Natural attenuation of sulfonamides and metabolites in contaminated groundwater - Review, advantages and challenges of current documentation techniques.

Sulfonamides are applied worldwide as antibiotics. They are emerging contaminants of concern, as their presence in the environment may lead to the spread of antibiotic resistance genes. Sulfonamides are present in groundwater systems, which suggest their persistence under certain conditions, highlighting the importance of understanding natural attenuation processes in groundwater. Biodegradation is an essential process, as degradation of sulfonamides reduces the risk of antibiotic resistance spreading. In this review, natural attenuation, and in particular assessment of biodegradation, is evaluated for sulfonamides in groundwater systems. The current knowledge level on biodegradation is reviewed, and a scientific foundation is built based on sulfonamide degradation processes, pathways, metabolites and toxicity. An overview of bacterial species and related metabolites is provided. The main research effort has focused on aerobic conditions while investigations under anaerobic conditions are lacking. The level of implementation in research is laboratory scale; here we strived to bridge towards field application and assessment, by assessing approaches commonly used in monitored natural attenuation. Methods to document contaminant mass loss are assessed to be applicable for sulfonamides, while the approach is limited by a lack of reference standards for metabolites. Furthermore, additional information is required on relevant metabolites in order to improve risk assessments. Based on the current knowledge on biodegradation, it is suggested to use the presence of substituent-containing metabolites from breakage of the sulfonamide bridge as specific indicators of degradation. Microbial approaches are currently available for assessment of microbial community's capacities, however, more knowledge is required on indigenous bacteria capable of degrading sulfonamides and on the impact of environmental conditions on biodegradation. Compound specific stable isotope analysis shows great potential as an additional in situ method, but further developments are required to analyse for sulfonamides at environmentally relevant levels. Finally, in a monitored natural attenuation scheme it is assessed that approaches are available that can uncover some processes related to the fate of sulfonamides in groundwater systems. Nevertheless, there are still unknowns related to relevant bacteria and metabolites for risk assessment as well as the effect of environmental settings such as redox conditions. Alongside, uncovering the fate of sulfonamides in future research, the applicability of the natural attenuation documentation approaches will advance, and provide a step towards in situ remedial concepts for the frequently detected sulfonamides.

Quantification of contaminant mass discharge from point sources in aquitard/aquifer systems based on vertical concentration profiles and 3D modeling.

Point sources with contaminants, such as chlorinated solvents, per- and polyfluoroalkyl substances (PFAS), or pesticides, are often located in low-permeability aquitards, where they can act as long-term sources and threaten underlying groundwater resources. We demonstrate the use of a 3D numerical model integrating comprehensive hydrogeological and contamination data to determine the contaminant mass discharge (CMD) from an aquitard into the underlying aquifer. A mature point source with a dissolved chlorinated solvent in a clayey till is used as an example. The quantitative determination is facilitated by model calibration to high-resolution vertical concentration profiles obtained by direct-push sampling techniques in the aquifer downgradient of the contaminant source zone. The concentration profiles showed a plume sinking with distance from the source characteristic for such aquitard/aquifer settings. The sinking is caused by the interplay between infiltrating water and horizontal groundwater flow. The application of 3D solute transport modeling on high-resolution profiles allowed for determining the infiltration rate, the hydraulic conductivity in the aquitard, and, ultimately, the CMD. Different source zone conceptualizations demonstrate the potential effects of fractures and sorption in source zones in aquitards on CMD development. Fractures in the aquitard had a minor influence on the current CMD determined with the presented approach. Still, fractures with hydraulic apertures larger than 10 µm were crucial for the temporal development of the CMD and plume. A thorough characterization of the source zone conditions combined with high-resolution concentration profiles and detailed modeling is valuable for shedding light on the probable future development of groundwater contamination arising from sources in aquitard/aquifer settings and evaluating remedial actions.

A novel concept for estimating the contaminant mass discharge of chlorinated ethenes emanating from clay till sites.

Interest in using contaminant mass discharge (CMD) for risk assessment of contaminated sites has increased over the years, as it accounts for the contaminant mass that is moving and posing a risk to water resources and receptors. The most common investigation of CMD involves a transect of multilevel wells; however, this is an expensive undertaking, and it is difficult to place it in the right position in a plume. Additionally, infrastructure at the site needs to be considered. To derive an initial CMD estimate at a contaminated site and to allow for the prioritization of further investigations and remedial actions, the ProfileFlux method has been developed. It is targeted at former industrial sites with a source zone in a low conductivity layer with primarily vertical flow overlying an aquifer with primarily horizontal groundwater flow. The ProfileFlux method was developed for mature chlorinated solvent plumes, typically originating from more than 30 to 50-year-old spills, as the usage of chlorinated solvents is primarily historical. Thus, it is assumed that the contaminant had time to distribute in the low conductivity layer by mainly diffusive processes. Today the contamination is continuously released to the underlying aquifer, where advection and dispersive (other than diffusive) processes are of higher importance. The approach combines high-resolution, depth-discrete vertical concentration profiles and a simple 2D flow and transport model to estimate CMD by comparing measured and simulated concentration profiles. The study presented herein includes a global sensitivity analysis, in order to identify crucial field parameters, and of particular importance in this regard are source length, groundwater flux and infiltration. The ProfileFlux method was tested at a well-examined industrial site primarily contaminated with trichloroethylene, thereby allowing a comparison between CMD from the ProfileFlux method and the traditional transect method. CMD was estimated at 117-170 g/year, when using the ProfileFlux method, against 143 g/year with the transect method, thus validating ProfileFlux method's ability to estimate CMD. In addition, applying the method identified weak points in the conceptual site model. The method will be incorporated into a user-friendly online tool directed at environmental consultants and decision-makers working on the risk assessment and prioritization of contaminated sites with the specific hydrogeological conditions of an aquifer with an overlying low permeability layer.

Dataset for laboratory treatability experiment with activated carbon and bioamendments to enhance biodegradation of chlorinated ethenes.

This dataset describes the outcome of a laboratory trichloroethene (TCE) treatability experiment with liquid activated carbon and bioamendments. The treatability experiment included unamended microcosms, bioamended microcosms with a Dehalococcoides containing culture and electron donor, and bioamended microcosms including liquid activated carbon (PlumeStop®). Data were collected frequently over an 85-day experimental period. Data were collected for the following parameters: redox sensitive species, chlorinated ethenes, non-chlorinated end-products, electron donors, compound specific isotopes, specific bacteria and functional genes. The reductive dechlorination of TCE could be described by a carbon isotope enrichment factor (εC) of -7.1 ‰. In the amended systems, the degradation rates for the TCE degradation were 0.08-0.13 d-1 and 0.05-0.09 d-1 determined by concentrations and isotope fractionation, respectively. Dechlorination of cis-DCE was limited. This dataset assisted in identifying the impact of different bioamendments and activated carbon on biodegradation of chlorinated ethenes. The dataset is useful in optimising design and setup for future laboratory and field investigations. This study provides novel information on the effect of low dose liquid activated carbon on chlorinated ethenes degradation by applying isotopic and microbial techniques, and by linking the outcome to a field case study. The data presented in this article are related to the research article "Assessment of chlorinated ethenes degradation after field scale injection of activated carbon and bioamendments: Application of isotopic and microbial analyses" (Ottosen et al., 2021).

Assessment of chlorinated ethenes degradation after field scale injection of activated carbon and bioamendments: Application of isotopic and microbial analyses.

Over the last decade, activated carbon amendments have successfully been applied to retain chlorinated ethene subsurface contamination. The concept of this remediation technology is that activated carbon and bioamendments are injected into aquifer systems to enhance biodegradation. While the scientific basis of the technology is established, there is a need for methods to characterise and quantify the biodegradation at field scale. In this study, an integrated approach was applied to assess in situ biodegradation after the establishment of a cross sectional treatment zone in a TCE plume. The amendments were liquid activated carbon, hydrogen release donors and a Dehalococcoides containing culture. The integrated approach included spatial and temporal evaluations on flow and transport, redox conditions, contaminant concentrations, biomarker abundance and compound-specific stable isotopes. This is the first study applying isotopic and microbial techniques to assess field scale biodegradation enhanced by liquid activated carbon and bioamendments. The injection enhanced biodegradation from TCE to primarily cis-DCE. The Dehalococcoides abundances facilitated characterisation of critical zones with insufficient degradation and possible explanations. A conceptual model of isotopic data together with distribution and transport information improved process understanding; the degradation of TCE was insufficient to counteract the contaminant input by inflow into the treatment zone and desorption from the sediment. The integrated approach could be used to document and characterise the in situ degradation, and the isotopic and microbial data provided process understanding that could not have been gathered from conventional monitoring tools. However, quantification of degradation through isotope data was restricted for TCE due to isotope masking effects. The combination of various monitoring tools, applied frequently at high-resolution, with system understanding, was essential for the assessment of biodegradation in the complex, non-stationary system. Furthermore, the investigations revealed prospects for future research, which should focus on monitoring contaminant fate and microbial distribution on the sediment and the activated carbon.

Natural attenuation of a chlorinated ethene plume discharging to a stream: Integrated assessment of hydrogeological, chemical and microbial interactions.

Attenuation processes of chlorinated ethenes in complex near-stream systems result in site-specific outcomes of great importance for risk assessment of contaminated sites. Additional interdisciplinary and comprehensive field research is required to enhance process understanding in these systems. In this study, several methods were combined in a multi-scale interdisciplinary in-situ approach to assess and quantify the near-stream attenuation of a chlorinated ethene plume, mainly consisting of cis-dichloroethene (cis-DCE) and vinyl chloride (VC), discharging to a lowland stream (Grindsted stream, Denmark) over a monitoring period of seven years. The approach included: hydrogeological characterisation, reach scale contaminant mass balance analysis, quantification of contaminant mass discharge, streambed fluxes of chlorinated ethenes quantified using Sediment Bed Passive Flux Meters (SBPFMs), assessment of redox conditions, temporal assessment of contaminant concentrations, microbial analysis, and compound-specific isotope analysis (CSIA). This study site exhibits a special attenuation behaviour not commonly encountered in field studies: the conversion from an initially limited degradation case (2012-16), despite seemingly optimal conditions, to one presenting notable levels of degradation (2019). Hence, this study site provides a new piece to the puzzle, as sites with different attenuation behaviours are required in order to acquire the full picture of the role groundwater-surface water interfaces have in risk mitigation. In spite of the increased degradation in the near-stream plume core, the contaminant attenuation was still incomplete in the discharging plume. A conceptualization of flow, transport and processes clarified that hydrogeology was the main control on the natural attenuation, as short residence times of 0.5-37 days restricted the time in which dechlorination could occur. This study reveals the importance of: taking an integrated approach to understand the influence of all attenuation processes in groundwater - surface water interactions; considering the scale and domain of interest when determining the main processes; and monitoring sufficiently both spatially and temporally to cover the transient conditions.

Transport of citrate and polymer coated gold nanoparticles (AuNPs) in porous media: Effect of surface property and Darcy velocity.

With the release of nanoparticles (NPs) into the subsurface, it is imperative to better understand the fate and transport of NPs in porous media. Three types of stable AuNPs were used as model NPs to investigate the impact of surface coatings (type and coverage) and water velocity on the NP transport in a porous media (column studies). The NPs were electrostatic stabilized citrate AuNPs and sterically stabilized AuNPs with amphiphilic block co-polymer (PVA-COOH) in two particle/polymer ratios (weak vs. strong stabilization). The citrate AuNPs transport was sensitive to ionic changes in the mixing front of the plume, where destabilization occurred, and will therefore depend on the size/type of release. Blocking of deposition sites by aggregates was seen to facilitate transport, whereby a higher flow velocity (larger shadow zone) also resulted in better transport. The polymeric surface coating had great impact with steric repulsion as a main force contributing to the transport of NPs in the porous media. Sufficient polymer coating was crucial to obtain highly unfavorable attachment conditions (very low α) where the enhanced NP mobility was independent of the water velocity (comparable to solute tracer). Without sufficient steric stabilization, the transport and recovery was significantly reduced compared to the solute tracer, but increased with increasing water velocity. This highlights the importance of sufficient surface coating to achieve enhanced mobility, but also the increased risk of spreading to down-gradient receptors. For the (weakly) sterically stabilized NPs, the loss of polymer through ligand exchange with the porous media negates transport.

Chlorinated ethene plume evolution after source thermal remediation: Determination of degradation rates and mechanisms.

The extent, mechanism(s), and rate of chlorinated ethene degradation in a large tetrachloroethene (PCE) plume were investigated in an extensive sampling campaign. Multiple lines of evidence for this degradation were explored, including compound-specific isotope analysis (CSIA), dual C-Cl isotope analysis, and quantitative real-time polymerase chain reaction (qPCR) analysis targeting the genera Dehalococcoides and Dehalogenimonas and the genes vcrA, bvcA, and cerA. A decade prior to this sampling campaign, the plume source was thermally remediated by steam injection. This released dissolved organic carbon (DOC) that stimulated microbial activity and created reduced conditions within the plume. Based on an inclusive analysis of minor and major sampling campaigns since the initial site characterization, it was estimated that reduced conditions peaked 4 years after the remediation event. At the time of this study, 11 years after the remediation event, the redox conditions in the aquifer are returning to their original state. However, the DOC released from the remediated source zone matches levels measured 3 years prior and plume conditions are still suitable for biotic reductive dechlorination. Dehalococcoides spp., Dehalogenimonas spp., and vcrA, bvcA, and cerA reductive dehalogenase genes were detected close to the source, and suggest that complete, biotic PCE degradation occurs here. Further downgradient, qPCR analysis and enriched δ13C values for cis-dichloroethene (cDCE) suggest that cDCE is biodegraded in a sulfate-reducing zone in the plume. In the most downgradient portion of the plume, lower levels of specific degraders supported by dual C-Cl analysis indicate that the biodegradation occurs in combination with abiotic degradation. Additionally, 16S rRNA gene amplicon sequencing shows that organizational taxonomic units known to contain organohalide-respiring bacteria are relatively abundant throughout the plume. Hydraulic conductivity testing was also conducted, and local degradation rates for PCE and cDCE were determined at various locations throughout the plume. PCE degradation rates from sampling campaigns after the thermal remediation event range from 0.11 to 0.35 yr-1. PCE and cDCE degradation rates from the second to the third sampling campaigns ranged from 0.08 to 0.10 yr-1 and 0.01 to 0.07 yr-1, respectively. This is consistent with cDCE as the dominant daughter product in the majority of the plume and cDCE degradation as the time-limiting step. The extensive temporal and spatial analysis allowed for tracking the evolution of the plume and the lasting impact of the source remediation and illustrates that the multiple lines of evidence approach is essential to elucidate the primary degradation mechanisms in a plume of such size and complexity.

Mobility of electrostatically and sterically stabilized gold nanoparticles (AuNPs) in saturated porous media.

The stability of gold nanoparticles (AuNPs) stabilized electrostatically with citrate or (electro)sterically by commercially available amphiphilic block copolymers (PVP-VA or PVA-COOH) was studied under various physicochemical conditions. Subsequently, the mobility of the AuNPs in porous media (sand) was investigated in column studies under environmental relevant physicochemical conditions. Electrostatically stabilized AuNPs were unstable under most physicochemical conditions due to the compression of the electrical double layer. Consequently, aggregation and deposition rapidly immobilized the AuNPs. Sterically stabilized AuNPs showed significantly less sensitivity towards changes in the physicochemical conditions with high stability, high mobility with negligible retardation, and particle deposition rate coefficients ranging an order of magnitude (1.5 × 10-3 to 1.5 × 10-2 min-1) depending on the type and amount of stabilizer, and thereby the surface coverage and attachment affinity. The transport of sterically stabilized AuNPs is facilitated by reversible deposition in shallow energy minima with continuous reentrainment and blocking of available attachment sites by deposited AuNPs. The stability and mobility of NPs in the environment will thereby be highly dependent on the specific stabilizing agent and variations in the coverage on the NP. Under the given experimental conditions, transport distances of the most mobile AuNPs of up to 20 m is expected. Due to their size-specific plasmonic properties, the easily detectable AuNPs are proposed as potential model or tracer particles for studying transport of various stabilized NPs under environmental conditions.

A modeling approach integrating microbial activity, mass transfer, and geochemical processes to interpret biological assays: An example for PCE degradation in a multi-phase batch setup.

The rate at which organic contaminants can be degraded in aquatic environments is not only dependent upon specific degrading bacteria, but also upon the composition of the microbial community, mass transfer of the contaminant, and abiotic processes that occur in the environment. In this study, we present three-phase batch experiments of tetrachloroethene (PCE) degradation by a consortium of organohalide-respiring bacteria, cultivated alone or in communities with iron- and/or sulfate-reducers. We developed a modeling approach to quantitatively evaluate the experimental results, comprised of chemical and biomolecular time series data. The model utilizes the IPhreeqc module to couple multi-phase mass transfer between gaseous, organic and aqueous phases with microbial and aquatic geochemical processes described using the geochemical code PHREEQC. The proposed approach is able to capture the contaminant degradation, the microbial population dynamics, the effects of multi-phase kinetic mass transfer and sample removal, and the geochemical reactions occurring in the aqueous phase. The model demonstrates the importance of aqueous speciation and abiotic reactions on the bioavailability of the substrates. The model-based interpretation allowed us to quantify the reaction kinetics of the different bacterial guilds. The model further revealed that the inclusion of sulfate-reducing bacteria lowers the rate of PCE degradation and that this effect is moderated in the presence of iron-reducing bacteria.

Identification of abiotic and biotic reductive dechlorination in a chlorinated ethene plume after thermal source remediation by means of isotopic and molecular biology tools.

Thermal tetrachloroethene (PCE) remediation by steam injection in a sandy aquifer led to the release of dissolved organic carbon (DOC) from aquifer sediments resulting in more reduced redox conditions, accelerated PCE biodegradation, and changes in microbial populations. These changes were documented by comparing data collected prior to the remediation event and eight years later. Based on the premise that dual C-Cl isotope slopes reflect ongoing degradation pathways, the slopes associated with PCE and TCE suggest the predominance of biotic reductive dechlorination near the source area. PCE was the predominant chlorinated ethene near the source area prior to thermal treatment. After thermal treatment, cDCE became predominant. The biotic contribution to these changes was supported by the presence of Dehalococcoides sp. DNA (Dhc) and Dhc targeted rRNA close to the source area. In contrast, dual C-Cl isotope analysis together with the almost absent VC (13)C depletion in comparison to cDCE (13)C depletion suggested that cDCE was subject to abiotic degradation due to the presence of pyrite, possible surface-bound iron (II) or reduced iron sulphides in the downgradient part of the plume. This interpretation is supported by the relative lack of Dhc in the downgradient part of the plume. The results of this study show that thermal remediation can enhance the biodegradation of chlorinated ethenes, and that this effect can be traced to the mobilisation of DOC due to steam injection. This, in turn, results in more reduced redox conditions which favor active reductive dechlorination and/or may lead to a series of redox reactions which may consecutively trigger biotically induced abiotic degradation. Finally, this study illustrates the valuable complementary application of compound-specific isotopic analysis combined with molecular biology tools to evaluate which biogeochemical processes are taking place in an aquifer contaminated with chlorinated ethenes.

Characterization of chlorinated solvent contamination in limestone using innovative FLUTe® technologies in combination with other methods in a line of evidence approach.

Characterization of dense non-aqueous phase liquid (DNAPL) source zones in limestone aquifers/bedrock is essential to develop accurate site-specific conceptual models and perform risk assessment. Here innovative field methods were combined to improve determination of source zone architecture, hydrogeology and contaminant distribution. The FACT™ is a new technology and it was applied and tested at a contaminated site with a limestone aquifer, together with a number of existing methods including wire-line coring with core subsampling, FLUTe® transmissivity profiling and multilevel water sampling. Laboratory sorption studies were combined with a model of contaminant uptake on the FACT™ for data interpretation. Limestone aquifers were found particularly difficult to sample with existing methods because of core loss, particularly from soft zones in contact with chert beds. Water FLUTe™ multilevel groundwater sampling (under two flow conditions) and FACT™ sampling and analysis combined with FLUTe® transmissivity profiling and modeling were used to provide a line of evidence for the presence of DNAPL, dissolved and sorbed phase contamination in the limestone fractures and matrix. The combined methods were able to provide detailed vertical profiles of DNAPL and contaminant distributions, water flows and fracture zones in the aquifer and are therefore a powerful tool for site investigation. For the limestone aquifer the results indicate horizontal spreading in the upper crushed zone, vertical migration through fractures in the bryozoan limestone down to about 16-18m depth with some horizontal migrations along horizontal fractures within the limestone. Documentation of the DNAPL source in the limestone aquifer was significantly improved by the use of FACT™ and Water FLUTe™ data.

Stable carbon isotope analysis to distinguish biotic and abiotic degradation of 1,1,1-trichloroethane in groundwater sediments.

The fate and treatability of 1,1,1-TCA by natural and enhanced reductive dechlorination was studied in laboratory microcosms. The study shows that compound-specific isotope analysis (CSIA) identified an alternative 1,1,1-TCA degradation pathway that cannot be explained by assuming biotic reductive dechlorination. In all biotic microcosms 1,1,1-TCA was degraded with no apparent increase in the biotic degradation product 1,1-DCA. 1,1,1-TCA degradation was documented by a clear enrichment in (13)C in all biotic microcosms, but not in the abiotic control, which suggests biotic or biotically mediated degradation. Biotic degradation by reductive dechlorination of 1,1-DCA to CA only occurred in bioaugmented microcosms and in donor stimulated microcosms with low initial 1,1,1-TCA or after significant decrease in 1,1,1-TCA concentration (after∼day 200). Hence, the primary degradation pathway for 1,1,1-TCA does not appear to be reductive dechlorination via 1,1-DCA. In the biotic microcosms, the degradation of 1,1,1-TCA occurred under iron and sulfate reducing conditions. Biotic reduction of iron and sulfate likely resulted in formation of FeS, which can abiotically degrade 1,1,1-TCA. Hence, abiotic degradation of 1,1,1-TCA mediated by biotic FeS formation constitute an explanation for the observed 1,1,1-TCA degradation. This is supported by a high 1,1,1-TCA (13)C enrichment factor consistent with abiotic degradation in biotic microcosms. 1,1-DCA carbon isotope field data suggest that this abiotic degradation of 1,1,1-TCA is a relevant process also at the field site.

Effects of bioaugmentation on enhanced reductive dechlorination of 1,1,1-trichloroethane in groundwater: a comparison of three sites.

Microcosm studies investigated the effects of bioaugmentation with a mixed Dehalococcoides (Dhc)/Dehalobacter (Dhb) culture on biological enhanced reductive dechlorination for treatment of 1,1,1-trichloroethane (TCA) and chloroethenes in groundwater at three Danish sites. Microcosms were amended with lactate as electron donor and monitored over 600 days. Experimental variables included bioaugmentation, TCA concentration, and presence/absence of chloroethenes. Bioaugmented microcosms received a mixture of the Dhc culture KB-1 and Dhb culture ACT-3. To investigate effects of substrate concentration, microcosms were amended with various concentrations of chloroethanes (TCA or monochloroethane [CA]) and/or chloroethenes (tetrachloroethene [PCE], trichloroethene [TCE], or 1,1-dichloroethene [1,1-DCE]). Results showed that combined electron donor addition and bioaugmentation stimulated dechlorination of TCA and 1,1-dichloroethane (1,1-DCA) to CA, and dechlorination of PCE, TCE, 1,1-DCE and cDCE to ethane. Dechlorination of CA was not observed. Bioaugmentation improved the rate and extent of TCA and 1,1-DCA dechlorination at two sites, but did not accelerate dechlorination at a third site where geochemical conditions were reducing and Dhc and Dhb were indigenous. TCA at initial concentrations of 5 mg/L inhibited (i.e., slowed the rate of) TCA dechlorination, TCE dechlorination, donor fermentation, and methanogenesis. 1 mg/L TCA did not inhibit dechlorination of TCA, TCE or cDCE. Moreover, complete dechlorination of PCE to ethene was observed in the presence of 3.2 mg/L TCA. In contrast to some prior reports, these studies indicate that low part-per million levels of TCA (< 3 mg/L) in aquifer systems do not inhibit dechlorination of PCE or TCE to ethene. In addition, the results show that co-bioaugmentation with Dhc and Dhb cultures can be an effective strategy for accelerating treatment of chloroethane/chloroethene mixtures in groundwater, with the exception that all currently known Dhc and Dhb cultures cannot treat CA.

The impact of bioaugmentation on dechlorination kinetics and on microbial dechlorinating communities in subsurface clay till.

A molecular study on how the abundance of the dechlorinating culture KB-1 affects dechlorination rates in clay till is presented. DNA extracts showed changes in abundance of specific dechlorinators as well as their functional genes. Independently of the KB-1 added, the microbial dechlorinator abundance increased to the same level in all treatments. In the non-bioaugmented microcosms the reductive dehalogenase gene bvcA increased in abundance, but when KB-1 was added the related vcrA gene increased while bvcA genes did not increase. Modeling showed higher vinyl-chloride dechlorination rates and shorter time for complete dechlorination to ethene with higher initial concentration of KB-1 culture, while cis-dichloroethene dechlorination rates were not affected by KB-1 concentrations. This study provides high resolution abundance profiles of Dehalococcoides spp. (DHC) and functional genes, highlights the ecological behavior of KB-1 in clay till, and reinforces the importance of using multiple functional genes as biomarkers for reductive dechlorination.

Identification of chlorinated solvents degradation zones in clay till by high resolution chemical, microbial and compound specific isotope analysis.

The degradation of chlorinated ethenes and ethanes in clay till was investigated at a contaminated site (Vadsby, Denmark) by high resolution sampling of intact cores combined with groundwater sampling. Over decades of contamination, bioactive zones with degradation of trichloroethene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA) to 1,2-cis-dichloroethene (cis-DCE) and 1,1-dichloroethane, respectively, had developed in most of the clay till matrix. Dehalobacter dominated over Dehalococcoides (Dhc) in the clay till matrix corresponding with stagnation of sequential dechlorination at cis-DCE. Sporadically distributed bioactive zones with partial degradation to ethene were identified in the clay till matrix (thickness from 0.10 to 0.22 m). In one sub-section profile the presence of Dhc with the vcrA gene supported the occurrence of degradation of cis-DCE and VC, and in another enriched δ(13)C for TCE, cis-DCE and VC documented degradation. Highly enriched δ(13)C for 1,1,1-TCA (25‰) and cis-DCE (-4‰) suggested the occurrence of abiotic degradation in a third sub-section profile. Due to fine scale heterogeneity the identification of active degradation zones in the clay till matrix depended on high resolution subsampling of the clay till cores. The study demonstrates that an integrated approach combining chemical analysis, molecular microbial tools and compound specific isotope analysis (CSIA) was required in order to document biotic and abiotic degradations in the clay till system.

A remediation performance model for enhanced metabolic reductive dechlorination of chloroethenes in fractured clay till.

A numerical model of metabolic reductive dechlorination is used to describe the performance of enhanced bioremediation in fractured clay till. The model is developed to simulate field observations of a full scale bioremediation scheme in a fractured clay till and thereby to assess remediation efficiency and timeframe. A relatively simple approach is used to link the fermentation of the electron donor soybean oil to the sequential dechlorination of trichloroethene (TCE) while considering redox conditions and the heterogeneous clay till system (clay till matrix, fractures and sand stringers). The model is tested on lab batch experiments and applied to describe sediment core samples from a TCE-contaminated site. Model simulations compare favorably to field observations and demonstrate that dechlorination may be limited to narrow bioactive zones in the clay matrix around fractures and sand stringers. Field scale simulations show that the injected donor is expected to be depleted after 5 years, and that without donor re-injection contaminant rebound will occur in the high permeability zones and the mass removal will stall at 18%. Long remediation timeframes, if dechlorination is limited to narrow bioactive zones, and the need for additional donor injections to maintain dechlorination activity may limit the efficiency of ERD in low-permeability media. Future work should address the dynamics of the bioactive zones, which is essential to understand for predictions of long term mass removal.

Dissolution and transport of coal tar compounds in fractured clay-rich residuum.

We investigated the dissolution and transport of organic contaminants from a crude coal tar mixture in a monolith of fractured clay-rich residuum. An electrolyte solution was eluted through the residuum monolith containing a small emplaced source of coal tar under biologically inhibited and mildly acidic conditions. Concentrations of 10 coal tar compounds, representing mono-, poly-, and heterocyclic aromatic hydrocarbons that constitute crude coal tar were monitored in the effluent over a period of 377 days. Most compounds appeared in the effluent within the first 0.1 pore volume eluted indicating the importance of rapid dissolution and transport through the fracture networks. The concentrations continued to rise but did not reach the corresponding effective solubility limit in most cases. Compounds that were less soluble and those that were more susceptible to sorption or matrix diffusion eluted at a much slower rate. Analysis of contaminant concentrations in microcore residuum samples indicated that all 10 compounds had spread throughout the entire monolith and had diffused into the fine-grained matrix between fractures. These data suggest that the predominantly fine pore structure did not appear to inhibit coal tar dissolution and subsequent transport, even though only a small portion of tar was in direct contact with fractures and macropores that control most flow.

Sorption of chlorinated solvents and degradation products on natural clayey tills.

The sorption of chlorinated solvents and degradation products on seven natural clayey till samples from three contaminated sites was investigated by laboratory batch experiments in order to obtain reliable sorption coefficients (K(d) values). The sorption isotherms for all compounds were nearly linear, but fitted by Freundlich isotherms slightly better over the entire concentration range. For chloroethylenes, tetrachloroethylene (PCE) was most strongly sorbed to the clayey till samples (K(d)=0.84-2.45Lkg(-1)), followed by trichloroethylene (TCE, K(d)=0.62-0.96Lkg(-1)), cis-dichloroethylene (cis-DCE, K(d)=0.17-0.82Lkg(-1)) and vinyl chloride (VC, K(d)=0.12-0.36Lkg(-1)). For chloroethanes, 1,1,1-trichloroethane (1,1,1-TCA) was most strongly sorbed (K(d)=0.2-0.45Lkg(-1)), followed by 1,1-dichloroethane (1,1-DCA, K(d)=0.16-0.24Lkg(-1)) and chloroethane (CA, K(d)=0.12-0.18Lkg(-1)). This is consistent with the order of hydrophobicity of the compounds. The octanol-water coefficient (logK(ow)) correlated slightly better with logK(d) values than logK(oc) values indicating that the K(d) values may be independent of the actual organic carbon content (f(oc)). The estimated logK(oc) or logK(d) for chlorinated solvents and degradation products determined by regression of data in this study were significantly higher than values determined by previously published empirical relationships. The site specific K(d) values as well as the new empirical relationship compared well with calculations on water and soil core concentration for cis-DCE and VC from the Rugårdsvej site. In conclusion, this study with a wide range of chlorinated ethenes and ethanes--in line with previous studies on PCE and TCE--suggest that sorption in clayey tills could be higher than typically expected.