Landfill leachate plumes pose serious threats to groundwater systems. In many cases, pollutant concentration gradients are steep such that plume boundaries can be missed by sparse groundwater sampling. In this work, a high-resolution transient electromagnetic (TEM) survey was conducted to produce a 3D resistivity model across an area of 240 hectares up to depths of 90 m in a sandy aquifer downgradient of an old landfill. An electrically conductive anomaly was identified in the resistivity model, and targeted groundwater sampling confirmed the presence of landfill leachate plume with elevated ionic strength related to high concentrations of inorganic pollutants (e.g., chloride). Although organic contaminants arising from the landfill such as sulfonic acid, sulfonamides, and barbiturates were also detected in groundwater samples, a clear relationship with the geophysical data was not observed. Nonetheless, a petrophysical relationship was successfully established between co-located resistivity data and groundwater electrical conductivity measurements, allowing delineation of the plume boundary. The landfill leach plume can be seen trending northwestward from the landfill. This approach demonstrates the ability of TEM mapping to rapidly detect conductive plumes at scales relevant to groundwater management. Ultimately, such applications can be useful in reducing the time and cost of contaminant management efforts.

Per- and polyfluoroalkyl substances (PFAS) leaching from landfills without liner or leachate collection pose a significant risk to groundwater and drinking water resources. This study developed a conceptual understanding of long-term PFAS leaching from legacy landfills into the subsurface by reviewing PFAS occurrence data and investigating the PFAS leaching from two landfill types: dry landfills with an unsaturated zone below, and wet landfills without one. The landfill processes were simulated in a 1D vertical PFAS landfill leaching model, followed by a 2D subsurface model with PFAS transport through the unsaturated and saturated zone. Scenario modelling evaluated the impact of sorption and infiltration on PFAS release over time. Leaching longevity was controlled by the retention processes within the landfill and the underlying unsaturated zone, landfill type, and the temporal release pattern of PFAS. Dry landfills exhibited substantial retention of long-chain PFAS due to air-water interfacial sorption, leading to prolonged leaching over decades or centuries. Contaminant mass discharge estimated downgradient of a landfill showed peak attenuation and delayed arrival of long-chain PFAS, indicating the importance of long-term monitoring and providing a tool for risk management. Landfill history, hydrological conditions and depth to the groundwater were identified as key factors controlling PFAS release and spreading and should be considered in site investigations. This study provides novel insights into PFAS leaching behaviour across different landfill types, highlighting the critical role of the unsaturated zone, and presents a comprehensive modelling framework for site-specific risk assessment from the source to a potential receptor.

Understanding the fate and transport of PFAS in the subsurface is essential for groundwater and contaminated site management. Most international studies focused on the transport in homogeneous sands, while other hydrogeological settings are less well studied. In this contribution, we investigate the impact of different glacial geological settings typically found in the Northern Hemisphere, possibly containing fractures and heterogeneities, on PFAS leaching through unsaturated glacial sediments. We have implemented a vertical cross-section model that simulates transient groundwater flow and PFAS transport through the variably-saturated zone, accounting for sorption to the solid phase and to air-water interfaces. The model was tested on measured breakthrough curve data from saturated and unsaturated laboratory column experiments considering PFAS with different chain lengths. Model parameters were obtained from a comprehensive literature review, laboratory studies, and field investigations of contaminated sites. The model was used to investigate the leaching of PFAS with different chain lengths through different setups with glacial sediment. We observed that the hydrogeological setting determines the magnitude of the air-water interfacial area and, thus, the retention of surface-active PFAS like PFOS and PFOA. Further, the model outcomes demonstrated a chromatographic separation of PFAS with different chain lengths due to different retention mechanisms. The longer-chained PFAS were retained more strongly in the unsaturated zone, while shorter-chained compounds were mobile. Low-permeability clay-rich layers and inclusions generally provided less retention for surface-active PFAS due to a typically higher water saturation and, thus, smaller interfacial area compared to high-permeability media like sands. Fractures and heterogeneities may lead to the formation of preferential flow paths and thereby a potential bypassing of the unsaturated zone, where sorption to the air-water interface could occur. On the other hand, matrix diffusion may slow the rate of plume expansion by retaining PFAS in low-permeability layers. Over time, back diffusion from the matrix can result in long-term release to the groundwater. The modeling investigations based on realistic data conducted in this study led to an improved understanding of the transport of short- and longer-chained PFAS in variably saturated glacial geological settings. Our findings allowed analyzing the influence of key parameters and processes on PFAS fate and transport.

This note contains information on dispersion and selection of dispersivity values in aquifers, with focus on its relation to the spreading of contaminants in groundwater at contaminated sites. The note assumes a basic knowledge of transport in aquifers including the advection/dispersion equation and Darcy’s law.

The widespread occurrence of micropollutants like the antibiotic sulfadimidine in the environment has become a growing concern. Compound-specific stable isotope analysis (CSIA) offers a powerful tool for tracking the fate of such pollutants, but its application is often limited by low sensitivity. To address this limitation, a large-scale solid-phase extraction method was developed to extract, enrich, and isolate sulfadimidine for δ13C- and δ34S-CSIA. Each step of the method was carefully evaluated, ensuring no detectable isotope artifacts. The limit of quantification was determined as 1.1 nmol of carbon and 1.2 nmol of sulfur directly injected on the column. Applied to groundwater samples from a contaminated site in Denmark, the method allowed for the analysis of concentrations as low as 0.17 mg/L, with a concentration factor of up to 10,000 used to enrich sulfadimidine. This is the first study to analyze δ13C and δ34S for sulfadimidine in aquifer water samples and highlights the potential of CSIA for tracking sulfadimidine transformations in contaminated water environments.

Knowledge of contaminant distribution and transport of contaminant plumes in groundwater is important for effective remediation. Tedious and expensive laboratory analyses could be supplemented with optical measurements such as fluorescence to offer a rapid alternative with the potential for on-site measurements. Here, we explore the applicability of fluorescence spectroscopy as an on-site alternative to identifying the extent of a groundwater contaminant plume in Grindsted, Denmark. We show that three abundant contaminants (sulfanilamide, sulfaguanidine, and sulfanilic acid) emit very strong, but highly similar fluorescence distinct from the naturally occurring organic matter. The limit of detection for the sum of these three contaminants was 14 and 142 μg/L using benchtop measurements and handheld sensors, respectively. We demonstrate that low-volume solid-phase extractions can be a tool to lower the detection limits through the selective enrichment of contaminants. However, the co-occurrence of natural and anthropogenic fluorescent organic matter presents a significant challenge for the reliable quantification of contaminants. The high similarity between investigated fluorescent contaminants poses a significant challenge for machine learning approaches that are commonly used to increase sensitivity and selectivity. Nonetheless, the results demonstrate how fluorescence spectroscopy can be applied as a viable indicator and classification tool to identify pharmaceutical contamination in groundwater, as well as surface waters.

Contaminated legacy sites pose a risk to the environment and human health worldwide. The accurate characterization and monitoring of subsurface contamination is crucial for effective risk assessment, management and prioritization of contaminated sites and ultimately ensures a more efficient and sustainable use of the allocated resources.Contaminant mass discharge (CMD) integrates two important features of contaminant risk: concentration and mobility. CMD is increasingly being incorporated into risk assessments of contaminated sites as an alternative to point-value concentration-based risk assessment. The CMD is estimated by interpolating and integrating multilevel point measurements of concentration and flow across a control plane of interest. However, the geological settings at contaminated sites are typically subject to large heterogeneities resulting in complex hydrogeological conditions and significant spatial variability in the CMD, which combined with limited data availability renders it impossible to determine exact or error-free estimates.We present a geostatistical method for quantification of CMD uncertainties in a multilevel control plane downstream a contaminated site aimed at practical implementation, with focus on the interpolation and associated uncertainty related to the concentration measurements.The method uses geostatistical conditional simulation and applies an analytical solution of a macro-dispersive transport equation to simulate the spatially varying global mean. A Box-cox transformation is employed to ensure non-negative concentration values and account for skewness. The method is a development of that presented by Troldborg et al. (2012). We have refined the parameter identification by applying a Markov-Chain Monte Carlo (MCMC) algorithm for parameter sampling and furthermore constrained the prior sampling distributions to ensure the posterior is linked to conceptual site-specific knowledge. This links the CMD estimation to the conceptual site model and allows for source-zone data and geologic knowledge to be incorporated into the CMD estimate, which increases credibility, especially for low sampling density transects. The MCMC algorithm efficiently explores the high-dimensional parameter space, generating a statistically representative sample of geostatistical, transformation and transport-model parameters, thereby characterizing the uncertainty associated with model parameter identification in heterogeneous geologic settings. The result of the conditional simulations is an ensemble of concentration realizations that all honor the measured concentration data and capture the spatial variability of the contaminant plume.The method has successfully been applied to determine the CMD uncertainty at multiple contaminated sites. It is firstly demonstrated at a site with substantial data and prior knowledge, and secondly at two sites to assess the challenges related to prior knowledge, sampling density and different hydrogeological conditions.The proposed method represents a practical solution for quantifying CMD uncertainty at contaminated sites. By combining MCMC sampling and geostatistics, it overcomes the limitations of traditional deterministic methods and provides involved stakeholders with probabilistic estimates for better informed remediation and risk assessment practice when managing contaminated soil- and groundwater. ReferencesTroldborg, M., Nowak, W., Lange, I. V., Pompeia Ramos dos Santos, M. C., Binning, P. J., and Bjerg, P.L. (2012). Application of bayesian geostatistics for evaluation of mass discharge uncertainty at contaminated sites. Water Resources Research, 48(9):W09535. DOI: 10.1029/2011WR011785

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Contaminant mass discharge (CMD) estimation involves combining multilevel concentration and flow measurements to quantify the contaminant mass passing through a control plane downgradient of a point source. However, geological heterogeneities and limited data introduce uncertainties that complicate CMD estimation and risk assessment. Although CMD is increasingly used in groundwater management, methods for quantifying and handling these uncertainties are still needed. This study develops and tests a CMD estimation method based on Bayesian geostatistics to quantify CMD uncertainties using data from a control plane perpendicular to the contaminant plume. By combining geostatistical conditional simulations of the spatial concentration distribution with the flow, an ensemble of CMD realizations is generated, from which a cumulative distribution function is derived. A key element of this approach is the use of a macrodispersive transport model to simulate the spatial concentration trend. This ensures that the estimated concentration reflects the expected physical behavior of the contaminant plume while also allowing the integration of site-specific conceptual information. The method is applicable to plumes with dissolved contaminants, such as chlorinated solvents, petroleum hydrocarbons, Per- and polyfluoroalkyl substances (PFAS) and pesticides. Site-specific conceptual understanding is used to inform the prior probability density functions of the structural model parameters and to define acceptable simulated concentration limits. We applied the method at three sites contaminated with chlorinated ethenes, demonstrating its robustness across varying information levels and data availability. Our results shows that strong site-specific conceptual knowledge and high sampling density constrain the CMD uncertainty (CV = 21 %) and results in estimated model parameters and a spatial concentration distribution that agrees well with the conceptual model. For a site with less data and limited conceptual knowledge, CMD and concentration distribution estimates are still feasible, though with higher uncertainty (CV = 41 %). Extending the method to account for multiple source zones and complex plume migration improved parameter identification and reduced the 95 % CMD confidence interval by 11 % ([4950-8750] to [5090-8480] g yr-1), while also providing a spatial concentration distribution in better agreement with the plume conceptualization. This study highlights the importance of integrating site-specific conceptual knowledge in CMD estimation, particularly for less-sampled sites. The method can furthermore assist in identifying remediation targets, evaluating remedial effectiveness, and optimizing sampling strategies.

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.

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.

The contaminant mass discharge is a relevant metric to evaluate the risk that a groundwater plume poses to water resources. However, this assessment is often vitiated by a high uncertainty inherent to the assessment method and often limited number of measurement points to carry out the assessment. Direct-Push techniques in combination with profiling tools and dedicated sampling can be an interesting alternative to increase the measurement point density and hence reduce the mass discharge uncertainty. The main objective of our study was to assess if DP logging and sampling could be employed to get a reasonable estimate of contaminant mass discharge in a large sulfonamide contaminant plume (> 1500 m wide), compared to a more traditional approach based on monitoring wells. To do so, an Hydraulic Profiling Tool (HPT) logging with a dedicated site calibration was used to estimate the hydraulic conductivity field. The sulfonamide concentrations were inferred from the compound fluorescence properties measured by laboratory spectrofluorometry (λEx / λEm = 255/340 nm) and a dedicated log-log linear regression model. Our results show that HPT-derived hydraulic conductivity values are in good agreement with the monitoring well results, and within the order of magnitude reported in similar studies or indirect geophysical techniques. Fluorescence appears as a powerful proxy for the sulfonamide concentration levels. Ultimately, the contaminant mass discharge estimate from HPT and fluorescence techniques lies within a factor 2 from the estimate by monitoring wells, with 549 [274-668] and 776 [695-879] kg/yr respectively. Overall, this study highlights that DP logging tools combined with indirect methods (correlation with fluorescence) could provide a relevant contaminant mass discharge estimate for some optically active substances, given that a proper calibration phase is carried out.

The release of anthropogenic chemicals to streams, stemming from contaminated sites, direct application (rural/urban) or accidental release, represents a significant threat to water resources and thus the health of humans and aquatic ecosystems. Predicting the transport and fate of chemicals is key to quantify contaminant concentrations and develop environmental quality standards (EQS). Tracer tests are a well-established tool for such hydrological investigations in various water-based systems. In stream settings, such experiments have predominantly investigated longitudinal mixing and flow velocities by measuring the tracer concentration in a few discrete locations; few studies have focused on the transversal mixing properties. Recent progress in hyperspectral remote sensing from unmanned aerial systems (UAS) allows advancing the two-dimensional monitoring of tracer tests, by mapping the tracer concentration with a high spatial resolution in narrow streams with difficult accessibility. So far, such methods have only been demonstrated in controlled settings or in ocean waters, but not in optically complex streams. In this study, we evaluated the performance of a miniaturized hyperspectral imaging system and a consumer grade camera on board of an UAS, to map the concentration of the fluorescent tracer Rhodamine WT in a stream impacted by a contaminated site. In order to estimate tracer concentrations from the remotely sensed data, a band ratio of the red and blue band was used for the photo camera, while a vector based method, estimating the spectral angle in regards to a reference spectrum was applied for the hyperspectral data. The photo camera performed well, but it only mapped reliably the concentration in sections of the stream exposed to direct sunlight (R 2 : 0.83; nRMSE: 10.2 %), failing to map the concentration in all locations, which included locations where the direct sunlight was blocked by riparian trees (R 2 : 0.17; nRMSE: 28.7 %). In contrast, the advanced spectral information allowed the hyperspectral-based system to map the concentration well in all sections of the stream (R 2 : 0.76; nRMSE: 15.1 %), regardless of illumination changes. This demonstrated the advantage of optical cameras measuring water-leaving irradiance from hundreds of contiguous narrow spectral bands that also allow detecting finer spectral absorption and emission features. The results presented here would help to improve the knowledge about mixing of contaminants in streams, i.e. to predict the location of fully transversal mixing for contaminant sites discharging to streams via groundwater-surface interactions, as well as general assumptions behind mixing and dilution models.

Heterogeneous glacial deposits dominate large parts of the Northern Hemisphere. In these landscapes, high-resolution characterization of the geology is crucial for understanding contaminant transport. Geological information is mostly obtained from multiple boreholes drilled during a site investigation, but such point-based data alone do not always provide the required resolution to map small-scale heterogeneity between boreholes. Crosshole ground penetrating radar (GPR) is suggested as a tool for adding credible geological information between boreholes at contaminated site investigations in industrial sites where infrastructure, such as electrical installations, can pose a challenge to other geophysical methods. GPR data are sensitive to the dielectric permittivity and the bulk electrical conductivity, which can be related to the distribution of water content and sand/clay occurrences. Here we present a detailed crosshole GPR dataset collected at an industrial contaminated site in a clay till setting. The data are processed using a novel inversion approach where information on changes in the velocity and attenuation of the radar signal are obtained independently. The GPR results are compared to borehole logs, grain size analyses, and relative permeability data from the site. The GPR data analysis provided valuable information on the understanding of the lateral geological variability. A silt layer with a thickness of a few decimeters, likely important for flow characterization, was confirmed and resolved by GPR data. Our findings suggest that crosshole GPR has the potential for contributing with high-resolution geological information by filling the data gap between boreholes, thereby becoming a relevant tool in contaminated site investigations.

Sulfonamides are widely used antiobiotics and a threat to water resources and related ecosystems. While the direct discharge from untreated sewer or wastewater to surface water is a well-known pathway to the aquatic environment, only a limited number of studies have looked at the discharge and fate of sulfonamides from a contaminant plume to surface water so far.In this study, we investigated a sulfonamide contaminant plume discharging to a stream in Denmark. The sulfonamides (originating from a former production facility), are transported through a multilayered sandy aquifer and discharge to a groundwater-fed stream located down gradient. Our objectives were to evaluate if a screening using fluorescence properties could be used to delineate the sulfonamide contaminant plume and support the contaminant mass discharge estimation (both by transect method and in-stream measurements).Direct push technics in combination with a fluorescence screening allowed a relatively unexpensive coarse delineation of high concentrations areas (as opposed to laboratory analysis) down to 15 m.b.g.s. and ergo optimization of monitoring wells / screen locations in the transect. Chemical analyses were combined with slug test and hydraulic gradient estimates via continuous monitoring to quantify the sulfonamide flux and its temporal variations in a 2 km-long transect along the stream (24 monitoring wells with 3 - 6 screens).The estimated sulfonamide mass discharge (transect based) is in good agreement with the mass discharge calculated from in-stream measurements, highlighting the relevance of the screening approach to select appropriate measurement point locations. Furthermore, the comparison between the flux in both stream and groundwater compartments shows that the degradation of sulfonamides seems relatively limited in the near-stream and hyporheic zone, with the exception of the sulfanilic acid. The results of this study will be used for the prioritization of remedial actions along the main discharge zones.

A new methodology was developed to support contaminant mass discharge (CMD)‐based risk assessment of groundwater contamination downgradient of point source zones. Geoelectrical cross‐borehole induced polarization (IP) data were collected at a site undergoing in situ remediation of chlorinated solvents for determining 2D hydraulic conductivity (K) distributions with an inversion model resolution of 0.15 m (vertically) x 0.50 m (horizontally) in three control planes from 10 to 20 m depth. Additionally, 18 slug tests and 31 grain size distribution analyses (GSA) from the control planes, were used for K‐estimation. The geometric means and variance of the IP, slug test, and GSA derived K‐estimates were consistent with previously studied sandy aquifers. Furthermore, the vertical variation in K between two geological settings, a sandy till and a meltwater sand formation, was clearly identified by the IP K‐estimates. The vertical variation was backed up by hydraulic profiling tool (HPT) measurements. Random realizations of CMD were simulated based on the cross‐borehole IP derived K‐values. For comparison, the CMD was also estimated with a geostatistical conditional simulation approach, using the data from slug tests and GSAs. The high IP resolution captured the small scale variations in K across the transects and led to CMD predictions with a narrow uncertainty interval, whereas slug test and GSA either under‐ or overestimated the magnitude of the areas with the highest CMD. Applying the geophysical cross‐borehole method for estimating K‐distributions in addition to traditional methods would improve CMD‐based risk assessment and evaluation of remediation performance at contaminated sites.

This study investigated methods for predicting the duration and impact on groundwater quality from persistent and mobile organic compounds (PMOCs) at a drinking water well field affected by multiple contaminant sources. The fungicide metabolite N,N-dimethylsulfamide (DMS), which frequently occurs above the Danish groundwater quality criterion (0.1 mu g/L), was used as an example. By combining contaminant mass discharge (CMD) estimations, modeling, and groundwater dating, a number of important discoveries were made. The current center of contaminant mass was located near the source area. The CMD at the well field was predicted to peak in 2040, and an effect from the investigated sources on groundwater quality could be expected until the end of the 21st century. A discrepancy in the current CMD at the well field and the estimated arrival time from the studied source area suggested an additional pesticide source, which has not yet been thoroughly investigated. The presence of the unknown source was supported by model simulations, producing an improved mass balance after inclusion of a contaminant source closer to the well field.The approach applied here was capable of predicting the duration and impact of DMS contamination at a well field at catchment scale. It furthermore shows potential for identification and quantification of the contribution from individual sources, and is also applicable for other PMOCs. Predicting the duration of the release and impact of contaminant sources on abstraction wells is highly valuable for water resources management and authorities responsible for contaminant risk assessment, remediation, and long-term planning at water utilities.

Environmental pollution with Persistent and Mobile Organic Compounds (PMOC) from anthropogenic activities is an increasing cause for concern. These compounds are readily leached to groundwater aquifers and are likely to resist degradation, putting pressure on groundwater resources. Pesticides can form PMOCs upon degradation in the environment. The PMOC N,N-dimethylsulfamide (DMS) was the most frequently detected pesticide metabolite in Danish drinking water wells in 2020, although the pesticidal use of the last parent compound (tolylfluanid) ended in 2007. This study aimed to improve the understanding of the leaching of the PMOC DMS from clayey tills by combining a review of compound properties, sources and use, comprehensive field observations and numerical flow and solute transport modeling. The modeling explored the mechanisms of DMS retention during vertical transport in clayey till and the fingerprint in the underlying aquifer. The results were supported by detailed field observations at an agricultural site with strawberry production. Porewater samples were collected from clayey till to a depth of 12 m bgs by a custom designed installation method of suction cups. Groundwater sampling (249 samples) was designed to provide vertical concentration profiles at various distances from the presumed sources. The review of properties showed that the parent compounds and intermediates degrade quickly in topsoil, releasing the highly persistent and mobile DMS. We tested the effect of fractures on transport with different hydraulic apertures and a scenario without fractures by numerical modeling. The results showed that the presence of fractures can smooth the breakthrough curve below the clayey till, leading to faster breakthrough, lower maximum concentration, and several decades of prolonged leaching in simulations with the largest aperture (20 & mu;m). The fracture-matrix interaction is a possible explanation for the observed delay of leaching from clayey till. The vertical concentration profiles in groundwater were used for identifying the sources at the field site and testing source strengths. Assigning one point source (200 & mu;g/L) and two diffuse sources (40-50 & mu;g/L) to the model produced vertical concentration profiles that compared well with observed field data in clayey till and the aquifer. All results were integrated into a conceptual model for the environmental fate of PMOCs in soil and groundwater. The findings of this study imply that the presence of fractures in clayey till should be considered in conceptual site models, since they can substantially prolong the leaching of PMOCs to groundwater. The integration of comprehensive field investigations and numerical modeling is key to understand the fate of PMOCs in complex field systems with different source types. Together with widespread occurrences of PMOCs in groundwater systems, the results highlight the need for improved approval procedures for pesticides and biocides which considers their persistent and mobile metabolites.