Search

Seawater intrusion in coastal aquifers is a significant problem that can be addressed through the construction of subsurface dams or physical cut-off barriers. An alternative method is the use of microbially induced carbonate precipitation (MICP) to reduce the hydraulic conductivity of the porous medium and create a physical barrier. However, the effectiveness of this method depends on various factors, and the scientific literature presents conflicting results, making it challenging to generalise the findings. To overcome this challenge, a statistical and machine learning (ML) approach is employed to infer the causes for the reduction in hydraulic conductivity and identify the optimum MICP parameters for preventing seawater intrusion. The study involves data curation, exploratory analysis, and the development of various models to fit the input data (k-Nearest Neighbours – kNN, Support Vector Regression – SVR, Random Forests – RF, Gradient Boosting – XgBoost, Linear model with interaction terms, Ensemble learning algorithms with weighted averages – EnL-WA and stacking – EnL-Stack). The models performed reasonably well in the region where permeability reduction is sensitive to carbonate increase capturing the permeability reduction profile with respect to cementation level while demonstrating that they can be used in initial assessments of the specific conditions (e.g., soil properties). The best performing algorithms were the EnL-Stack and RF followed by XgBoost and SVR. The MICP method is effective in reducing hydraulic conductivity provided that the various biochemical parameters are optimised. Critical biochemical parameters for successful MICP formulations are the bacterial optical density, the urease activity, calcium chloride concentration and flow rate as well as the interaction terms across the properties of the porous media and the biochemical parameters. The models were used to identify the optimum MICP formulation for various porous media properties and the maximum permeability reduction profiles across cementation levels have been derived. • MICP effectively reduces hydraulic conductivity to generate cut-off walls. • Machine learning models predict hydraulic conductivity for various MICP parameters. • Critical biochemical factors are OD 600 , urease activity and CaCl 2 concentration. • Interactions between porous media properties and biochemical factors are critical. [ABSTRACT FROM AUTHOR]

The high-level nuclear waste, HLW, from Swedish and Finnish reactors will be deposited in crystalline rock at depths around 500 m. The waste is enclosed in steel canisters protected against corrosion by a 5 cm thick copper shell, which ensures a lifetime far longer than 100 000 years. Should some canister be breached any leaking nuclides will have decayed to so low activity that even if they reached the biosphere, they would cause minimal risk to humans. The cost of the copper is significant. The dismantling of the nuclear reactors, with induced activity must also be disposed of and this waste volume is much larger than that of the HLW, which makes it impossible to protect it in the same way. This paper explores if by locating the waste at larger depth where the ground water is more saline, and where the hydraulic conductivity of the rock is lower up-flow of contaminated water can be ensured to be negligible because the denser water at larger depth counteracts up-flow due to negative buoyancy. Several processes that could cause local up-flow are addressed, such as infiltration of meteoric water, impact of surface topology, heat production of the waste, geothermal gradient, salinity gradient, hydraulic conductivity heterogeneities and salt migration between seeping water and salt in matrix pore water. Flow and transport simulations using data from extensive field investigations over more than ten years with scores of km deep boreholes suggest that a HLW repository at around one km depth may be sufficient to hinder up-flow to the biosphere. • Natural saline water gradients hinder contaminant up-welling. • Narrow channels in fractures form dense 3D channel networks, which promotes mass exchange between mobile water and matrix pore water and retards solute transport. • Contaminants in waters at large depths are delayed from reaching the biosphere for very long times. [ABSTRACT FROM AUTHOR]

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. • 3D solute transport modeling of chlorinated ethenes in source and plume. • High-resolution vertical concentration profiles by direct-push field methods. • Model calibration to concentration profiles reproduces characteristic plume sinking. • Quantification of contaminant mass discharge, infiltration, hydraulic conductivity. • Source conceptualization and fractures crucial for temporal CMD evolution. [ABSTRACT FROM AUTHOR]

This paper presents an analytical study of organic contaminants transport in a cut-off wall and aquifer dual-domain system, considering the effects of the inlet boundary conditions and cut-off structural arrangements. The comprehensive sensitivity analysis of parameters focusing on the breakthrough time, attenuation time and cumulative concentration are presented using the Monte Carlo simulation and Sobol global method. The simplified constant inlet boundary condition can lead to an excessively conservative prediction of the contaminant breakthrough compared to the 'finite mass' and 'decaying source' boundary conditions. The cut-off wall hydraulic performance can be enhanced by reducing the contaminant's head loss, shape factor, half-life and cut-off wall hydraulic conductivity while increasing the retardation factor. The contaminant's half-life can largely influence the maximum contaminant concentration, attenuation time and breakthrough time. For example, the maximum contaminant concentrations for T 1/2 = 1.4 years and T 1/2 = 100 years are 13 and 123 times greater than that for T 1/2 = 0.1 year, respectively. The influence of the variation of shape factor on the breakthrough curve should be taken into consideration. Altering the structural arrangement of the cut-off wall to accommodate a smaller shape factor increases the contaminant breakthrough time. The proposed solution allows the analysis of a cut-off wall and aquifer system with different inlet boundary conditions and structural arrangements of the cut-off wall. • An analytical model of organic contaminants transport in a CAS is proposed. • Monte Carlo simulation and Sobol global method are used for sensitivity analysis. • Different inlet boundary conditions and structural arrangements are investigated. • A constant contaminant concentration leads to conservative predictions. • Altering the structural arrangement can enhance the performance of the barrier. [ABSTRACT FROM AUTHOR]

Toxic organic contaminants in groundwater are pervasive at many industrial sites worldwide. These contaminants, such as chlorinated solvents, often appear as dense non-aqueous phase liquids (DNAPLs). To design efficient remediation strategies, detailed characterization of DNAPL Source Zone Architecture (SZA) is required. Since invasive borehole-based investigations suffer from limited spatial coverage, a non-intrusive geophysical method, direct current (DC) resistivity, has been applied to image the DNAPL distribution; however, in clay-sand environments, the ability of DC resistivity for DNAPLs imaging is limited since it cannot separate between DNAPLs and surrounding clay-sand soils. Moreover, the simplified parameterization of conventional inversion approaches cannot preserve physically realistic patterns of SZAs, and tends to smooth out any sharp spatial variations. In this paper, the induced polarization (IP) technique is combined with DC resistivity (DCIP) to provide plausible DNAPL characterization in clay-sand environments. Using petrophysical models, the DCIP data is utilized to provide tomograms of the DNAPL saturation (S N) and hydraulic conductivity (K). The DCIP-estimated K / S N tomograms are then integrated with borehole measurements in a deep learning-based joint inversion framework to accurately parameterize the highly irregular SZA and provide a refined DNAPL image. To evaluate the performance of the proposed approach, we conducted numerical experiments in a heterogeneous clay-sand aquifer with a complex SZA. Results demonstrate the standalone DC resistivity method fails to infer the DNAPL in complex clay-sand environments. In contrast, the combined DCIP technique provides the necessary information to reconstruct the large-scale features of K / S N fields, while integrating DCIP data with sparse but accurate borehole data results in a high resolution characterization of the SZA. • Standalone DC resistivity cannot distinguish DNAPL SZs from clay-sand soils. • Combined DC-IP can discern between DNAPL and adjacent soils. • Integrating DCIP with borehole data improves the characterization of SZs and lithology. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) has impacted groundwater resources and can pose a serious threat to human health and the environment. Its fate in groundwater is complex and challenging to predict, as it is affected by adsorption to sediments, complexation with aqueous phase ligands, and variations in hydraulic conductivity. In this study, a 2D reactive transport model based on MODFLOW and RT3D is used to simulate published experimental results of cadmium migration without and with EDTA present in a flow cell containing high- and low-permeability zones (i.e., HPZs and LPZs). The model is then extended to conceptual flow cells with more complex LPZ configurations. Simulation results generally match the experimental data well, and analysis of experimental and simulated Cd effluent concentration profiles shows that EDTA enhances Cd removal from LPZs relative to water alone. Simulation results indicate that faster Cd removal is due to EDTA complexation with adsorbed Cd in LPZs, which enhances its solubilization and subsequent back diffusion. Lastly, simulation results show that with increasing LPZ heterogeneity more Cd is retained in flow cells, and EDTA is more effective in enhancing Cd removal relative to water alone; these results are attributed to more LPZ-HPZ interfaces that enhance Cd mass transfer into LPZs during contamination, and enhance EDTA mass transfer into LPZs to promote cleanup. Overall, the results highlight the promise of using EDTA to remove Cd from heterogeneous sites, but caution is advised due to model simplicity and lack of consideration of changes in solution pH, redox potential, or competing cations. [Display omitted] • Effects of EDTA on cadmium migration in high- and low-permeability system need to be simulated. • Reactions between aqueous or adsorbed forms of Cd and EDTA can be presented via RT3D. • EDTA enhances solubilization and subsequent back diffusion of adsorbed cadmium. • The more heterogeneous system enhances cadmium accumulation but favors cleanup via EDTA. [ABSTRACT FROM AUTHOR]

Hydrocarbon fate and transport in various categories of peatlands is complicated by the botanical origin, and thus variations in the hydraulic structures and surface chemistry of its peat soils. There has been no systematic evaluation of the role of different peat types on hydrocarbon migration. Thus, two-phase, and three-phase flow experiments were performed for living and partially decomposed peat cores from bog, fen, and swamp peatlands. Numerical simulations of water drainage were performed using HYDRUS-1D, diesel-water and diesel-water-air flow using MATLAB Reservoir Simulation Toolbox (MRST). Five water table (WT) fluctuations were imposed to explore its potential to reduce residual diesel saturation in peat columns. Our results demonstrate a good match of the relative water permeability (k rw) – saturation (S) relations estimated using the unsaturated hydraulic conductivity- S relation derived from HYDRUS-1D modeling of two-phase flow, and k rw – S from MRST for three-phase flow, for all tested peat columns. Thus, we recommended using two-phase system based k rw – S predictions if multiphase data are unavailable for peatland sites' spill management plans. We found the discharge of water and diesel both increase with increasing hydraulic conductivity, while residual water and diesel were within the range of 0.42–0.52 and of 0.04–0.11, respectively. High diesel discharge rates suggest that quick spill-response is required to manage its spread in peatlands. Up to 29% of residual diesel saturation was yielded by the five WT fluctuations, and thus we strongly recommend WT manipulation as a first step towards diesel decontamination progression in peatlands. [Display omitted] • This work is the first to report the role of different peat types on NAPL flow. • Rapid flow of NAPL in the near-surface bog peat followed by the fen and swamp. • Water table fluctuations reduce residual NAPL saturation by ∼29%. • Water table manipulation advised as first step towards NAPL spill in peatlands. [ABSTRACT FROM AUTHOR]

Quantifying VOC transport from contaminated groundwater to streams is challenging and important for understanding off-site migration of VOCs, cross-media contamination (groundwater to surface water and eventually air), and potential impacts on downstream ecosystems and human populations. A streambed point sampling approach was used to quantify fluxes of water and 14 VOCs from groundwater to an urban stream in North Carolina, USA, during summer (June 2015) and winter (January 2016). The approach is unique in coupling measurements of vertical hydraulic conductivity, vertical hydraulic head gradient, and groundwater VOC concentration at each individual sampling point, reducing or eliminating some potential concerns with other Darcian methods for quantifying VOC inputs to streams. Most results were consistent with discharge of two main VOC plumes on opposite sides of the stream. Plume 1 from the west side was dominated by cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) at mean concentrations of 19 and 11 μg L−1, respectively. Plume 2 from the east side was dominated by benzene (mean concentration 56 μg L−1). Plume 2 was not previously known, and the improved sampling approach allowed VOC discharge from both plumes to be quantified simultaneously. For 13 of the 14 detected VOCs, the mean VOC flux from groundwater to the stream (f VOC) was higher in January 2016 than in June 2015, mainly because groundwater flux was higher in January. The only exception was cDCE, the most abundant VOC in Plume 1, which had mean f VOC values of 9.8 and 9.5 mg m−2 d−1 in June 2015 and January 2016, respectively. Benzene was the most abundant VOC in Plume 2 and had mean f VOC values of 11 and 37 mg m−2 d−1 in June 2015 and January 2016, respectively. High groundwater flux drove almost all the occurrences of high VOC flux. For a given VOC, the flow-weighted mean concentration (with each VOC concentration weighted by the upward groundwater flux at the VOC sampling point) was generally larger than the unweighted mean concentration. Thus, flow-weighting of concentrations gave a more accurate indication of the average VOC concentration in net groundwater discharge to the stream. An estimate of total VOC mass discharge from groundwater to the study reach of the stream, 3.6 kg of VOC per year, was based on the f VOC results and streambed area in the reach. The bulk of this discharge was due to benzene, cDCE, and VC, with individual mass discharges of 2.1, 0.83, and 0.40 kg yr−1, respectively. Estimates of maximum potential VOC degradation in the streambed suggest that the 3.6 kg yr−1 estimate of mass discharge was not sensitive to potential degradation of VOCs in the streambed sediments above the groundwater sampling depth. • Improved Darcian streambed approach quantified VOC flux from groundwater to stream. • Hydraulic conductivity, head gradient, and VOC concentration at every sampling point. • Second plume discovered on opposite side of stream from known plume. • Integration of flux yielded VOC mass discharge: 3.6 kg yr−1 for ∼240 m2 of streambed. • Flux and mass discharge estimates not sensitive to retardation or degradation. [ABSTRACT FROM AUTHOR]

To investigate the effect of pH on bioclogging process during managed aquifer recharge, three laboratory-scale column experiments were conducted and the relative hydraulic conductivity, bacterial cell number and the concentrations of polysaccharide, protein and EPS were measured under pH 5, 7, and 9, respectively. High-throughput sequencing was also used to determine the characteristics of bacterial community under different pH conditions. The development of bioclogging was rather different for the case of pH 5. 7, and 9; i.e., the growth process and number of bacteria differed with pH. The shortest growth period and lowest number of bacteria were observed at pH 5. In addition, the difference in bacterial EPS concentration was mainly associated with the polysaccharides. The variation in pH led to different bacterial community composition and diversity. The acid-resistant Elizabethkingia and Bacillus were abundant at pH 5, while Chryseobacterium and Klebsiella had relatively high abundances at pH 7. In contrast, the basophilic Exiguobacterium accounted for >80% of the total bacterial abundance at pH 9. This work is of great significance to explore bioclogging mechanism during MAR process, and provides insights and guidances for field-based managed aquifer recharge. • Evolution characteristics at different pH conditions was revealed. • The role of pH on bioclogging phenomenon is determined. • Relationship between EPS content and aquifer clogging was clarified. [ABSTRACT FROM AUTHOR]

Permeable adsorptive barriers (PABs) consisting of individual (compost, zeolite, and brown coal) and composite (brown coal-compost and zeolite-compost) adsorbents were evaluated for their hydraulic performance and effectiveness in removing aqueous benzene using batch and column experiments. Different adsorption isotherms and kinetic models and different formulations of the equilibrium advection-dispersion equation (ADE) were evaluated for their capabilities to describe the benzene sorption in the media. The batch experiments showed that the adsorption of benzene by the adsorbents was favourable and could be adequately described by the Freundlich and Langmuir isotherms and the pseudo-second-order kinetic model. Particle attrition and structural reorganization occurred in the columns, possibly introducing preferential flow paths and resulting in slight changes in the final hydraulic conductivity values (4.3 × 10−5 cm s−1–1.7 × 10−3 cm s−1) relative to the initial values (4.2 × 10−5 cm s−1–2.14 × 10−3 cm s−1). Despite the fact that preferential flow appeared to have an impact on the performance of the investigated adsorbents, the brown coal-compost mixture proved to be the most effective adsorbent. It significantly delayed benzene breakthrough (R = 29), indicating that it can be applied as a low-cost effective adsorbent in PABs for sustainable remediation of benzene-contaminated groundwater. The formulated transport models could fairly describe the behaviour of benzene in the investigated media under dynamic flow conditions; however, model refinement and additional experimental studies are needed before pilot/full-scale applications to improve the fits and verify the benzene removal processes. Our results generally demonstrate how such studies can be useful in evaluating potential reactive barrier materials. • Sorption of benzene by low-cost permeable adsorptive barriers was investigated. • Compost, zeolite, brown coal, and their mixtures were used as the adsorbents. • Lab experiments and numerical simulations were used to evaluate the adsorbents. • Brown coal-compost mixture proved to be the most effective adsorbent. • Hydraulic conductivity of brown coal-compost mixture remained practically the same. [ABSTRACT FROM AUTHOR]

Millions of tonnes of coarse tailings sand are produced every year as a byproduct of the bitumen extraction process in the Athabasca Oil Sands Region. These tailings materials contain residual quantities of mobile solutes, which can be transported through groundwater to downgradient terrestrial and aquatic ecosystems. The anticipated ubiquity of coarse tailings sand on the post-mined landscape necessitates the characterization of its hydraulic and transport properties. Hydraulic conductivity and dispersivity was evaluated at multiple scales, and included the first field-scale tracer test conducted in a tailings sand aquifer. Average hydraulic conductivity derived using laboratory cores, single-well response tests, and the tracer test were 3.2 m d−1, 2.9 m d−1, and 3.4 m d−1, respectively. These measurements demonstrated close agreement and were consistent with expectations of a material that experiences some grain-size segregation and homogenization due to the oil sands process and the nature of deposition. The field-scale tracer test appeared to obtain the asymptotic dispersivity of the coarse tailings sand aquifer, reaching a maximum value of 0.5 m after 18 m of displacement. Coarse tailings in the oil sands that experience similar processes of segregation, settling, and deposition on the reclamation landscape could be expected to have similar hydraulic properties. • Large volumes of tailings sand must be reintegrated into the mine closure landscape. • The industrial processing and deposition of coarse tailings minimizes heterogeneity. • Multiple methods demonstrated an average hydraulic conductivity of 1.2 × 10−5 m/s. • Moment analysis of natural gradient tracer test indicated a dispersivity of 0.49 m. [ABSTRACT FROM AUTHOR]

Clay liners are widely used as porous membrane barriers to control solute transport and to prevent the leakage of leachate both in horizontal and vertical flow scenarios, such as the isolated base and ramps of sanitary landfills. Despite the primary importance of saturated hydraulic conductivity in a reliable simulation of fluid flow through clay barriers, there is no model to predict hydraulic conductivity of clayey soils permeated with saline aqueous solutions because most of the current models were developed for pure water. Therefore, the main motivation behind this study is to derive semi-empirical models for simulating the hydraulic conductivity of clayey soils in the presence of arbitrary solute concentrations in addition to deionized water. In order to achieve this goal, a relatively comprehensive dataset of 842 measured hydraulic conductivities was retrieved from the experimental literature, where almost 44% of them are related to certain solute concentrations. Afterwards, two modelling approaches were introduced; the first one is a modified form of Mbonimpa et al.'s (2002) model, in which the constants are adjusted to take into consideration the variations in liquid limit due to a change in solute concentration. A modification term was added to the model for the sake of accuracy. In the second approach, a new form of solute concentration-dependent hydraulic conductivity function was proposed, where special attention was given to void ratio and adaptive liquid limit as effective parameters. The results revealed that hydraulic conductivity predictions could be erroneous if the influence of solute concentrations in permeating fluid is ignored. An error analysis was conducted to examine the models' applicability and deviations. A blind independent set of data, including 132 data points, was also used to verify models. On the other hand, both newly proposed models could predict the hydraulic conductivity for a variety of soils, salt species, and concentrations well. Therefore, the proposed modelling approaches are somehow unique by considering the salinity of the pore fluid in addition to deionized water. More importantly, both models are comprised of easy-to-measure parameters with clear physics-based implications. • Two hydraulic conductivity models considering pore fluid salinity are proposed. • New models are benchmarked against a dataset including 842 laboratory measurements. • 3D surface correlating conductivity with void ratio and liquid limit is generated. • All parameters have physical meaning and can be determined by conventional methods. • Verification of models using a blind set of data including 132 data points. [ABSTRACT FROM AUTHOR]

Coastal aquifers are a vital water source for the more than one billion people living in coastal regions around the globe. Due to the intensity of economic activities and density of population, these aquifers are highly susceptible not only to seawater intrusion, but also to anthropogenic contamination, which may contaminate the aquifer and submarine groundwater discharge. Identification and localization of contaminant source characteristics are needed to reduce contamination. The techniques of contaminant source identification are based on numerical models that require the knowledge of the hydrodynamic properties of aquifers. Thus, the challenging topic of contaminant source and aquifer characterization (CSAC) is widely developed in the literature. However, most of the existing studies are concerned with inland aquifers with relatively uniform groundwater flow. Coastal aquifers are influenced by density-driven seawater intrusion, tidal forces, and water injection and abstraction wells. These phenomena create complex flow and transport patterns, which render the CSAC especially challenging and may explain why CSAC has never been addressed in coastal settings. The presented study aims to provide an efficient methodology for the simultaneous identification of contaminant source characteristics and aquifer hydraulic conductivity in coastal aquifers. For this purpose, the study employs numerical modeling of density-dependent flow and multiple-species solute transport, to develop trained and validated artificial neural network metamodels, and then employs these metamodels in a version of the ensemble Kalman filter (EnKF) termed the 'constrained restart dual EnKF (CRD-EnKF)' algorithm. We show that this variant of the EnKF can be successfully applied to CSAC in the complex setting of coastal aquifers. Furthermore, the study analyzes the influence of common issues in CSAC monitoring, such as the effect of non-ideal monitoring network distributions, measurement errors, and multi-level vs. single level monitoring wells. • Contaminant source and aquifer characterization is performed in a coastal aquifer setting. • The aquifer is affected by density-dependent flow, tidal forces, and injection/abstraction. • The constrained-restart-dual ensemble Kalman filter is employed. • Effect of non-ideal monitoring networks, measurement errors, and multi vs. single-level monitoring wells are analyzed. [ABSTRACT FROM AUTHOR]

Steam injection technology is commonly used to rapidly remove volatile and semi-volatile organic pollutants in aquifers, and its remediation effect is highly related to steam migration and temperature distribution. However, systematic studies on steam migration and temperature distribution across different types of aquifers are lacking. In this study, the steam migration and temperature distribution in an aquifer were investigated through a series of two-dimensional sandbox experiments with different groundwater velocities, steam injection flow rates, and stratigraphic structures. The experimental results indicated that the temperature distribution in the aquifer was related to the formation permeability and steam injection flow rate. When the hydraulic conductivity of the aquifer was lower than 10−3 cm·s−1, the heating zone in the aquifer had an H-shaped distribution, and when it was higher than 10−2 cm·s−1, the heating zone had a V-shaped distribution for a high steam injection flow rate (1 kgh−1), and an H-shaped distribution for a low injection flow rate (0.5 kgh−1). Under the same injection steam flow rate, the total area of the heating zone in the aquifers with different media was in the following order of sand particle size: coarse sand > fine sand > medium sand. Owing to the heat pipe and heat dispersion effects, the heating zone area in the fine sand aquifer was larger than that of the medium sand aquifer. Groundwater velocity did not affect the area of the heating zone. With the increase in groundwater velocity, the heating zone expanded downstream. In layered heterogeneous aquifers, the upper fine and lower coarse structures formed a steam-blocking interface, resulting in steam accumulation and temperature increase in the lower layer. These findings are significant for improving our understanding of steam migration and temperature distribution in aquifers, leading to an improved design and prediction of the steam remediation required for mitigating aquifer pollution. • The shape of the heating zone was related to aquifer permeability and steam flow rate. • The heating zone area in fine sand increased due to heat pipe and thermal dispersion effects. • Interface effect led to steam accumulation and temperature increase in lower layer. • Groundwater velocity did not affect the total area of the heating zone. [ABSTRACT FROM AUTHOR]

Immobile water is defined as a firmly bound thin film of water surrounding clay particles that somehow resembles solid features. The characteristics and amount of immobile water depend on pore water chemistry, the type of clay, and the contact area of water with particles. Seepage can occur only when the hydraulic gradient increases sufficiently. To date, no accurate method is available to determine the immobile water content. Therefore, an approach is proposed for direct measurement of the immobile water content, which applies vegetable oil to displace water in saturated compacted clay-rich materials. Under a high hydraulic gradient, the vegetable oil completely displaces the free porewater in the clay-rich materials, and the remaining residual water is assumed to be immobile water. By using carbon tetrachloride (CCl 4) as an extracting agent, the oil in the permeated sample was subjected to extraction and distillation, and the total liquid content was determined by the oven-drying method. As a final step, the immobile water content in the sample was calculated. A high-speed centrifugation method was used to determine the immobile water content from the corresponding soil water potential, which accounted for approximately 634.3 kPa. It was found that the immobile water of the clay-rich materials was essentially equal to the bound water. The effective porosity of clay was obtained based on the immobile water content. By comparing the relationship between porosity, effective porosity and hydraulic conductivity, the effective porosity revealed an improved linear relationship with log (k). The test results show the oil seepage method to be useful and reliable for the quantification of the immobile water content of saturated compacted clay-rich materials. • The method of measuring immobile water content of compacted clay is proposed. • The effective porosity of clay is obtained by immobile water. • As illite and montmorillonite increases, immobile water content is increasing and effective porosity n e is decreasing. • The effective porosity n e has better linear relationship than porosity n with log(k). [ABSTRACT FROM AUTHOR]

Uniform and multi-dispersed grain packs have been frequently used to conceptually study flow in porous media. Numerical simulations were previously used to address the effect of particle shape on characteristics, such as pore space fractal dimension, moisture characteristic curve (MCC) and saturated hydraulic conductivity (SHC) of grain packs. However, experimental observations are still required since fractal-based approaches have been extensively proposed to model various properties in porous media. In this study, 16 angular sand and 16 spherical glass bead samples with different particle size distributions (PSDs) from well- to poorly-sorted were packed. The MCC was measured using the combination of sandbox and pressure plates methods. The pore space fractal dimension (D MCC), calculated from the measured MCC, ranged from 0.80 to 2.86 in sand and from ‐0.18 to 2.81 in glass bead packs, which indicated that D MCC may be negative in homogeneous media (e.g., glass bead packs) consistent with several studies in the literature. Results showed greater D MCC for the sand packs than the glass bead packs with the same geometric mean diameter values and PSDs. This clearly demonstrated the effect of particle shape on D MCC in the studied packs. The critical path analysis (CPA) approach was used to estimate the SHC measured using the constant-head method. We found that the CPA estimated the SHC accurately, within a factor of four of the measurements on average. Although the CPA is theoretically known to be accurate in media with broad pore size distributions, we experimentally found that it estimated the SHC in various types of grain packs reasonably well. • Particle characteristics effect on pore space was investigated via synthetic media. • Critical path analysis was applied to estimate saturated hydraulic conductivity. • Results showed good agreement between theoretical estimations and measurements. [ABSTRACT FROM AUTHOR]

Accurate characterization of hydraulic parameters is vital for modeling subsurface flow and transport. In the past decade, ensemble-based methods have been widely applied in estimating unknown parameters from state measurements. However, these methods require sufficiently large ensemble sizes to guarantee the accuracy of the ensemble averaged parameter sensitivities, leading to heavy computational burdens especially in large-scale problems. Although different surrogates have been introduced to alleviate the computational burden, the sensitivity information therein is still calculated by sampling the surrogate. Therefore, the sampling error is still inevitable. In this study, we propose an adaptive Gaussian process (GP) based iterative smoother (GPIS) algorithm in which the parameter sensitivity indices are analytically derived from the GP surrogate. During the iterations, the GP surrogate is adaptively refined by taking the updated parameter realizations as new base points. Both numerical and experimental cases are conducted to test the effectiveness of GPIS. We also compare its performance in estimating the heterogeneous hydraulic conductivity field with that of its prototype iterative ensemble smoother (IES) and our previously developed GP based iterative ensemble smoother (GPIES). Results show that, using the GP-derived sensitivity indices, GPIS shows advantages over GPIES in terms of both estimation accuracy and computational efficiency. Although subsurface flow and transport problems are considered in this work, the proposed method can be equally applied in other hydrological problems. • The Gaussian process (GP) is combined with the iterative smoother (IS) for data assimilation. • The parameter sensitivity indices in GPIS are analytically derived from the GP surrogate. • GPIS is more accurate and efficient than the previously proposed GPIES algorithm. [ABSTRACT FROM AUTHOR]

Hexavalent chromium (Cr(VI)) in groundwater impose serious health problems for human society. This study investigates the potential of using calcium (Ca) bentonite amended with sodium hexametaphosphate (SHMP) as a backfill constituent material in the soil-bentonite slurry trench wall to envelop the Cr(VI) impacted groundwater. The hydraulic conductivity (K) and consolidation of backfill comprising of 80 wt% sand and 20 wt% SHMP-amended Ca-bentonite were determined via flexible-wall permeameter tests and oedometer tests, respectively. Microstructure characterizations of the amended bentonites before and after contamination were also explored. The results indicated that when the permeated liquid changed from tap water to Cr(VI) solution, the tested specimens exhibited a 1.0 to 1.2-fold variation in short-term K , with all K values fall in range of 2.1 × 10−10 to 2.5 × 10−10 m/s. This mild variation may be attributed to terminate the tests without achieving chemical equilibrium. On the other hand, the Cr(VI) solution had insignificant effect on consolidation of the amended backfill, which is attributed to the dominated incompressible sand matrix skeleton in the backfill that withstood the consolidation pressure and shield the negative effects of the contaminated solution. The microstructure images revealed that the Cr(VI) resulted in relatively strong interlink between particles. Overall, the SHMP-amended bentonite is promising for enhancing Cr(VI) containment performance of the soil-bentonite slurry trench wall backfills. • Novel SHMP amended backfill exhibited excellent containment of Cr(VI) contamination. • Cr(VI)-contaminated groundwater slightly increased short-term hydraulic conductivity. • Cr(VI) contamination slightly affected consolidation property of the backfill. • Cr(VI) contamination yielded negligible change in microstructure of bentonite. [ABSTRACT FROM AUTHOR]

In the contaminant remediation of groundwater, the release history of contaminant sources and hydraulic conductivity field are two key parameters that need to know, but their actual values are difficult to obtain and can only be inversely identified by limited measured data. However, the process of solving the inverse problem needs to repeatedly call the forward model of contaminant transport, which is very time-consuming, especially for the high-dimensional inverse problems. In this study, based on the training data generated from a prior range of parameters (the release strength of contaminant sources and hydraulic conductivity at pilot points), the self-organizing maps (SOM) algorithm was employed to construct the surrogate model for the numerical model of contaminant transport in a simplified hypothetical aquifer, then the surrogate model was used to retrieve jointly the contaminant strength of sources and the hydraulic conductivity at pilot points, and the Kriging method of geostatistics was further used to process the estimated K-values at pilot points to obtain the hydraulic conductivity field. Also, to investigate the ability of the SOM-based surrogate model for retrieving both contaminant source strengths and hydraulic conductivity, we gradually expanded the prior range and increased the number of inversion terms in each prior range. Moreover, the robustness of the SOM-based surrogate model for inversion was illustrated by proposing the scarcity of data and different degrees of measurement error in the limited actual observation data. When the actual observation data is reduced by 2/3, the Root Mean Square Error (RMSE) of retrieving source strengths and hydraulic conductivity at pilot points are 1.07 and 0.09, respectively. The results indicated the SOM-based surrogate model shows remarkable inversion precision and robustness, and an accurate estimation of the actual hydraulic conductivity field could be obtained by the Kriging method based on that. • SOM algorithm is adopted to construct the surrogate model of contaminant transport • Jointly retrieving the contaminant source and hydraulic conductivity at pilot points by the surrogate model • Obtaining an accurate estimation of hydraulic conductivity field by coupling geostatistics method and SOM algorithm • Investigating the robustness of the surrogate model for inversion [ABSTRACT FROM AUTHOR]

Some remediation techniques, such as thermal remediation, can significantly change the soil properties. These changes can be beneficial or detrimental to the sequential application of Nature-based Solutions. This work evaluated the effects of thermal remediation on the properties of two tropical soils (Technosol and Oxisol), and discuss how these changes might impact both biotic and abiotic degradation processes. Bench tests using disturbed samples were performed under oxic and anoxic conditions, whereas 3D physical models were used to simulate the heat distribution along undisturbed samples. The changes in soils texture, density, hydraulic conductivity, iron concentration, mineralogy and microbiota were evaluated. The properties of Oxisol were more affected than those of Technosol due to the higher levels in Fe(III), organic carbon and finer texture. When heated in the range of 120 to 300 °C under oxic and anoxic conditions, the Fe(II) content and the magnetism intensity increased in Oxisol, probably due to the formation of magnetite. Under oxic conditions, the burning of Oxisol organic matter promoted an anoxic atmosphere, favoring the formation of Fe(II). However, the continuous increase of the temperature (>300 °C) lead to the decrease of Fe(II) due to the transformation of magnetite to maghemite, and then to hematite. The heating process also promoted some minerals decomposition and cementation of the clay fraction, increasing the soil texture. Bacterial populations were impacted, but showed ability to recover at 60 °C. However, above 100 °C no culturable cells were recovered and at temperatures above 270 °C soil sterilization occurred. The changes observed, especially in Oxisol samples, indicated that mild heating (between 120 and 240 °C), in turn, can increase the potential for abiotic degradation of some contaminants, such as chlorinated solvents. Therefore, heating conditions up to 240 °C during thermal remediation can be defined as to promote beneficial changes in soil properties, increasing its potential for natural attenuation by abiotic processes even when the microbiota is affected, and improving its sustainability. [Display omitted] • Physical and chemical properties of tropical soils are impacted by heating above 120 °C • Increased temperatures can lead to the transformation of Fe(III) into Fe(II) • Abiotic degradation of organics might be favored by heating from 120 to 250 °C • Biotic degradation is impacted by heating above 60 °C [ABSTRACT FROM AUTHOR]