DNAPL Research Articles

A Case Study of Deep DNAPL Contamination in Marine Soft Clays

Aquifers in China’s southeast coastal areas are protected by the overlying marine soft clays formed in Holocene transgression. However, a fundamental understanding of the characteristics of DNAPL (dense nonaqueous phase liquid) contamination in marine soft clays is limited. The study was conducted on the site of a former pharmaceutical factory in Shanghai, where serious 1,1,2-trichloroethane (DNAPL) contamination was detected up to the depth of 22.0 m below the existing grade. Partitioning calculation method was used to identify the presence of pure phase, and the results showed that pure 1,1,2-trichloroethane was accumulated at the interface of the upper soft silty clay and lower soft clay. The vertical transport was believed to be the pure DNAPL displacing pore water following the principle of two-phase flow, rather than the convection and diffusion of aqueous phase. The contamination (NAPL-soil interaction) impacted the soil properties slightly, and this effect could not account for the deep contamination. Soil structure analysis showed that these clays contained a proportion of large interaggregate pores, providing pathways for the transport of pure DNAPL. Considering their flocculated structure, the marine soft clays are not capable to perform as a barrier to prevent the downward migration of pure DNAPL.

Treatment of a Complex Emulsion of a Surfactant with Chlorinated Organic Compounds from Lindane Wastes under Alkaline Conditions by Air Stripping.

Surfactant-enhanced aquifer remediation is commonly applied in polluted sites with dense non-aqueous phase liquids (DNAPLs). This technique transfers the contamination from subsoil to an extracted emulsion, which requires further treatment. This work investigated the treatment of a complex emulsion composed of a nonionic surfactant and real DNAPL formed of chlorinated organic compounds (COCs) and generated as a lindane production waste by air stripping under alkaline conditions. The influence of the surfactant (1.5-15 g·L-1), COC concentrations (2.3-46.9 mmol·L-1), and temperature (30-60 °C) on the COC volatilization was studied and modeled in terms of an apparent constant of Henry at pH > 12. In addition, the surfactant stability was studied as a function of temperature (20-60 °C) and surfactant (2-10 g·L-1), COC (0-70.3 mmol·L-1), and NaOH (0-4 g·L-1) concentrations. A kinetic model was successfully proposed to explain the loss of surfactant capacity (SCL). The results showed that alkali and temperature caused the SCL by hydrolysis of the surfactant molecule. The increasing surfactant concentration decreased the COC volatility, whereas the temperature improved the COC volatilization. Finally, the volatilization of COCs in alkaline emulsions by air stripping (3 L·h-1) was performed to evaluate the treatment of an emulsion composed of the COCs (17.6 mmol·kg-1) and surfactant (3.5 and 7 g·L-1). The air stripping was successfully applied to remove COCs (>90%), reaching an SCL of 80% at 60 °C after 8 h. Volatilization can remove COCs from emulsions and break them, enhancing their further disposal.

Feasibility evaluation of novel anionic-nonionic gemini surfactants for surfactant-enhanced aquifer remediation

Surfactant-enhanced aquifer remediation (SEAR) can effectively remove residual dense non-aqueous phase liquids. In this work, a series of environment-friendly anionic-nonionic gemini surfactants, namely, ethylene glycol succinate polyoxyethylene sulfonate (GEOnS-m, n = 3, 5 and 7; m = 10 and 12), were synthesized and evaluated for application in SEAR. The critical micelle concentrations of GEOnS-m were 2.14 × 10−3–1.62 × 10−2 mM, and the molar solubilization ratios of GEOnS-m for tetrachloroethylene were 1.93–3.44, which were 1.5–5 times higher than traditional surfactants. The GEOnS-m solutions remained stable at 5–25 °C, 50 mM salt, and pH = 5.0–9.0. The micelle size decreased as the temperature decreased, which reduced the solubilization capacity (24.9%–37.0%) of the GEOnS-m solutions. Inorganic salts could improve the solubilization capacity of the GEOnS-m solutions by promoting micelle polymerization and increasing the hydrophobic core volume for solubilizing tetrachloroethylene. This work found that 0.80%–4.00% of GEOnS-m adsorbed on the media during simulated column flushing, which was less than the traditional surfactants due to stronger electrostatic repulsion between the micelles and the media. Meanwhile, the solubilization capacity of GEOnS-m was positively correlated with the hydrophobic carbon chain length, while, the adsorption amount in the media and application cost of GEOnS-m were positively correlated with the EO number and negatively correlated with the hydrophobic carbon chain length. Comprehensive evaluation results revealed that GEO3S-12 was preferred for SEAR. This work not only developed suitable surfactants for SEAR but also attempted to further the rational design of surfactants for diverse applications by combining different functional groups into one molecule.

Multi‐response optimization of process parameters for remediation of tetrachloroethylene pools by surfactants: Application of Taguchi design of experiment and Artificial Neural Network

AbstractWithin the scope of the study, the effectiveness of the experimental conditions was tested by performing a multi‐response Taguchi experimental design for the optimization of the minimum cost remediation performance with Tween 80, Methyl beta cyclodextrine (MCD) and Sodium dodecyl sulfate (SDS) from tetrachloroethylene (PCE) contaminated porous media. Tween 80, MCD and SDS were extensively used in cosmetic industry as emulsifier. Both time of remediation and cost of remediation were studied as two separate response variables in three replicate experiments conducted according to the Taguchi L9 orthogonal experimental design. In the multi‐response Taguchi analysis, the sensitivity analysis was performed by systematically changing the weights determined for two separate response variables in the calculation of total loss of quality (TNQLj). Optimum experimental conditions were determined with the help of the calculated multi‐response signal/noise (S/N) ratios (MRSN). The results show that the type of Flushing Agent is the most important factor in optimizing the remediation time and remediation cost for the removal of dense non‐aqueous phase liquid (DNAPL) PCE mass. Flushing rate is considered to be the least contributing factor. Furthermore, the results of the analysis of variance (ANOVA) showed that all parameters used in the system had a significant effect on the experimental results and the Taguchi method could explain 97.15% of the Remediation Time and 92.03% of the Remediation Cost. Afterwards, the data obtained from the experiments performed according to the experimental design were modelled using Artificial Neural Network (ANN) to estimate the remediation performance and remediation cost without performing new experiments.

Hydrodynamically-enhanced transfer of dense non-aqueous phase liquids into an aqueous reservoir

The use of surfactants represents a viable strategy to boost the removal yield of Dense Non-Aqueous Phase Liquids (DNAPLs) from groundwater and to shorten the operational timing of the remediation process. Surfactants, in general, help in reducing the interfacial tension at the DNAPL/water interface and enhance the solubility of the pollutant in the water phase through the formation of dispersed systems, such as micelles and emulsions. In this paper, we show that a suitable choice of a surfactant, in this case belonging to the bio-degradable class of ethoxylated alcohols, allows for the formation of hydrodynamic interfacial instabilities that further enhances the dissolution rate of the organic pollutant into the water phase. In a stratified configuration (denser organic phase at the bottom and lighter water phase on top), the instabilities appear as upward-pointing fingers that originate from the inversion of the local density at the interface. This inversion stems from the synergetic coupling of two effects promoted by the ethoxylated surfactant: i) the enhanced co-solubility of the DNAPL into the water (and viceversa), and (ii) the differential diffusion of the DNAPL and the surfactant in the aqueous phase. By dissolving into the DNAPL, the surfactant also reduces locally the surface tension at the liquid–liquid interface, thereby inducing transversal Marangoni flows. In our work, we carefully evaluated the effects of the concentration of different surfactants (two different ethoxylated alcohols, sodium dodecylsulphate, cetyltrimethyl ammonium bromide, N-tetradecyl-N, N-dimethylamine oxide and bis(2-ethylhexyl) sulfosuccinate sodium salt) on the onset of the instabilities in 3 different DNAPLs/water stratifications, namely chloroform, trichloroethylene and tetrachloroethylene, with a special emphasis on the trichloroethylene/water system. By means of a theoretical model and nonlinear simulations, supported by surface tension, density and diffusivity measurements, we could provide a solid explanation to the observed phenomena and we found that the type of the dispersed system, the solubility of the DNAPL into the water phase, the solubility of the surfactant in the organic phase, as well as the relative diffusion and density of the surfactant and the DNAPL in the aqueous phase, are all key parameters for the onset of the instabilities. These results can be exploited in the most common remediation techniques.

Experimental Study on Residual and Dissolution of DNAPL in Transparent Fracture

Dense nonaqueous phase liquid (DNAPL) is one of the main pollution sources of the underground environment. After DNAPL enters the underground environment, the migration and diffusion process has experienced a variety of media, resulting in large-scale and long-term pollution. To better understand the residual and dissolution process of DNAPL in fractures, the visual experiment of DNAPL residual and dissolution in the fracture was carried out using the transparent fracture model made of glass material, and the changes of DNAPL migration morphology and residual distribution in the crack were obtained. The results showed that the migration front of DNAPL in the fracture was finger shaped in the process of displacement of water phase by DNAPL phase, which was consistent with the state in porous media. When the process of water phase displacing DNAPL phase was over, discrete and aggregated DNAPL droplets remained in the fracture. The residual DNAPL was mainly concentrated in the area ranged from 0.3 mm to 0.8 mm. The dissolution rate of DNAPL in the fracture changed from fast to slow, and there was an obvious tailing period. The increased velocity of water phase flow significantly shortened the time of dissolution process. The DNAPL with hydrolysis reaction accounted for only 0.86% on average in the dissolution process. The findings of this study are helpful to the remediation of the underground environment.

Mass-flux Reduction and Mass Removal Relationships (MFR-MR) of Pool-dominated Multicomponent DNAPLs in Heterogeneous Geological System

<p>Contamination of groundwater by chlorinated solvents such as trichloroethylene (TCE, tetrachloroethylene (PCE), and 1,1-dichloroethylene (1,1DCE) is a widespread problem that can seriously threaten human health and the environment. These compounds are often present as dense nonaqueous phase liquid (DNAPL) mixtures in the subsurface that serve as a long-term groundwater contamination sources. The objective of this study is to test the effectiveness of different in-situ flushing agents (SDS, Tween 80, MCD, and water) for the recovery of DNAPL mixtures (TCE, PCE, 1,1DCE) entrapped in heterogeneous aquifer media. A series of batch tests were first performed to assess the multiphase behavior of water-DNAPL mixture-reagent system. Intermediate scale tank experiments were then performed to compare the performance of the reagent solutions. The flushing breakthrough curves were also used to investigate the impact of solubilization agents on mass-flux reduction/mass-removal behavior relationships. Performance of agents on DNAPL mixture source zone remediation was found to be in the following order: Tween 80>SDS>MCD>>Water. Individual DNAPL constituents exhibited transient effective solubilities that depend on the reagent contents and DNAPL compositions. Although high fractions of DNAPL source zones in heterogeneous porous media were removed through chemical flushing (SDS, Tween 80, MCD, and water), the heterogeneous system exhibited an extended multi-step concentration behavior. The results emphasized that in the early stage, some portion of the organic liquid is hydraulically accessible (matrix) whereas the later stage of mass removal was controlled by the more poorly accessible mass (pool) associated with higher-saturation zones. Our results also showed that the distribution and the emplacement of organic liquid, flow-field heterogeneity, sorption and desorption, and flushing solution all influenced the DNAPL mixture removal. The results from this study underline the need for understanding the multiphase behaviour of a system, particularly when DNAPL mixtures are present, prior to field-scale remediation implementation.</p>

Experimental study of electrical complex resistivity in a 2D multiphase porous medium under non‐isothermal conditions: Application to soil remediation monitoring

AbstractIn this decade, electro‐geophysical methods are widely used in different environmental subjects. Studies on soil remediation when polluted by dense non‐aqueous phase liquids (DNAPLs) has become a certain need for all countries. Geoelectrical methods have shown their potential to facilitate evaluating decontamination processes. Our challenge in this study was to understand how coupled temperature and saturation changes affect electro‐geophysical parameters in a contaminated 2D sample. The primary objective was to evaluate the efficiency and potential of spectral‐induced polarization (SIP) for monitoring the recovery of DNAPLs in contaminated porous media. A set of 2D tank experiments investigated the impacts of temperature and saturation changes on the electrical complex resistivity of a saturated porous medium under non‐isothermal conditions. The measurements were made with a coal‐tar and water fluid pair in a porous medium that has been simulated by 1 mm glass beads. Hot water was circulated around the tank and an immersion heater used to heat the porous medium in the tank at different stages. The SIP technique (also called complex resistivity) was used to measure the complex electrical resistivity of a medium in the frequency domain. The experimental results for a simple drainage case were validated using numerical modelling. The complex electrical resistivity was used to obtain the saturation field before and after imbibition. For this purpose, the generalized Archie's law obtained for the same fluid pair with 1D cells (with a vertical flow) was used. Our results from electrical resistivity measurements for saturation fields are in accordance with 2D tank images and can illustrate the saturation change with pointwise resistivity measurements. The results show that saturation change has the primary role in electrical resistivity variation compared to temperature (5%–7%). We also studied the effects of temperature change on the Cole–Cole parameters, and the results confirm our previous findings with the same variation trend in these parameters. The results from varying electrical complex resistivity from the 2D tank (with vertical and horizontal flow) in the laboratory conditions will help us to understand the coupled temperature and saturation effects on complex resistivity in a real polluted site case.

Inversion-based identification of DNAPLs-contaminated groundwater based on surrogate model of deep convolutional neural network

Abstract This paper combines theoretical analysis with practical examples to examine outstanding issues in research on the inversion-based identification of dense non-aqueous phase liquids (DNAPLs) in groundwater. We first generalize the relevant geological and hydrogeological conditions to establish a conceptual model of groundwater contamination. We then use it to formulate a preliminary model of the contamination of groundwater by DNAPLs based on multi-phase flow to describe the mechanism of migration of these pollutants. Following this, a surrogate model is established by training and validating the deep convolutional neural network (DCNN) based on training samples and samples for verification. Finally, the surrogate model is embedded into an optimization model as an equality constraint and the particle swarm optimization (PSO) algorithm is used to solve it.

Acute Toxicity Evaluation of Lindane-Waste Contaminated Soils Treated by Surfactant-Enhanced ISCO.

The discharge of lindane wastes in unlined landfills causes groundwater and soil pollution worldwide. The liquid waste generated (a mixture of 28 chlorinated organic compounds, COCs) constitutes a dense non-aqueous phase liquid (DNAPL) that is highly persistent. Although in situ chemical oxidation (ISCO) is effective for degrading organic pollutants, the low COCs solubility requires high reaction times. Simultaneous injection of surfactants and oxidants (S-ISCO) is a promising technology to solve the limitation of ISCO treatment. The current work studies the remediation of highly polluted soil (COCs = 3682 mg/kg) obtained at the Sardas landfill (Sabiñáñigo, Spain) by ISCO and S-ISCO treatments. Special attention is paid to acute soil toxicity before and after the soil treatment. Microtox®, modified Basic Solid-Phase Test (mBSPT) and adapted Organic Solvent Sample Solubilization Test (aOSSST) were used for this scope. Persulfate (PS, 210 mM) activated by alkali (NaOH, 210 mM) was used in both ISCO and S-ISCO runs. A non-ionic and biodegradable surfactant selected in previous work, Emulse®3 (E3, 5, and 10 g/L), was applied in S-ISCO experiments. Runs were performed in soil columns filled with 50 g of polluted soil, with eight pore volumes (Pvs) of the reagents injected and 96 h between successive Pv injections. The total treatment time was 32 days. The results were compared with those corresponding without surfactant (ISCO). After remediation treatments, soils were water-washed, simulating the conditions of groundwater flux in the subsoil. The treatments applied highly reduced soil toxicity (final soil toxicity equivalent to that obtained for non-contaminated soil, mBSPT) and organic extract toxicity (reduction > 95%, aOSSST). Surfactant application did not cause an increase in the toxicity of the treated soil, highlighting its suitability for full-scale applications.

A combined search method based on a deep learning combined surrogate model for groundwater DNAPL contamination source identification

Ensemble Kalman filter (EnKF) and optimization methods are two mainstream methods in groundwater contamination source identification, but most researchers usually only use or improve on one method. However, each method has strengths and weaknesses, and using one method alone may cause inaccuracy of the identification results for complex situations. Therefore, it is necessary to explore the combination of methods. In this paper, for the optimization method, to further enhance the solution precision of the optimization model for groundwater dense non-aqueous phase liquid (DNAPL) contamination source identification (GDCSI), we first constructed an improved butterfly optimization algorithm by introducing the dynamic switching probability mechanism in the previous butterfly optimization algorithm. Next, the EnKF method and optimization method (based on the improved butterfly optimization algorithm) were used respectively for GDCSI, to assess the strengths and weaknesses of the methods. Then, according to the strengths and weaknesses of the EnKF and optimization methods, the two methods were merged to build a more robust and practical combined search method (CSM) for GDCSI, to further enhance the identification accuracy and effectiveness. In addition, we innovatively applied the EnKF method to combine two single deep learning surrogate models: the deep belief neural network surrogate model and the deep residual neural network surrogate model. And a deep learning combined surrogate model was established, which further enhanced the approximation precision and applicability to the groundwater DNAPL contamination multiphase flow numerical simulation model. Finally, the results of CSM were compared with those of two single methods to analyze the former's effectiveness. The results showed that the CSM significantly improved the identification accuracy and effectiveness of GDCSI compared with any single method.

Formation of in-situ microemulsion and its efficiency for residual PCE removal in low temperature aquifers

Microemulsion is used extensively for enhanced oil recovery due to its efficient solubilization. Recently, in-situ microemulsion has been considered to have great potential for remediation of dense non-aqueous phase liquid contaminated aquifers. However, the temperature of aquifers is substantially below that of oilfields (30–120 °C), and the influence of temperature variation on in-situ microemulsion formation and solubilization is unclear. In this work, the efficient in-situ microemulsion for low temperature aquifers were established via the ε-β fishlike phase diagram; the solubilization capacity and size distribution of microemulsion at low temperature of about 10 °C were investigated; and the remediation efficiency of microemulsion flushing for residual tetrachloroethylene (PCE) removal was evaluated. Microemulsion precursor composition of 6.0 wt% compound surfactants (Tween80: SDS=3:2), 8.0 wt% isopropyl alcohol (IPA) and 3.0 wt% NaCl was prepared to form microemulsion with PCE in situ. The PCE apparent solubility was determined to be 80.28 g L−1 at 10 °C, which was three orders of magnitude higher than that in water at the same temperature, about 0.06 g L−1. The solubilization capacity was inversely proportional to temperature and IPA concentration, whereas proportional to Na+ concentration. The microemulsion droplets size were enlarged with the increase of IPA concentration, Na+ concentration and PCE solubilization by DLS experiments. Besides, sand column flushing using various eluents (water, micelle solution, in-situ microemulsion) was conducted to determine the removal efficiency for residual PCE. It was determined that in-situ microemulsion formulations can achieve removal efficiency of 97.1% for residual PCE, which was higher than that flushed via water and micelle solution, about 54.6% and 1.1%. In summary, the results confirm great feasibility of in-situ microemulsion for DNAPLs polluted aquifer remediation.

The effect of sand fractional wettability on SDBS-enhanced PCE immiscible mobilization in porous media.

Fractional wettability is common in the dense non-aqueous phase liquids (DNAPL) contaminated sites. However, it is still unclear how fractional wettability affects surfactant-enhanced DNAPL immiscible mobilization in saturated porous media. The macro-contact angle of the fractional wettability media was measured. The results of column experiments showed that the entrapped tetrachloroethene (PCE) saturations after sodium dodecyl benzene sulfonate (SDBS) flooding were lower in the media where NAPL-wet sand was present compared with those in water-wet media. In the media which contained 25% octadecyltrichlorosilane (OTS)-treated sand, the entrapped PCE saturations decreased to the minimum, and the decrease was much larger in fine sand media. The SDBS-enhanced PCE recoveries were jointly affected by fractional wettability, particle size, and interfacial tension (IFT). When NAPL-wet sand was present and SDBS concentration was just 0.125g⋅L-1, the SDBS-enhanced PCE recoveries increased significantly. As the SDBS concentration continues to increase to 0.5g⋅L-1, they only increased slightly. In the fine sand media, the SDBS-enhanced PCE recoveries were higher, and they increased more obviously with the increase of NAPL-wet sand fractions. The influence weight of fractional wettability on SDBS-enhanced PCE recoveries was the largest (47.09%) under the experimental conditions. These findings indicate that it is important to consider fractional wettability characteristics when establishing a DNAPL immiscible mobilization strategy, because it is not sufficient to consider only IFT reduction, especially in media with finer pore structures.

LED visible light assisted photochemical oxidation of HCHs in aqueous phases polluted with DNAPL

This work focuses on removing hexachlorocyclohexanes (HCHs) found in groundwater polluted with dense non-aqueous phase liquids (DNAPLs) by photo-oxidation with hydrogen peroxide or persulfate using LED visible light and ferrioxalate as the catalyst. Single oxidation tests were also performed to evaluate the contribution of LED-vis light on HCHs removal. Results show that it is possible to attain the degradation of HCHs up to 85% in 420 min with persulfate, whereas percentages lower than 40% are obtained when using hydrogen peroxide. Using both oxidants in the presence of ferrioxalate and LED visible light promotes the generation of hydroxyl and sulfate radicals under circumneutral pH values, which are the main responsible species for HCHs removal. Specifically, an oxidant conversion higher than 50% was achieved during the photochemical treatment with both oxidants, whereas conversions below 20% were obtained in the absence of LED visible light irradiation. On the other hand, DNAPL produced as liquid residuum of lindane production contains other chlorinated organic compounds (COCs), which are susceptible to being oxidized by hydroxyl and sulfate radicals, generating competitive oxidation reactions. The final conversion of chlorbenzenes reaches values close to 100% and HCHs are only effectively removed when persulfate is used as the oxidant. This better performance indicates that the photo-oxidation of DNAPL polluted groundwater with LED-vis light should be carried out with persulfate to ensure the removal of more dangerous COCs. This confirms the excellent ability of sulfate radicals for C-Cl bond breakdown.

An approach for quantification of the heterogeneity of DNAPL source zone geometries

Many studies have investigated the migration and entrapment processes of source zones from dense non-aqueous phase liquid (DNAPL) contamination under different conditions. However, the characterization of occupying area by source zone (or source shape) in water-saturated aquifers is still rudimentarily considered. In this study, we demonstrated this issue (1) by providing a brief review of existing approaches for source shape consideration, (2) by proposing an approach with simple shape parameters based on the non-uniformity of source widths, and (3) by providing exemplary applications of our proposed approach on shapes already published in previous research works. Our literature review suggested that the source zone in mathematical approaches is generally characterized as simple geometrical shapes (arbitrary lines or rectangles) or system-defined parameters that contrast to complex and discontinuous shapes observed in the real world. But the characterization of such complex shapes is still not possible with acceptable efforts. Therefore, we proposed an approach to parameterize the source shape by considering the variation of width and midpoints over the depth of the entire source zone and formulate four parameters based on population statistics (mean, standard deviation). To illustrate the suitability of our approach, we applied it to the results of lab experiments, and by analyzing these complex shapes, we highlighted the potential for improving the characterization techniques of non-uniformity of the source zones.

Dense non-aqueous phase liquid chlorinated contaminant detected far from the source release area in an aquifer

Here we confirm migration of chlorinated solvents tetrachloroethene and trichloroethene, and co-disposed elemental mercury as dense non-aqueous phase liquid in an aquifer at a scale larger than previously documented in the literature, over 650 m from original surface discharge. This finding enhances the conceptual model explaining extensive contaminated plume persistence by adding structurally controlled dense non-aqueous phase liquid to known mechanisms such as source dissolution and matrix back-diffusion. Following injections of oxidants at a depth greater than 50 m, which effectively destroyed dissolved chlorinated contaminants during a groundwater remediation pilot test in South Carolina, we measured excess chloride higher than attributable to pre-test dissolved concentrations of chlorinated contaminants, and oxidative releases of mercury, which is an opportunistic tracer of non-aqueous phase solvent transport when in its elemental form. The results suggest potentially targeted destruction strategies in disconnected non-aqueous phase liquid accumulation areas may reduce remediation timeframe and support cleanup of sites previously considered technically impracticable.

Comprehensive study of acute toxicity using Microtox® bioassay in soils contaminated by lindane wastes

This research studies the acute toxicity of real contaminated soils (topsoil and subsoil) with hazardous chlorinated organic compounds (COCs) from lindane manufacturing wastes. The Microtox® bioassay was used to determine the toxicity of soils (modified Basic Solid Phase Test), soil elutriates (Basic Test), and organic extracts (adapted Organic Solvent Sample Solubilization Test), in which hydrophobic organic compounds are soluble. The acute toxicity of these persistent contaminants (hexachlorocyclohexanes, HCH isomers, as particulate matter in topsoil, and COCs, from dense non-aqueous phase liquid, DNAPL, in subsoil) and the commercial compounds were also measured. Soils tested showed different contaminant levels (topsoil: 0.9–1149 mg/kg and subsoil: 20–9528 mg/kg). Soil contaminants distribution, concentration and acute toxicity were highly related to the contamination source (HCHs or DNAPL). Soils, organic extracts, and subsoil elutriates presented high toxicity, highlighting the need for remediation of these sites. EC50 was calculated in the three-test applied for the soils tested. EC50 vs. COCs concentration in soils and soil elutriates showed an asymptotic trend, explained by the low pollutants solubility in the aqueous phase. Contrarily, EC50 vs. soil COCs concentration was more linear in the case of the organic extracts. This test was the most reliable from statistical analysis. The three methods reveal interesting and complementary information and are necessary for a complete overview of the acute toxicity of contaminated soils.

Integration of Deep Learning‐Based Inversion and Upscaled Mass‐Transfer Model for DNAPL Mass‐Discharge Estimation and Uncertainty Assessment

AbstractThe challenges posed by high‐resolution characterization of dense nonaqueous phase liquid (DNAPL) source zone architecture (SZA) have motivated the development of simpler upscaled models that rely on domain‐averaged metrics to capture the average mass discharge downstream of source zones (SZ). However, SZA is highly irregular, making estimation of these domain‐averaged metrics from sparse borehole data extremely difficult. Poor estimation of SZ metrics means that upscaled models cannot reproduce the multistage effluent concentrations. Bayesian inversion methods can be used to obtain accurate estimates of SZ metrics and their uncertainties from sparse data, and from there, upscaled models can better reproduce multistage effluent concentrations. This work presents a framework for integrating a deep‐learning‐based 3D SZA inversion method named Convolutional Variational AutoEncoder—Ensemble Smoother with Multiple Data Assimilation (CVAE‐ESMDA) with a process‐based (PB) upscaled mass‐transfer model. This framework can utilize sparse SZ data to estimate directly mass discharge without multiphase modeling. First, CVAE‐ESMDA estimates the SZA by conditioning on sparse data, which is then used as input in the upscaled model for mass‐discharge estimation. We evaluated our framework on two real and 30 synthetic bench‐scale experiments, with significantly different SZAs and multistage effluent concentrations. The results demonstrate that the CVAE‐based inversion method captures the temporal variations in SZ metrics better than standard ordinary kriging. With the improved SZ metrics, the PB model more accurately reproduces the salient patterns of the multistage mass‐discharge profiles and associated uncertainty. This approach can be used to provide valuable input for risk‐based decision making in remediation applications.

Determining effective diffusion coefficients of chlorohydrocarbons in natural clays: Unique results from highly resolved controlled release field experiments

This study aims to precisely determine the effective diffusion coefficients of chlorohydrocarbons in low permeable units under in-situ field conditions. To this end, two controlled release field experiments using TCE and PCE as dense non-aqueous phase liquids (DNAPLs) were conducted in two natural clayey deposits. Several months to years after the controlled DNAPL release, highly resolved concentration profiles were determined for the chlorohydrocarbons that had diffused into the clayey deposits. Effective diffusion coefficients for TCE and PCE were then determined by calibrating a 3D numerical and 1D analytical model, respectively, to the measured high-resolution concentration profiles. The simulations revealed that the effective diffusion coefficients vary by as much as a factor of four within the same low permeability unit being consistent with observed small-scale heterogeneities. The determined chlorohydrocarbon effective diffusion coefficients were further used to determine the equivalent thickness of DNAPL that would completely dissolve in an idealized, parallel-plate fracture by diffusion transport into clayey deposits for the time periods of the controlled release field experiments. The equivalent TCE and PCE DNAPL film thicknesses ranged between 36 and 581 μm, respectively, comparable and exceeding fracture apertures measured in naturally fractured clay rich deposits. Hence, films of DNAPL initially contained within fractures in clay-rich deposits can completely dissolve away within a few months to a few years due to diffusion. This stored contaminant mass poses a risk to adjacent aquifers if it is re-released due to diffusion out of the matrix after source depletion or remediation.

Numerical simulations of high viscosity DNAPL recovery in highly permeable porous media under isothermal and non-isothermal conditions

We developed a decimetric size model based on coupling generalized Darcy's law and heat-transfer equations to model viscous dense non-aqueous phase liquid (DNAPL) pumping through highly permeable porous media under non-isothermal conditions. The presence of fingering and non-wetting phase ganglia was modeled through an unsteady capillary diffusion coefficient and an arbitrary heterogeneous permeability field. The model was validated using existing experimental data of a simple case, an oil injection in a 2D tank packed with glass beads. Next, we compared the results of this model against a DNAPL extracting situation in the 2D tank to better understand the two-phase flow behavior in highly permeable porous media. We found that natural convection during heating plays an essential role in heat transfer, especially in the wetting phase zone. By adding the dynamic effect (unsteady conditions) we were better able to describe the presence of the ganglia in porous media. We observed good agreement between modeled and experimental oil saturation curves until the breakthrough point, with a mean relative error of about 10% for low and high flow rates, and 8% and 16% after breakthrough for low and high flow rates, respectively. Extracting viscous oil at low flow rates and high temperature generates less fingering and is well described by the generalized Darcy's law. The remobilization of residual non-wetting ganglia after the breakthrough point at the outlet is, however, difficult to simulate using the generalized Darcy's law. In the end, we treated this issue by using a perturbed permeability field to simulate the observed fingering in the 2D tank.