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.

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.

Process-based modeling of electrokinetic-enhanced bioremediation of chlorinated ethenes.

This study presents a process-based modeling analysis of electrokinetic-enhanced bioremediation (EK-Bio) to illuminate the complex interactions between physical, electrostatic and biogeochemical processes occurring during the application of this remediation technique. The features of the proposed model include: (i) multidimensional electrokinetic transport in saturated porous media by electromigration and electroosmosis, (ii) charge interactions, (iii) degradation kinetics, (iv) microbial populations dynamics of indigenous and specialized exogenous degraders, (v) mass transfer limitations, and (vi) geochemical reactions. A scenario modeling investigation is presented, which was inspired by an EK-Bio pilot application conducted in a clayey aquitard at the Skuldelev site (Denmark) contaminated by chlorinated ethenes.

Cross-borehole tomography with full-decay spectral time-domain induced polarization for mapping of potential contaminant flow-paths.

Soil contamination from industrial activities is a large problem in urban areas worldwide. Understanding the spreading of contamination to underlying aquifers is crucial to make adequate risk assessments and for designing remediation actions. A large part of the northern hemisphere has quaternary deposits consisting of glacial clayey till.

Electrokinetics applied in remediation of subsurface soil contaminated with chlorinated ethenes - A review.

Electrokinetics is being applied in combination with common insituremediation technologies, e.g. permeable reactive barriers, bioremediation and in-situ chemical oxidation, to overcome experienced limitations in remediation of chlorinated ethenes in low-permeable subsurface soils.

A Bayesian belief network approach for assessing uncertainty in conceptual site models at contaminated sites.

A key component in risk assessment of contaminated sites is in the formulation of a conceptual site model (CSM). A CSM is a simplified representation of reality and forms the basis for the mathematical modeling of contaminant fate and transport at the site. The CSM should therefore identify the most important site-specific features and processes that may affect the contaminant transport behavior at the site.

Risk-based prioritization of ground water threatening point sources at catchment and regional scales.

Contaminated sites threaten ground water resources all over the world. The available resources for investigation and remediation are limited compared to the scope of the problem, so prioritization is crucial to ensure that resources are allocated to the sites posing the greatest risk. A flexible framework has been developed to enable a systematic and transparent risk assessment and prioritization of contaminant point sources, considering the local, catchment, or regional scales (Danish EPA, 2011, 2012).

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.

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.

Natural attenuation processes in landfill leachate plumes at three Danish sites.

This article provides an overview of comprehensive core and fringe field studies at three Danish landfill sites. The goal of the research activities is to provide a holistic description of core and fringe attenuation processes for xenobiotic organic compounds in landfill leachate plumes. The approach used is cross-disciplinary, encompassing integration of field-scale observations at different scales, field injection experiments, laboratory experiments, and reactive solute transport modeling.

Risk assessment and prioritisation of contaminated sites on the catchment scale.

Contaminated sites pose a significant threat to groundwater resources worldwide. Due to limited available resources a risk-based prioritisation of the remediation efforts is essential. Existing risk assessment tools are unsuitable for this purpose, because they consider each contaminated site separately and on a local scale, which makes it difficult to compare the impact from different sites.

Fringe-controlled natural attenuation of phenoxy acids in a landfill plume: integration of field-scale processes by reactive transport modeling.

Data obtained from a field study of an aquifer contaminated by landfill leachate and related laboratory experiments suggest that natural attenuation of phenoxy acid herbicides such as mecoprop (MCPP) occurs in the transition zone between the anaerobic plume core and the overlying aerobic water body. The location of this transition zone is assumed to be primarily controlled by vertical transverse dispersion processes occurring downstream of the pollution source. A reactive transport modeling study was carried out to evaluate this conceptual model.

Oxygen-enhanced biodegradation of phenoxy acids in ground water at contaminated sites.

The effects of adding oxygen to anaerobic aquifer materials on biodegradation of phenoxy acid herbicides were studied by laboratory experiments with aquifer material from two contaminated sites (a former agricultural machinery service and an old landfill). At both sites, the primary pollutants were phenoxy acids and related chlorophenols. It was found that addition of oxygen enhanced degradation of the six original phenoxy acids and six original chlorophenols.

Identification of a reactive degradation zone at a landfill leachate plume fringe using high resolution sampling and incubation techniques.

Vertical small-scale variation in phenoxy acid herbicide degradation across a landfill leachate plume fringe was studied using laboratory degradation experiments. Sediment cores (subdivided into 5 cm segments) were collected in the aquifer and the sediment and porewater were used for microcosm experiments (50 experiments) and for determination of solid organic carbon, solid-water partitioning coefficients, specific phenoxy acid degraders and porewater chemistry. Results from a multi-level sampler installed next to the cores provided information on the plume position and oxygen concentration in the groundwater.

Can degradation products be used as documentation for natural attenuation of phenoxy acids in groundwater?

In situ indicators of degradation are important tools in the demonstration of natural attenuation. A literature survey on the production history of phenoxy acids and degradation pathways has shown that metabolites of phenoxy acid herbicides also are impurities in the herbicide products, making the bare presence of these compounds useless as in situ indicators. These impurities can make up more than 30% of the herbicides.

In situ exposure to low herbicide concentrations affects microbial population composition and catabolic gene frequency in an aerobic shallow aquifer.

The aim of this study was to evaluate how the in situ exposure of a Danish subsurface aquifer to phenoxy acid herbicides at low concentrations (<40 micro g l(-1)) changes the microbial community composition. Sediment and groundwater samples were collected inside and outside the herbicide-exposed area and were analyzed for the presence of general microbial populations, Pseudomonas bacteria, and specific phenoxy acid degraders. Both culture-dependent and culture-independent methods were applied.

Effect of exposure history on microbial herbicide degradation in an aerobic aquifer affected by a point source.

The effects of in situ exposure to low concentrations (micrograms per liter) of herbicides on aerobic degradation of herbicides in aquifers were studied by laboratory batch experiments. Aquifer material and groundwater were collected from a point source with known exposure histories to the herbicides mecoprop (MCPP), dichlorprop, BAM, bentazone, isoproturon, and DNOC. Degradation of the phenoxy acids, mecoprop and dichlorprop, was observed in five of six sampling points from within the plume.