Canal dikes in low-lying polders, as well as in other regions worldwide, are critical infrastructure for flood protection and water management. The subsurface water conditions can cause dike failures during excessive rainfall and prolonged periods of drought. There is a lack of multi-year monitoring of subsurface water conditions in canal dikes and an insufficient understanding of their geohydrological behavior. This study provides and analyses a novel multiyear data set of soil moisture and hydraulic heads (from February 2020 until March 2023) from a monitoring network covering various canal dikes with different characteristics in the western Netherlands. The data, including two extremely dry summers, highlight the impact of meteorological variations on the subsurface water conditions. Non-hydrostatic hydraulic head levels were observed during droughts that can be detrimental to dike stability and that are often not accounted for in safety assessments for drought situations. The effectiveness of various meteorological drought indicators applied to subsurface water conditions was evaluated: the precipitation deficit is the most reliable measure and outperforms the standardized drought indicators (SPEI and SPI). The drought recovery of dikes was analyzed to understand seasonal transitions and the sequence of different failure mechanisms, during dry and wet situations. This analysis also reveals differences between meteorological, soil moisture, and groundwater droughts, highlighting soil's storage capacity after drought and the limitations of meteorological drought indicators as proxies for soil moisture and groundwater. The insights from this study enhance assessments, inspection procedures and the identification of weak spots of dikes and other earthworks of infrastructure.@en

This study quantifies the field hydraulic performance of a dual-functionality landfill cover, combining microbial methane oxidation with water diversion using a capillary barrier. The investigated 500 m2 test field, constructed on a landfill in the Netherlands, consisted of a cover soil optimised for methane oxidation, underlain by a sandy capillary layer and a gravelly capillary block. Outflows from these layers were measured between 2009 and 2023. Average precipitation was 848 mm/a, evapotranspiration, diverted infiltration and breakthrough amounted to 504 (59.4 %), 282 (33.3 %) and 62 (7.3 %) mm/a, respectively. On average, the capillary barrier diverted 82 % of the inflow into the capillary layer. Breakthrough occurred mainly from October to March when evapotranspiration was low and the maximum water storage capacity of the cover soil was reached. During this period, inflow into the capillary barrier exceeded its diversion capacity, caused by the relatively high hydraulic conductivity of the cover soil due to its optimisation for gas transport. The diversion capacity declined drastically in the year after construction and increased again afterwards. This was attributed to suffusion of sand from the capillary layer into the capillary block and subsequent washout to greater depths or the influence of iron precipitates at the bottom of the capillary layer. The effect of a more finely grained methane oxidation layer on the hydraulic and methane oxidation performance should be investigated further. These measures could further improve the combined performance of the dual functionality landfill cover system under the given conditions of a temperate climate.@en

Bio-mediated methods, such as microbially induced carbonate precipitation, are promising techniques for soil stabilisation. However, uncertainty about the spatial distribution of the minerals formed and the mechanical improvements impedes bio-mediated methods from being translated widely into practice. To bolster confidence in bio-treatment, non-destructive characterisation is desired. Seismic methods offer the possibility to monitor the effectiveness and mechanical efficiency of bio-treatment both in the laboratory and in the field. To aid the interpretation of shear wave velocity measurements, this study uses the discrete element method to examine the small-strain stiffness of bio-cemented sands. Bio-cemented specimens with different characteristics, including properties of the host sand (void ratio, uniformity of particle size distribution) and properties of the precipitated minerals (distribution pattern, content, Young’s modulus), are modelled and subjected to static probing. The mechanisms affecting the small-strain properties of cemented soils are investigated from microscopic observations. The results identify two mechanisms controlling the mechanical reinforcement associated with bio-cementation, namely the number of effective bonds and the ability of a single bond to improve stiffness. The results show that the dominant mechanism varies with the properties of the host sand. These results support the use of seismic measurements to assess the mechanical efficiency and effectiveness of bio-mediated treatment.

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In-situ stabilization of waste bodies can be achieved by the infiltration of water or recirculation of leachate into the landfill, which is thought to enhance the microbial degradation of waste organics by (re-)moisturizing dry zones and flushing out metabolic products of organic matter decay. The success of in-situ stabilization should reflect in initially accelerated and thereafter reduced rates of anaerobic waste organic matter decay rates. This paper compares the methane generation that was modelled using the Afvalzorg multiphase model without the added effect of leachate recirculation with actually extracted methane in the landfill and gas generation on sampled wastes following five years of leachate recirculation on a Dutch landfill. Laboratory incubations revealed a methane potential between 0.03 kg CH4/t dw and 15.8 kg CH4/t dw for 365 days. Clear trends with respect to depth, moisture content, total organic carbon or share in hard plastics did not emerge as overall waste heterogeneity was high and likely obfuscated the correlation analysis. The results showed a recovery efficiency of 30.4% for 2021, with 0.07 kg CH4/t dw for the recovered methane and 0.23 kg CH4/t dw for the predicted methane in compartment 3. The average methane potential measured in the laboratory was almost twice as high as the remaining methane potential predicted for the period of 2021-2093. The discrepancy could be due to (i) enhanced waste degradability as a result of five years of recirculation, (ii) enhancing effects of material perturbation during sampling and/or (iii) impeded on-site methane generation and gas and water transport limitations due to presence of plastics. Overall, the laboratory incubations demonstrate a significant potential for waste biodegradation still residing in the waste.@en

Nitrogen undergoes multiple biogeochemical transformations during waste degradation, which depend on speciation, prevailing geochemical boundary conditions, and waste surface properties. This study developed a waste biodegradation model with high flexibility in accommodating reaction pathways to assess different process dynamics. The model was applied to landfill simulator reactors operating anaerobically. Model results show that dilution with adsorption matches the experimental dissolved NH4+ concentration (C/N=25) at the early experimental stages. Also, NH4+ binding decreases due to competition with Ca2+, and the model better captures the dissolved NH4+ behavior when CaSO4 is present in solution. Mass removal due to sampling and posterior dilution are the main mechanisms to reduce NH4+ concentration in the leachate. The model highlights the role of nitrogen sorption as the main mechanism for nitrogen accumulation in the solid phase of municipal solid waste.@en

Anaerobic sediment organic matter decay generates methane, delays sediment consolidation, reduces sediment density, viscosity and shear strength, all impacting the sediment rheological parameters and the navigable depth. This study quantifies the share of anaerobically and aerobically degradable sediment organic matter (SOM) in a depth profile and along a transect through the tidal river Elbe in the section of the Port of Hamburg. From exponential organic matter decay functions, organic matter decay rates (mg C g_TOC⁻¹ d⁻¹) were derived and clustered with a k-Means Cluster Analysis. The reactivity of different (kinetic) organic matter pools along the river transect were characterized based on their biodegradation rates. A fast, medium, slowly and non-degradable pool (pools 1–4) were identified based on the measured organic matter lability. SOM lability decreased from upstream to downstream, evidenced by the decreasing amount of the easily degradable pool 1 material from upstream to downstream. The size of the slowly degradable pool 3, assumed to be associated with SOM bound to the mineral particles, did not show any spatial gradient and is therefore suggested to represent a baseline share of hardly accessible SOM in the investigation area (about 12%−16% of TOC). Total degradability thus appears to be governed by the amount of SOM present in addition to this basis (pool 3), which in turn follows a source gradient and an age gradient from upstream to downstream. The recalcitrant pool 4 was the largest at any part of the harbour, for any depth, and for both, anaerobic and aerobic conditions (about 75%−85% of TOC). This indicates that the sediment in the investigation area, including the uppermost fluidic and freshly settled layers, mostly comprises stabilised organic matter and contributes largely to storage of organic carbon. Differently sized anaerobic SOM pools with depth were observed as well as seasonal changes of the easily degradable SOM pool 1. The degradability was larger in upper sediment layers, it was also larger under aerobic conditions (by about 10% of TOC) but the differences between aerobic and anaerobic decay decreased from upstream to downstream.

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This study presents a novel geotechnical engineering approach that utilizes naturally occurring processes to reduce soil permeability in-situ. This approach is inspired by a soil stratification process (Podzolization), where a low permeability layer is formed by metal-organic matter precipitates. In a field experiment, a direct aluminum-organic matter (Al-OM) floc injection was applied to create a continuous vertical flow barrier in a dike. Direct injection uses the shear-dependent size of Al-OM flocs. High-shear conditions (i.e., during injection) lead to the breakage of Al-OM flocs and thus allow their transportation in soils. When the injection stops and low-shear conditions prevail, the Al-OM flocs re-grow in size and block the pores, which ultimately reduces soil permeability. Two different Al-OM floc concentrations were applied in the field. Results show that a continuous flow barrier is only formed at lower concentrations; at higher concentrations a scattered permeability reduction was achieved. This demonstrates the viability of this approach in reducing soil permeability in-situ and shows that the spatial distribution of the flocs depends on input concentration.

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In-situ aeration of landfills accelerates biodegradation of waste organic matter and hence advances waste stabilization. The spatial outreach of aeration greatly affects stabilization efficiency. This study analyzed the spatial variability of gas composition and flow in 230 wells spread over four compartments of a Dutch landfill which is under in situ aeration since 2017, as well as the carbon extraction efficiency, tem-perature, and settlement. Flow rates and gas composition in the extraction wells varied strongly. The highest variability was observed in the compartment with the highest water tables with submerged filter screens for most wells, with low flow rates, and elevated ratios of CH₄ to CO₂, indicating predominance of anaerobic processes (compartment 11Z). The compartment with the most uniform distribution of gas flow rates, composition and lower ratios of CH₄ to CO₂, suggesting a significant share of aerobic carbon mineralization, also showed higher temperatures, a carbon extraction efficiency, and larger cumulative settlement, all indicative of enhanced microbial activity (compartment 11N). In this compartment, the amount of extracted carbon exceeded the carbon generation predicted from landfill gas modeling by the factor of 2 over the hitherto four years aeration. The effect of water tables on gas flow and the correlation between the flow, and the ratio of CH₄ to CO₂ appeared weak, indicating that also other factors than water tables influence gas concentration and flow. Future work includes stable isotope probing to analyze the significance of microbial respiration and microbial CH₄ oxidation for the composition of the final extracted gas mixture.

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Urbanization influences soil carbon (C) stocks and flows, which, in turn, affect soil-derived ecosystem services. This paper explores soil C storage in urban greenspaces in the Dutch city of The Hague along a transect from the suburban seaside towards the city centre, reflecting a toposequence from dune to peaty inland soils. C storage and C mineralisation potential were evaluated in relation to soil type and greenspace categories. Several soil-quality characteristics were measured, including dissolved organic C, pH, electrical conductivity, nitrogen, phosphorus, sulphur, calcium carbonate, and the water-holding capacity of the soil to evaluate what drives soil C storage in the urban context. The total SOC storage of the upper 30 cm of the greenspaces in The Hague (20.8 km2 with 37% greenspace) was estimated at 78.4 kt, which was significantly higher than assumed given their soil types. Degradability of soil organic matter in laboratory batch tests varied between 0.2 and 3 mg C gSOC−1 day−1. Degradability was highest in the seaside dune soils; however, extrapolated to the topsoil using the bulk density, topsoil C mineralization was higher in the urban forest. Soils beneath shrubs appeared to be hotspots for C storage, accounting for only 13% of the aerial cover but reflecting 24% of the total C storage. Land ownership, land use, greenspaces size, litter management and soil type did not result in significantly different C stocks, suggesting that processes driving urban soil C storage are controlled by different factors, namely land cover and the urbanization extent.@en

Abstract: The use of sediments as soils is an area of interest for Beneficial Use of dredged sediments. In this study the impact of the transition from sediments to soils is researched by looking at the seasonal and long year (10 year) change in pore water metal chemistry of sediments which are considered clean (class A) according to the Dutch soil directive. This study is based on a combination of geohydrological, geochemical and ecotoxicological risk models and validated against measured pore water concentrations for metals over an dry/wet period. The pore water metal concentrations are compared against standards and expressed as at Risk Characterization Ratio’s (RCR) values. The RCR values are high (> 1) during the first 3 years after the application of sediments as soil, especially at the end of the summer. The multi substances Potentially Affected Fraction (ms-PAF) shows a similar trend as the RCR values, although it takes 5 years before the combined calculated potential ecotoxicity is below the legal 40% threshold level. Translated to land use, it is advised to restrict land use for farming on soils where these clean (class A) sediments are applied for a five-year transition period. Article Highlights: Beneficial Use of sediments should take into account the different conditions when used as soils.Use of sediments as soils lead to a predicable seasonal and multiple year trend in metal concentrations in pore water.The predicted results in metal pore water concentrations are translated into an advice for temporal land use.

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The geometric variability of soil layers is a large source of uncertainty in the reliability assessment of dikes. Because direct samples of the subsurface soils are often insufficient to capture the complexity of the subsurface, geophysical methods provide a powerful source of complementary information. A combined approach to estimate the geometry of soil layers is presented. The approach combines local point data, i.e. data obtained from a CPT or a borehole log, and geophysical tomography in a universal cokriging framework. The approach uses the contact points between soil layers obtained from local point data and the orientations of the layers derived from geophysical tomography. To reduce subjectivity in the interpretation of tomographic images, an automated edge detection technique was used. The combined approach was applied to characterise two test sites where the presence of paleochannels locally change the geometry of soil layers. The results show that a combined approach enables the reduction of sampling efforts with an improved estimation of geometric variability.

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In order to reduce the environmental and financial burden for future generations, approaches are needed to shorten aftercare of landfills. Aeration of the waste-body is a promising approach, however, the poor understanding of transport of gas and water through a waste-body makes it difficult to design an effective aeration strategy. The aim of this study is to develop a tool to determine the optimal aeration strategy for landfills. This study presents a comparison of aeration strategies based on the air distribution they generate with a 3-D multiphase model. The implemented theory is based on parameter values obtained from (laboratory) experiments performed under conditions which are similar to those in a full scale landfill. Calibration with field scale gas extraction data from the Dutch pilot site Wieringermeer shows that the model gives a good description of the average gas flow under extraction. Scenario analyses for the case study landfill indicate that injection strategies reach a larger volume fraction of waste with a higher air flow compared with extraction strategies, especially at the bottom of the landfill. Extraction, however, supplies oxygen more homogeneously through-out the waste. An import design criterion is also the distance between the wells. Too large distances lead to ineffective treatment because too large volumes of waste/leachate remain untreated. In addition to the comparison of aeration strategies, an optimal aeration strategy for the pilot site is presented. A combination of (alternating) injection and extraction wells which are maximum 20m apart seems to be the optimal strategy.@en

Purpose: The microbial turnover of sediment organic matter (OM) in ports and waterways impacts water quality, sonic depth finding and presumably also rheological properties as well as greenhouse gas emissions, especially if organic carbon is released as methane. As a consequence, sediment management practices as a whole are affected. This study aimed to discern spatial OM degradability patterns in the Port of Hamburg and investigated correlations with standard analytical properties as a basis for future predictive modelling. Materials and methods: Sediments in the Port of Hamburg were repeatedly sampled at nine locations along an east-west transect using a 1-m corer. In a stratified sampling approach, layers of suspended particulate matter (SPM), fluid mud (FM), pre-consolidated sediment (PS) and consolidated sediment (CS) were identified and individually analysed for long-term aerobic and anaerobic degradation of organic matter, DNA concentration, stable carbon isotope signature, density fractions and standard solids and pore water properties. Results and discussion: The investigation area was characterised by a distinct gradient with a 10-fold higher OM degradability in upstream areas and lower degradability in downstream areas. Concomitantly, upstream locations showed higher DNA concentrations and more negative δ¹³C values. The share of bulk sediment in the heavy density fraction as well as the proportion and absolute amount of organic carbon were significantly larger at downstream locations. A depth and hence age-related gradient was found at individual locations, showing higher degradability of the upper, younger material, concomitant with higher DNA concentration, and lower OM turnover in the deeper, older and more consolidated material. Deeper layers were also characterised by higher concentrations of pore water ammonium, indicative of anaerobic nitrogen mineralisation. Conclusions: Organic matter lability is inversely linked to its stabilisation in organo-mineral complexes. The observed degradability gradient is likely due to the different OM quality in relation to its origin. Downstream OM enters the system with the tidal flood current from the direction of the North Sea whereas upstream locations receive OM originating from the catchment, containing more autochthonous, plankton-derived and more easily degradable components. At individual sampling points, depth-related degradability gradients reflect an age gradient, with easily degradable material in top layers and increasing stabilisation of OM in organo-mineral compounds with depth.

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Back-diffusion of chlorinated ethenes (CEs) from low-permeability layers (LPLs) causes contaminant persistence long after the primary spill zones have disappeared. Naturally occurring degradation in LPLs lowers remediation time frames, but its assessment through sediment sampling is prohibitive in conventional remediation projects. Scenario simulations were performed with a reactive transport model (PHT3D in FloPy) accounting for isotope effects associated with degradation, sorption, and diffusion, to evaluate the potential of CSIA data from aquifers in assessing degradation in aquitards. The model simulated a trichloroethylene (TCE) DNAPL and its pollution plume within an aquifer-aquitard-aquifer system. Sequential reductive dechlorination to ethene and sorption were uniform in the aquitard and did not occur in the aquifer. After 10 years of loading the aquitard through diffusion from the plume, subsequent source removal triggered release of TCE by back-diffusion. In the upper aquifer, during the loading phase, δ13C-TCE was slightly enriched (up to 2‰) due to diffusion effects stimulated by degradation in the aquitard. In the upper aquifer, during the release phase, (i) source removal triggered a huge δ13C increase especially for higher CEs, (ii) moreover, downstream decreasing isotope ratios (caused by downgradient later onset of the release phase) with temporal increasing isotope ratios reflect aquitard degradation (as opposed to downstream increasing and temporally constant isotope ratios in reactive aquifers), and (iii) the carbon isotope mass balance (CIMB) enriched up to 4‰ as lower CEs (more depleted, less sorbing) have been transported deeper into the aquitard. Thus, enriched CIMB does not indicate oxidative transformation in this system. The CIMB enrichment enhanced with more sorption and lower aquitard thickness. Thin aquitards are quicker flushed from lower CEs leading to faster CIMB enrichment over time. CIMB enrichment is smaller or nearly absent when daughter products accumulate. Aquifer CSIA patterns indicative of aquitard degradation were similar in case of linear decreasing rate constants but contrasted with previous simulations assuming a thin bioactive zone. The Rayleigh equation systematically underestimates the extent of TCE degradation in aquifer samples especially during the loading phase and for conditions leading to long remediation time frames (low groundwater flow velocity, thicker aquitards, strong sorption in the aquitard). The Rayleigh equation provides a good and useful picture on aquitard degradation during the release phase throughout the sensitivity analysis. This modelling study provides a framework on how aquifer CSIA data can inform on the occurrence of aquitard degradation and its pitfalls.@en

Using naturally occurring processes to modify the engineering properties of the subsurface has received increasing attention from industry and research communities as they aid in the development of cost-effective, robust and sustainable engineering technologies. In line with this trend, we propose to use precipitates of aluminum (Al) and organic matter (OM) to reduce soil permeability in-situ. This process is inspired by podzolization: a soil stratification process where a layer with low permeability is developed at depth via the precipitation of metal-OM complexes. In this study, the concept of applying Al-OM precipitates for in-situ soil permeability reduction was for the first time applied in the field. The aim of the field experiment was to create a cylindrical flow barrier in a sand layer at depth. In order to design and engineer the field application, we performed a series of scenario analyses with a site-specific 3D reactive transport model. This led to an in-situ engineering approach where a flow barrier was created by separate injection of Al and OM using a combined injection/extraction strategy. During the field application, the local variation of soil conditions required significant modifications to the design. Further scenario analyses with the model were conducted to adapt the original design and to understand the consequences of these modifications. The results show that a cylindrical flow barrier was created after an injection period of 8 days. The precipitation of Al-OM is a highly localized process, where large amount of precipitates is formed in the close vicinity of the injection filter screens. Evaluation of pumping tests that were performed after the injection activities revealed that the permeability of the treated sand was reduced to 2% of its original value. This first full-scale field test demonstrates that applying Al-OM precipitates is a suitable bio-based engineering tool to reduce soil permeability in-situ.@en

Microbially induced carbonate precipitation (MICP) through denitrification can potentially be applied as a bio-based ground improvement technique. Two strategies involving multiple batch treatments in a modified triaxial test setup were used to study the process efficiency. Both strategies aim to achieve 1 weight percentage (% by weight) of precipitated calcium carbonate (CaCO₃) and differ in number of flushes, hydraulic residence time, and substrate concentrations. In the experiment with few flushes and high substrate concentrations the microbial process was inhibited, only 0.28% by weight CaCO₃ was measured in the sand after 5 weeks of treatment. The regime with many flushes and low substrate concentrations stimulated microbial growth resulting in 0.65% by weight CaCO₃ within the same time period. Biomass growth and nitrogen gas production were stable throughout the experiment at low concentration, reducing the hydraulic conductivity of the sand, which eventually led to clogging. Precipitation rates up to 0.26% by weight/day CaCO₃ were observed. Applying a suitable substrate regime and residence time is important to limit inhibition and maintain the cell activity, allow for an efficient conversion, and generate a good precipitation rate.

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Long-term emissions of Municipal Solid Waste (MSW) landfills are a burden for future generations because of the required long-term aftercare. To shorten aftercare, treatment methods have to be developed that reduce long-term emissions. A treatment method that reduces emissions at a lysimeter scale is re-circulation of leachate. However, its effectiveness at the field scale still needs to be demonstrated. Field scale design can be improved by theoretical understanding of the processes that control the effectiveness of leachate recirculation treatment. In this study, the simplest and most fundamental sets of processes are distilled that describe the emission data measured during aerobic and anaerobic leachate recirculation in lysimeters. A toolbox is used to select essential processes with objective performance criteria produced by Bayesian statistical analysis. The controlling processes indicate that treatment efficiency is mostly affected by how homogeneously important reactants are spread through the MSW during treatment. A more homogeneous spread of i.e. oxygen or methanogens increases the total amount of carbon degraded. Biodegradable carbon removal is highest under aerobic conditions, however, the hydrolysis rate constant is lower which indicates that hydrolysis is not enhanced intrinsically in aerobic conditions. Controlling processes also indicate that nitrogen removal via sequential nitrification and denitrification is plausible under aerobic conditions as long as sufficient biodegradable carbon is present in the MSW. Major removal pathways for carbon and nitrogen are indicated which are important for monitoring treatment effectiveness at a field scale. Optimization strategies for field scale application of treatments are discussed.

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In this study the spatial distribution of dissolved metals in surface water is studied at nine locations in Lake Ketelmeer (the Netherlands). The measured dissolved metal concentrations are combined with the local water quality parameters for salinity, pH, alkalinity and DOC to calculate a FIAM Free Ion Activity Model (FIAM) and the Biotic Ligand Model (BLM) based bioavailable metal concentration. The BLM model is used for Cu, Ni, Pb and Zn and the FIAM model for Cd, Cr, Cu, Ni, Pb and Zn. To be able to compare the dissolved metal concentration with the FIAM or BLM based bioavailable metal concentration, an accepted reference standard can be used which is also corrected for the bioavailable concentration. Here the Water Framework Directive (WFD) Annual Average Quality Standard (AA-QS) is used, corrected for the FIAM and BLM based bioavailable metal concentration under reference conditions. This yielded a site specific Risk Characterization Ratio (RCRFIAM/RCRBLM). The FIAM model shows an exceedance of the site specific AA-QS for Cu (RCRFIAM of 1.8) and Pb (RCRFIAM of 1.5) in the northern middle part of the lake. This is due to a lower pH in this part of the lake. The BLM model was inconclusive with regard to spatial trends for Cu and Ni due to out of boundary conditions for the model. For locations where the BLM model was within the model boundary conditions, the RCRBLM could be as high as 7.5 for Cu and 3.2 for Ni. The main water quality parameter causing the high RCRBLM was the low DOC concentration. To establish if the locally increased RCR for Cu and Pb (FIAM) or Cu and Ni (BLM) poses an ecotoxicological risk to organisms the multi substances Potentially Affected Fraction (ms-PAF) model is used. The FIAM based ms-PAF indicates that the northern middle part of the lake has the highest chronic metal exposure risk, with an ms-PAF of 27%. The BLM based ms-PAF has a maximum of 45%, but lacks a spatial trend due to the missing BLM corrected Cu and Ni concentrations for some locations.@en

The utilization of natural processes for in situ permeability reduction has seen a growing interest in recent years since controlling infiltration or seepage of water is one of the most challenging tasks in water management and civil-engineering. We hereby propose a novel geoengineering tool for in situ permeability reduction, namely Soil Sealing by Enhanced Aluminum and organic matter Leaching (SoSEAL). SoSEAL makes use of the interaction between organic matter (OM) and aluminum (Al). Complexation and subsequent precipitation of OM by Al results in the formation of soil layers with reduced permeability; a process which is well known from podzols. This study demonstrates the suitability of the SoSEAL technique for permeability reduction in laboratory experiments. All experiments have been performed using humic acid (HUMIN P775, Humintech, Germany) as an OM source and aluminum chloride as the metal component. Batch experiments were conducted to study the interaction between OM and Al at various metal to organic carbon (M/C) ratios and pH levels. Results show that the precipitation of Al-OM flocs depends on the Al concentration and therefore the M/C ratio, which is well-known from OM removal in drinking water treatment. Precipitation of the hereby used humic acid starts to occur at a molar M/C ratio of 0.01 and almost all OM is removed at M/C ratios larger than 0.04. The size of the Al-OM flocs ranges between 20 and 1000 m, which enables them to cover micro- and mesopores in porous media and therefore reduce the permeability. In order to quantify the permeability reduction that can be achieved by Al-OM flocs, saturated column experiments were performed using sand with three different grain size distributions and applying various injection strategies to induce in situ mixing of the two separately injected components (i.e. Al and OM). We were able to reduce the hydraulic conductivity in the sand column to a range between 10 and 40% of its initial value. Results show that the reduction in permeability depends on several factors including the sand type, the injection technique, mixing and reaction of the two components in situ, and the orientation of the precipitation band. We conclude that the precipitation of Al-OM flocs induced by in situ mixing of Al and DOM can significantly reduce the permeability of different sand types. These results are the proof of principle of the SoSEAL concept. @en

Using naturally occurring processes to modify the engineering properties of the subsurface has gained increasing attention from industrial and research communities as they aid in the development of cost-effective, robust and sustainable engineering technologies. In line with this trend, we propose to use the interaction between aluminum (Al) and organic matter (OM) to reduce soil permeability in situ. This is inspired by podzolization: a soil stratification process where the mobilization of Al, iron and OM in the topsoil is followed by their precipitation at greater depth. In this study this newly developed engineering technique has been applied for the first time in the field. The aim of the field test was to create a cylindrical flow barrier (5 m i.d.) in a sand layer, located at a depth between 7 to 13 m below ground surface (bgs). A 3D reactive transport model was developed via the coupling between Darcy’s law and solutes transport, meanwhile Al-OM precipitation and its impact on permeability are included. The model was used to design and analyze the results from the field test. At the site Al and OM solutions were injected separately through 20 injection wells distributed in two circles with a radius of 2.5 m for Al injection and 3 m for OM injection. An extraction well was placed in the center of these two circles to control in situ mixing and precipitation of Al and OM and precipitation of these two components in a specific zone. Results demonstrated that after a period of 8 days, we successfully created a cylindrical flow barrier where precipitates formed in close vicinity of the injection filter screens. The permeability of the treated sand was reduced to 2.3 % of its original value. Pumping tests conducted 6 months after the treatment showed no change in the achieved permeability reduction, indicating the stability of the Al-OM precipitates during this period. Further investigation is however necessary to evaluate the long-term stability of the flow barrier. This field study demonstrates the viability of using Al and OM complexation and precipitation as an in situ engineering tool to reduce soil permeability. By separate injection of the two components and a combined injection/extraction strategy, we were able to induce in situ mixing of Al and OM and control the geometry of the formed flow barrier. @en