000 trees at numerous field sites also allowed testing of the tree genus and diameter effects on measured tree contaminant concentrations. Collectively, while these variables were found to significantly affect site-adjusted perchloroethylene (PCE) concentrations, the explanatory power of these effects was small (adjusted R(2) = 0.031). 90th quantile chemical concentrations in trees were significantly reduced by increasing Henry's constant and increasing hydrophobicity. Analysis of replicate tree core data showed no correlation between replicate relative standard deviation (RSD) and wood type or tree diameter, with an overall median RSD of 30%. Collectively, these findings indicate SPME is an appropriate technique for sampling and analyzing chlorinated solvents in wood and that phytoscreening is robust against changes in tree type and diameter.

The use of compound specific multi-isotope approach (C and Cl) in the characterization of a chlorinated ethenes contaminated fractured aquifer allows the identification of several sources and contaminant plumes, as well as the occurrence of biodegradation and mixing processes. The study site is located in Spain with contamination resulting in groundwater concentrations of up to 50mg/L of trichloroethene (TCE), the most abundant chlorinated ethene, and 7 mg/L of tetrachloroethene (PCE). The potential sources of contamination including abandoned barrels, an underground tank, and a disposal lagoon, showed a wide range in δ(13)C values from -15.6 to -40.5‰ for TCE and from -18.5 to -32.4‰ for PCE, allowing the use of isotope fingerprinting for tracing of the origin and migration of these contaminants in the aquifer. In contrast, there is no difference between the δ(37)Cl values for TCE in the contaminant sources, ranging from +0.53 to +0.66‰. Variations of δ(37)Cl and δ(13)C in the different contaminant plumes were used to investigate the role of biodegradation in groundwater. Moreover, the isotopic data were incorporated into a reactive transport model for determination of whether the isotope pattern observed downstream from the tank's source could be explained by the simultaneous effect of mixing and biodegradation. The results demonstrate that a multi-isotope approach is a valuable tool for characterization of complex sites such as fractured bedrock aquifer contaminated by multiple sources, providing important information which can be used by consultants and site managers to prioritize and design more successful remediation strategies., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Industrial chlorinated solvents continue to be among the most significant issues in groundwater (GW) pollution worldwide. This study assesses the effectiveness of eight novel oxidation treatments, including persulfate (PS), ferrous sulfate, sulfidated nano-zero valent iron (S-nZVI), and potassium ferrate, along with their combinations, for the potential in situ remediation of GW polluted with chlorinated solvents (1,2-dichloroethylene, trichloroethylene, and tetrachloroethylene). Our bench-scale results reveal that the combined addition of PS and S-nZVI can effectively eliminate trichloroethylene (10 µg/L), achieving removal rates of up to 80% and 92% within 1 h, respectively, when using synthetic GW. In the case of real GW, this combination achieved removal rates of 69, 99, and 92% for cis-1,2-dichloroethylene, trichloroethylene, and tetrachloroethylene, respectively, within 24 h. Therefore, this proposed remediation solution resulted in a significant reduction in the environmental risk quotient, shifting it from a high-risk (1.1) to a low-risk (0.2) scenario. Furthermore, the absence of transformation products, such as vinyl chloride, suggests the suitability of employing this solution for the in situ remediation of GW polluted with chlorinated solvents. [ABSTRACT FROM AUTHOR]

Industrial chlorinated solvents continue to be among the most significant issues in groundwater (GW) pollution worldwide. This study assesses the effectiveness of eight novel oxidation treatments, including persulfate (PS), ferrous sulfate, sulfidated nano-zero valent iron (S-nZVI), and potassium ferrate, along with their combinations, for the potential in situ remediation of GW polluted with chlorinated solvents (1,2-dichloroethylene, trichloroethylene, and tetrachloroethylene). Our bench-scale results reveal that the combined addition of PS and S-nZVI can effectively eliminate trichloroethylene (10 µg/L), achieving removal rates of up to 80% and 92% within 1 h, respectively, when using synthetic GW. In the case of real GW, this combination achieved removal rates of 69, 99, and 92% for cis-1,2-dichloroethylene, trichloroethylene, and tetrachloroethylene, respectively, within 24 h. Therefore, this proposed remediation solution resulted in a significant reduction in the environmental risk quotient, shifting it from a high-risk (1.1) to a low-risk (0.2) scenario. Furthermore, the absence of transformation products, such as vinyl chloride, suggests the suitability of employing this solution for the in situ remediation of GW polluted with chlorinated solvents.

Biodegradation and biotransformation of contaminants in groundwater commonly occurs naturally. However, natural biodegradation rates can be slow leading to elongated contaminant plumes and prolonged risks that demand greater remedial intervention. Enhancement of the biodegradation of contaminants in groundwater can be induced by the addition of amendments to change the geochemical conditions to those that are more favorable for indigenous or added biota. Enhancing biodegradation requires collocation of the contaminant of concern with the ‘right’ microbial communities under the ‘right’ geochemical conditions, so that the microbiota thrive and bio-transform, degrade or lock up the contaminant of interest. This is most easily achievable at laboratory or bench scale where mixing is easily performed, and mass transfer limitations are minimized. However, inducing such changes at field scale in aquifers is non-trivial - amendments do not easily mix into groundwater because it is a laminar (non-turbulent) and low-energy flow environment. Bioaugmentation of cultured or genetically modified organisms have also been considered to add to groundwater to enhance contaminant degradation rates. Here we provide an overview of research studies over approximately 40 years that highlight the progression of understanding from natural biodegradation of plumes in groundwater to active bioremediation efforts that have been variably successful at field scale. Investigated contaminants providing insights include petroleum hydrocarbons, chlorinated and brominated hydrocarbons, ammonium, metals, munition compounds, atrazine and per- and polyfluorinated alkyl substances. The redox and electron acceptor/donor conditions that are inducive to biodegradation for a range of contaminants are highlighted. Biodegradation is challenged by the availability of electron donors/acceptors in the core of plumes and on plume fringes. Cases for bioaugmentation are identified. A long history of investigations provides examples of the importance of amendment delivery mechanisms, scale-up from laboratory to field, and field-scale demonstration of the effectiveness of groundwater bioremediation technologies. Advantages and disadvantages of remedial approaches are tabulated. The value and contributions of integrative modelling advances are identified. The literature review and example cases provide a deep understanding of what scale of bioremediation might be achievable for groundwater plumes. Limitations to bioremediation strategies outlined here will help direct future efforts. Addressing the sources of groundwater plumes as well as bioremediation of the plume itself will achieve more effective outcomes. Twelve ‘lessons learnt’ are synthesized from the review.

Bioremediation is an active process for the detoxification of polluted ambient media employing the metabolism of microbes, while natural attenuation relies on physical, chemical and biological processes occurring without human intervention. A shallow aquifer (A0) was treated using a bioremediation approach through the amendment of whey to detoxify the most abundant contaminants: 1,1,2,2- tetrachloroethane (1,1,2,2-TeCA), perchloroethene (PCE) and trichloroethene (TCE). A deeper aquifer (A1), showing lower concentration of the contaminants, was left untreated. In A0, a concomitant decrease of more chlorinated molecules 1,1,2,2-TeCA, PCE and TCE and an increase of less halogenated molecules such as trichloroethane (1,1,2-TCA), cis-dichloroethene (cis-DCE) and vinyl chloride (VC) were observed, suggesting that a reductive dechlorination took place. In contrast, the aquifer A1 did not show a significant decrease of contaminants during this period. A metagenomic approach (shot gun and 16S rRNA gene) was then used to investigate the microbial population of the two aquifers. A massive presence of the dehalogenator Dehalococcoides mccartyi (D. mccartyi) and a spectrum of different Geobacter species were detected in A0, after the treatment. The metagenome assembly of shotgun (SG) data further indicated a significant presence of methanogenic archaea, most likely from class Methanomassiliicoccales, at a level comparable to that of D. mccartyi. Instead, A1 was characterized by the species Burkholderia, Curvibacter and Flavobacterium. These results indicate that the autochthonous microbial consortia reflected the geochemistry of the two aquifers, with a dominant population thriving in an anoxic and nutrient rich environment implicated in reductive dehalogenation in A0 and a more diverse population, not able to decompose the pollutants, in A1. [ABSTRACT FROM AUTHOR]

This study demonstrates a groundwater circulation enhanced electrobioremediation process for 1,2-dichloroethane (1,2-DCA) degradation in a simulated heterogeneous aquifer, in which a pair of electrodes were separately implanted between an injection well and an abstraction well and groundwater circulation carries anodic O2 and cathodic H2 sequentially into the aquifer forming oxidation and reduction regions. A two-dimensional tank was filled with interbedded sandy-clayed sediments to simulate the field aquifer conditions. It is expected that the interbedded clayed sediments could supply bacteria for biological degradation and also supply Fe(II) for chemical oxidation in the presence of O2 due to hydroxyl radical (•OH) production. At a constant circulation flow rate of ∼3.5 PVs/day, 1,2-DCA at an initial concentration of ∼8.07 mg/L was degraded to ∼0.65 mg/L (91.9% removal) after 33-day treatment, in which 80–100 mA current was applied to the electrodes after the initial stage of 8-day circulation. Batch experiments proved that 1,2-DCA degradation was mainly attributed to aerobic biological oxidation, with likely contribution of anaerobic reductive dechlorination in certain areas. Chemical 1,2-DCA oxidation was negligible. This study suggests that coupling groundwater circulation and separated electrolysis is effective for stimulating biological 1,2-DCA degradation in heterogeneous aquifers. [ABSTRACT FROM AUTHOR]

Petroleum hydrocarbon contaminants in groundwater are among the most impactful environmental problems in oil production in southern Iraq, especially Basra city. Petroleum hydrocarbon contaminants affect related projects surrounding the primary pollution site. Benzene, toluene, and dimethylbenzene are the most toxic pollutants affecting the removal of perchloroethene (Cl2C=CCl2) and trichloroethene (C2HCl3) in groundwater. These pollutants have high solubility in water, leading to their transport over long distances in groundwater and difficult removal. The influence of petroleum hydrocarbons on the chlorine removal of perchloroethene and trichloroethene was studied using a polytetrafluorethylene column packed with zero-valent iron (ZVI). Batch experiments were implemented to investigate the equilibrium supply of mixtures between the aqueous and solid stages in packed column systems. It was designated using the Freundlich isotherm expression, and the result showed that R2 was greater than 0.97 for benzene, toluene, and xylene. The column study noted that the reaction constant was decreased in all columns by approximately 48 % when the pore volume was between 50 and 205, which reflects the dechlorination priority of P-CE over T-CE. These findings indicate that benzene and toluene are more effective for adsorption on the ZVI particle surfaces owing to disparate influences. [ABSTRACT FROM AUTHOR]

The aim of this work was to gain insights about the feasibility of chlorinated solvents removal through biostimulated and bioaugmented biological processes in laboratory-scale permeable reactive barriers (PRBs) under anaerobic and aerobic conditions. The experimental plant consisted of two Plexiglas cylindrical columns filled with silica sand and fed with real groundwater contaminated by chlorinated solvents (mainly 1,2-dichloroethane, 1,2-DCA, at a concentration of 20 mg l−1). Column A simulated a PRB containing poly-β-hydroxybutyrate (PHB) powder as electron donor and worked under anaerobic conditions; in Column B an inlet air flow rate ensured aerobic conditions. Both columns were inoculated with dechlorinating bacterial consortia obtained by enrichment cultures from the same contaminated groundwater. Results from Column A showed that PHB can be fermented and used as a slow-releasing carbon source for sustaining reductive dechlorination, as revealed by acetate production up to 267 mg l−1 and 100% 1,2-DCA removal. The microbial community detected in Column A at the end of the experimental period was mainly enriched in sulfate reducing bacteria that could act as both fermenting and dechlorinating agents. Column B showed a slight lower 1,2-DCA removal efficiency (98%) likely related to the establishment of aerobic (co)metabolic processes. [Display omitted] • 98–100% (an)aerobic 1,2-DCA removal from a real contaminated groundwater in PRB. • PHB used as slow-release acetate source for anaerobic reductive dechlorination, 267 mg l−1. • Establishment of direct or cometabolic oxidative biodegradation in aerobic PRB. • Biodegradation confirmed by metagenomic detection of dehalogenating bacteria. • Feasibility of a bioremediation approach using bioaugmented and biostimulated PRBs. [ABSTRACT FROM AUTHOR]

Ozonation and four types of advanced oxidation processes, including O3/H2O2, O3/UV, O3/H2O2/UV, and UV/H2O2, were evaluated for the treatment of contaminated groundwater at a Superfund site in Simpsonville, South Carolina using bench-scale, batch ozone and UV apparatuses. Although the contaminants of concern were 1,4-dioxane, 1,1-dichloroethene, and trichloroethene, several other chlorinated organics as well as tetrahydrofuran were found in the groundwater samples. The O3/H2O2treatment with O3and H2O2doses of 6 and 1.5 mg/L, respectively, and the UV/H2O2treatment with UV and H2O2doses of 1,000 mJ/cm and 20 mg/L, respectively, were sufficient to degrade 200 µg/L of 1,4-dioxane, 110 µg/L of 1,1-dichloroethene, and 10 µg/L of trichloroethene below their performance standards of 10, 7, and 4 µg/L, respectively. Due to a high bromide concentration (0.35 mg/L) in the groundwater sample, bromate formation was found to be significant in ozone-based treatment, including O3/H2O2. [ABSTRACT FROM PUBLISHER]

Anaerobic, fixed film, bioreactors bioaugmented with a dechlorinating microbial consortium were evaluated as a potential technology for cost effective, sustainable, and reliable treatment of mixed chlorinated ethanes and ethenes in groundwater from a large groundwater recovery system. Bench- and pilot-scale testing at about 3 and 13,500 L, respectively, demonstrated that total chlorinated solvent removal to less than the permitted discharge limit of 100 μg/L. Various planned and unexpected upsets, interruptions, and changes demonstrated the robustness and reliability of the bioreactor system, which handled the operational variations with no observable change in performance. Key operating parameters included an adequately long hydraulic retention time for the surface area, a constant supply of electron donor, pH control with a buffer to minimize pH variance, an oxidation reduction potential of approximately −200 millivolts or lower, and a well-adapted biomass capable of degrading the full suite of chlorinated solvents in the groundwater. Results indicated that the current discharge criteria can be met using a bioreactor technology that is less complex and has less downtime than the sorption based technology currently being used to treat the groundwater. [ABSTRACT FROM AUTHOR]