Nonaqueous Phase Liquid Research Articles

Effects of source materials on desorption kinetics of carcinogenic PAHs from contaminated soils

Research investigating the desorptive behaviour of PAHs from contaminated soils often overlooked the effects of source materials, especially coal tar and coal tar pitch and materials alike. In this study, a refined experimental approach was adopted to establish a simple-to-complex continuum of systems that allow the investigation of desorption kinetics of benzo(a)pyrene (BaP) and 3 other carcinogenic PAHs (cPAHs) over an incubation period of 48 d. By comparing the modelled desorption parameters, elucidation of the effects of PAH source materials on their desorptive behaviour was achieved. Desorption of cPAHs from coal tar and pitch was enhanced when they were added to soils, with rapidly desorbing fraction (Frap) of BaP increased from 0.68% for pitch to 1.10% and 2.66% for pitch treated soils, and from 2.57% for coal tar to 6.24% for coal tar treated soil G and 8.76% for coal tar treated sand (1 d). At 1 d, desorption of target cPAHs from solvent and source material spiked soils generally followed the order of solvent > coal tar > pitch. Increases in Frap of cPAHs were observed in coal tar-treated soils after 48 d soil incubation (0.33%–1.16% for soil M, p ≥ 0.05, 6.24%–9.21% for soil G, p < 0.05) and was attributed to the continuous migration of coal tar as a non-aqueous phase liquid (NAPL) into soil pore structures. Slow desorption was dominated by source materials, whereas the extents and rates of rapid desorption (Frap and krap) were more controlled by the quantity of soil organic matter (SOM), rather than quality of SOM (as in solvent-spiked soils). The results of this study challenged the role of PAH source materials as ‘sinks’ and led to the proposed roles of coal tar and pitch and source materials alike as ‘reservoirs’ with a risk-driven perspective.

Update to the light nonaqueous phase liquid infiltration conceptual model

AbstractThe 1970s oil spill model described the infiltration of oil (light nonaqueous phase liquid or LNAPL) into the subsurface, resulting in an oil pancake depressing the water table within the capillary fringe. An update to the 1970s model is needed because, according to the discussion by Lenhard et al. on the work of Lenhard and Parker and Farr et al., “A key concept of their efforts was that LNAPL‐saturated ‘pancakes’ do not exist.” Lenhard and Parker and Farr et al. showed that the distribution of water, LNAPL, and air in the subsurface was a function of the LNAPL, water, and air pressures; fluid properties; and the pore‐size distribution of the porous medium, and that the fluid saturations can be calculated from fluid levels in a monitoring well. The 1970s oil spill infiltration model described that spilled LNAPL migrates downward through the vadose zone under the force of gravity with some lateral spreading. The vadose zone, where all of the liquid pressures are less than atmospheric pressure, becomes a three‐fluid zone consisting of variable saturations of air, water, and LNAPL, which together fully saturate the pore spaces. One important update to the 1970s model is that instead of the infiltrating LNAPL stopping at and depressing the water table, LNAPL penetrates the water table to a depth consistent with the gravitational and capillary forces experienced during LNAPL infiltration and creates a two‐fluid zone below the water table where LNAPL and water pressures are greater than atmospheric pressure. After the LNAPL release stops, LNAPL infiltration and migration will cease after reaching equilibrium. The updated LNAPL infiltration conceptual model, like the 1970s model, describes the situation where the LNAPL release has stopped and LNAPL infiltration and migration have ceased after reaching equilibrium. The volume of LNAPL released is also assumed to be sufficient to pass through the vadose zone and enter the saturated zone.

Thermal Treatment of Trichloroethene by Electrical Resistance Heating: Visualization of Gas Production in Coarse Layers

The effective implementation of in situ thermal treatment (ISTT) technologies requires understanding of gas production and migration in heterogenous media. However, investigations of the effects of high permeability contrast on gas formation, accumulation, and migration, as well as its potential effect on the redistribution of dense non-aqueous phase liquid (DNAPL), are relatively rare. In this study, electrical resistance heating (ERH) experiments were conducted in a thin sand-packed cell to simulate common yet not well-studied scenarios encountered during ISTT applications, such as coarse lenses surrounded by finer material. Two packing configurations were employed: 2 mm glass beads surrounded by 20/30 silica sand and 20/30 silica sand overlaying 40/50 silica sand. Each experiment contained an emplaced pool of trichloroethene (TCE) within the coarse material. If permeable material or pathways were present between the coarse lens and the upper cell boundary, the gas migrated along these pathways, and local DNAPL redistribution was limited to near the top of the pool before it vaporized. In contrast, if the coarse material was surrounded by finer material and contained a sufficient volume of DNAPL, the gas accumulated inside the coarse lens leading to DNAPL displacement from the lens. For five selected DNAPLs, this volume was estimated to be 0.1% to 0.5% of the total pore volume of the coarse material. The conceptual model developed in this study improves our understanding of this common geological scenario, demonstrating the importance of considering both lower- and higher-permeability material and their effects on multiphase flow during co-boiling, as well as the design of gas extraction systems during ISTT applications.

Predicting Interfacial Tension and Adsorption at Fluid-Fluid Interfaces for Mixtures of PFAS and/or Hydrocarbon Surfactants.

Many per- and polyfluoroalkyl substances (PFAS) are surface-active and adsorb at fluid-fluid interfaces. The interfacial adsorption controls PFAS transport in multiple environmental systems, including leaching through soils, accumulation in aerosols, and treatment methods such as foam fractionation. Most PFAS contamination sites comprise mixtures of PFAS as well as hydrocarbon surfactants, which complicates their adsorption behaviors. We present a mathematical model for predicting interfacial tension and adsorption at fluid-fluid interfaces for multicomponent PFAS and hydrocarbon surfactants. The model is derived from simplifying a prior advanced thermodynamic-based model and applies to nonionic and ionic mixtures of the same charge sign with swamping electrolytes. The only required model inputs are the single-component Szyszkowski parameters obtained for the individual components. We validate the model using literature interfacial tension data of air-water and NAPL (non-aqueous phase liquid)-water interfaces covering a wide range of multicomponent PFAS and hydrocarbon surfactants. Application of the model to representative porewater PFAS concentrations in the vadose zone suggests competitive adsorption can significantly reduce PFAS retention (up to 7 times) at some highly contaminated sites. The multicomponent model can be readily incorporated into transport models to simulate the migration of mixtures of PFAS and/or hydrocarbon surfactants in the environment.

Unraveling the Fate and Transport of DNAPLs in Heterogeneous Aquifer Systems—A Critical Review and Bibliometric Analysis

Dense nonaqueous phase liquid (DNAPL) in the subsurface environment beyond the permissible limit poses a threat to human health and a suite of ecological services. An accurate prediction of the concentration and mass fluxes of DNAPL at environmentally sensitive locations and their temporal variations can be obtained using robust and efficient fate and transport mathematical models. Thus, this study evaluated 412 articles published from 1990 to 2022 utilizing the Scopus® database to provide a quantitative overview of the present trends and future perspectives of the DNAPL transport research field, especially fate and transport models via bibliometric analysis. The major findings of the published literature based on the scale of the study and type of modeling framework, relationships of governing parameters with a scale of study, and recent developments in the mathematical models were discussed. The country-citation analysis revealed the USA and Canada as leading countries in DNAPL fate and transport research field. The findings of this study uncovered a need for studies considering low-permeability and stagnant regions, as well as the variable nature of the flow, transport, and reaction parameters to understand the complex plume evolution dynamics of DNAPLs under field-scale conditions. The nonlinear interactions between various flow and transport phenomena should be quantified during a model’s development by applying spatial- and time-varying global sensitivity analyses. The outcomes of the bibliometric survey, visual analysis, and concise review presented in this study can provide a wide range of references, emerging topics, and prospects to emphasize less focused on topics of DNAPL transport research.

Influence of porosity and temperature on light non-aqueous phase liquid diffusion in soils

Soil characteristics play an important role in distribution of light non-aqueous phase liquid (LNAPL) spilled from buried pipeline, and enhanced understanding of distribution is significant to the effective design of soil and groundwater remediation strategies. Therefore, distribution of diesel in soils with different porosity and temperature on the temporal evolution of the diesel migration following the saturation profiles of the two-phase flow in soils were investigated in this study. The diffusion ranges, areas and volumes in both the radial as well as in axial directions of leaked diesel in soils with different porosity and temperature increased with time. Soil porosities played an important role in the distributions when soil temperatures had no effect on distributions of diesel in soils. The distribution areas were 0.385 m2, 0.294 m2, 0.213 m2, and 0.170 m2 at 60 min when the soils porosities were 0.1, 0.2, 0.3, and 0.4, respectively. The distribution volumes were 0.177 m3, 0.125 m3, 0.082 m3, 0.060 m3 at 60 min when the soils porosities were 0.1, 0.2, 0.3, and 0.4, respectively. But the distribution areas were 0.213 m2 at 60 min when the soil temperatures were 286.15 K, 296.15 K, 306.15 K and 316.15 K, respectively. The distribution volumes were 0.082 m3 at 60 min when the soil temperatures were 286.15 K, 296.15 K, 306.15 K and 316.15 K, respectively. The calculation formulas of distribution areas and volumes of diesel in soils with different porosity and temperature for developing prevention and control strategies in the future were fitted. The seepage velocities of diesel changed sharply around the leakage port and decreased from about 4.9 m/s to 0 within a few millimeters in soils with different porosity. Additionally, the diffusion ranges of leaked diesel in soils with different porosity were different, indicating that soil porosity had a significant impact on seepage velocities and pressures. The seepage velocities fields and pressures fields of diesel in soils with different temperature were same at the leakage velocity of 4.9 m/s. And the study could provide some supports for determination of the safety zone and formulation of emergency response plans for LNAPL leakage accidents.

Numerical Modeling and Data‐Worth Analysis for Characterizing the Architecture and Dissolution Rates of a Multicomponent DNAPL Source

AbstractA numerical solute transport model was calibrated to a high‐resolution monitoring data set to characterize a multicomponent source of nonaqueous phase liquids (NAPLs) and evaluate the uncertainty of estimated parameters. The dissolution of NAPL mass was simulated using SEAM3D with parameter zones including adjustable NAPL saturations and mass transfer coefficients, representing the heterogenous architecture of the source zone. Source zone parameters were simultaneously estimated using PEST from aqueous‐phase concentrations measured in a multilevel monitoring transect and from mass recovery rates measured at extraction wells during a controlled field experiment. Data‐worth analyses, facilitated by PEST ancillary software, linked maximum aqueous‐phase concentrations of all compounds to reductions in the pre‐calibration uncertainty of mass transfer coefficients. In turn, decreasing concentrations of the most soluble NAPL fraction constrained the source mass estimation. The accurate estimation of model parameters was possible by removing concentrations measured during early NAPL dissolution stages, identified as drivers of model bias using the iterative ensemble smoother PESTPP‐iES. Although uncertainty analyses highlighted model limitations for representing sub‐grid‐scale heterogeneity of NAPL distribution and mass transfer rates, final stages of NAPL dissolution measured at multilevel ports eliminated parameter bias and produced long‐term projections of multi‐stage source zone depletion. Including mass discharge rates for model calibration further improved the accuracy of estimated residual source mass, complementing multilevel monitoring constraints on the saturation distribution and mass transfer coefficients.

Evaluation and Application of Fractal-Based Hydraulic Constitutive Model for Unsaturated Flow in Heterogeneous Soils

The physical-based hydraulic constitutive model developed by fractal theory has received much attention, however, its performance in two-phase flows has not been fully evaluated. This study evaluates the ability of fractal-based hydraulic constitutive models to describe the two-phase flow combining mass and momentum conservation equations. After that, we investigate the injection and displacement characteristics of the amendment at NAPL (non-aqueous phase liquids)-contaminated low-permeability sites based on this fractal model. Points of interest are the effect of the location of hydraulic fracturing in layered soils on amendment injection and distribution and the corresponding key parameters. Comparison with experimental results reveals that the fractal model describes the two-phase flow behavior very well. Fractal dimension and intrinsic permeability of soils have significant effects on the two-phase flow. For layered soils, the lower part of the low-permeability layer is preferably recommended as the main fracturing and injection zone to obtain the optimal injection efficiency. Besides, due to the lower flow resistance, hydraulic fractures across layers can be treated as fractures in highly permeable layers. This work provides new insights into the application of fractal theory in describing two-phase flows.

Mass transfer during surfactant-enhanced DNAPL remediation: Pore-scale experiments and new correlation

Surfactant-enhanced solubilization has been proven successful to remediate aquifers contaminated with non-aqueous phase liquids (DNAPLs). Understanding mass transfer from DNAPLs to the surfactant solution is highly important for improving remediation efficiency. However, the pore-scale mechanisms underlying the effect of surfactants on mass transfer behaviors have not been fully understood. Here we use microfluidic experiments to investigate the fundamental physical processes and quantify the impacts of surfactants on mass transfer rate at the pore scale. We find that surfactants can induce droplet breakup behaviors due to the interfacial disparity of mass transfer strength, resulting in the subsequent micro-movement of daughter blobs due to capillary force. Through image-based measurements, we find a significant inhibition of the mass transfer rates by surfactants at concentrations above the critical micelle concentration, and the inhibition becomes more pronounced as the concentration increases, which can be explained by an increase in stagnant film thickness. But the total mass transfer flux is improved because of the enhanced driving force caused by the surfactant-enhanced concentration gradient. In light of the observed physical processes and the behavior of mass transfer inhibition, we finally develop a new empirical model that explicitly considers the effects of surfactant concentration, DNAPL distribution, effective interface area, and flow rate to predict the mass transfer rate. This work provides pore-scale underpinnings of the mass transfer behaviors and a new mass transfer model considering the effect of surfactants, which improves the understanding of interphase mass transfer processes under the action of surfactants in subsurface hydrological and engineering systems.

Pyrolysis-A tool in the wastewater solids handling portfolio, not a silver bullet: Benefits, drawbacks, and future directions.

Pyrolysis is the process whereby carbonaceous materials, such as biosolids, are heated between 400°C and 900°C in the absence of oxygen. Three main products are generated: a solid product called biochar, a py-liquid that consists of aqueous phase and non-aqueous phase liquid, and py-gas. The biochar holds value as a beneficial soil amendment and sequesters carbon. The py-liquid is potentially hazardous and needs to be dealt with (including potentially reducing it on-site via catalysis or thermal oxidation). Py-gas can be used on-site for energy recovery. Pyrolysis has gained recent interest due to concern over per- and polyfluoroalkyl substances (PFAS) in biosolids. Although pyrolysis can remove PFAS from biosolids, it has been shown to produce PFAS that reside in py-liquid, and the fate in py-gas remains a knowledge gap. More research is needed to help close the PFAS and fluorine mass balance through pyrolysis influent and effluent products because pyrolysis alone does not destroy all PFAS. The moisture content of biosolids substantially affects the energy balance for pyrolysis. Utilities that already produce a dried biosolids product are in a better position to install pyrolysis. Pyrolysis has both defined benefits (solids reduction, PFAS removal from biosolids, and biochar production) as well as remaining questions (the fate of PFAS in py-gas and py-liquid, mass balance on nutrients, and py-liquid handling options) that will be answered through more pilot and full-scale demonstrations. Regulations and local policies (such as carbon sequestration credits) could affect pyrolysis implementation. Pyrolysis should be considered as an option in the biosolids stabilization toolbox with application being based on individual circumstances of a utility (e.g., energy, moisture content of biosolids, PFAS). PRACTITIONER POINTS: Pyrolysis has known benefits but limited full-scale operational data. Pyrolysis removes PFAS from biochar, but PFAS fate in gas phase is unknown. Moisture content of influent feed solids affects energy balance of pyrolysis. Policy on PFAS, carbon sequestration, or renewable energy could impact pyrolysis.

The effects of water table fluctuation on LNAPL deposit in highly permeable porous media: A coupled numerical and experimental study

Light Non-Aqueous Phase Liquid (LNAPL) flow on the water table is highly mobile and is sensitive to the fluctuation of groundwater. This process is highly complex and involves the migration of three immiscible phases (i.e. water, LNAPL and air) which need the explicit definition of multiple parameters. A coupled experimental and numerical simulation methodology is performed by using Time Domain Reflectrometer (TDR) and multiphase simulation of a controlled environment to mimic the water table fluctuation and its effect on the LNAPL residual saturation. TDR probes are installed in different locations of a 2D tank (i.e. a cuboid box with relatively low off-plane thickness) and the bulk permittivity of the phases are measured through artificially imposed boundary conditions. The bulk permittivity is then translated into saturation of the three different phases. The translated residual saturations along with the previously measured porous media properties (e.g. porosity and saturated permeability) are then inserted into the numerical simulator (i.e. COMSOL Multiphysics®) and the migration of the three phase in porous media is simulated. The numerical exponents and entry pressures needed for the simulation of the multiphase flow are estimated using the temporal experimental values. The exponents of water LNAPL relative permeability were estimated to be around 2 while the exponents gas LNAPL relative permeability were estimated to be closer to 3. The results, simulated with the optimized parameters, are then evaluated with pictures taken from the transparent face of the 2D tank different stages of the experiment. The temporal evolution of different phase saturation has been compared and validated between the experimental results obtained and interpreted by the TDR probe measurements and the simulations. The relative error stays in the 5 % confidence level for most reported points and only in the highly dynamic flow time steps the error reaches around 12% which are discussed in the text and is accepted due to the highly nonlinear nature of the problem.

Simulating field-scale thermal conductive heating with the potential for the migration and condensation of vapors

Thermal conductive heating (TCH) is an in-situ thermal treatment (ISTT) technology for treating non-aqueous phase liquid (NAPL) source zones. Numerical models can be useful tools for improving remedial performance, but traditional multiphase flow models are rarely used to simulate mass recovery during ISTT applications at the field scale due to their computational expense. This study developed a 3D model based on macroscopic invasion percolation to simulate the vaporization of NAPL, and the subsequent vapor migration and potential condensation at the field scale. The model was used to simulate the mass recovery of trichloroethene (TCE) from a NAPL source zone under seven scenarios of different heater placements, including three scenarios with an undersized target treatment zone (TTZ). Simulation results showed that TCH was effective in removing NAPL within the TTZ, but the treatment zone did not extend far from the perimeter heaters. In addition, during heating, NAPL condensation outside the TTZ due to the escaping vapor was observed in all scenarios. Overall, the resulting mass recovery was lower in the three scenarios with an undersized TTZ (91–95%) than in the other four scenarios (≈ 99%). Moreover, the locations of unrecovered/condensed NAPL could be inferred by monitoring mass recovery tailing at individual extraction wells.

Migration of DNAPL in Saturated Porous Media: Validation of High-Resolution Shock-Capturing Numerical Simulations through a Sandbox Experiment

This paper shows a comparison between experiments carried out in a laboratory-scale sandbox where the migration of a dense nonaqueous phase liquid (DNAPL), hydrofluoroether (HFE-7100), in a saturated porous medium was investigated, and validation was performed using high-resolution shock-capturing numerical simulations to resolve the nonlinear governing coupled partial differential equations of a three-phase immiscible fluid flow. The contaminant was released using a colored fluid as a tracer for a fixed time and pressures different from the atmospheric one into the saturated zone, first by using a column laboratory experiment, and then a sandbox-scale example with a hydraulic gradient. A digital image analysis procedure was used to determine the saturation distribution of the contaminant during its migration. These results are compared with the values determined for a DNAPL migration in a similar porous media through a numerical simulation. They show good agreement with the experimental results and also show that CactusHydro can follow the migration of a plume evolution very precisely and can also be used to evaluate the effects and environmental impacts deriving from leaks of DNAPL in saturated zones.

Impact of Nonaqueous Phase Liquid on Cr(VI) Removal by Nano Zerovalent Iron Particles: Effects of Contact Time, Pollution Load, and pH

Impact of Nonaqueous Phase Liquid on Cr(VI) Removal by Nano Zerovalent Iron Particles: Effects of Contact Time, Pollution Load, and pH

Temporal moment-based approach to understand the dissolved-phase LNAPL recovery and associated characteristics in the porous system under dynamic groundwater table conditions

Abstract The dissolved-phase hydrocarbon recovery can be the first step in decontaminating the soil–water system if spilled with light/dense non-aqueous phase liquid (L/D-NAPL). This study proposes a temporal moment-based approach to investigate the effectiveness of groundwater table manipulations for recovering dissolved-phase byproducts of light non-aqueous phase liquid (LNAPL) from the subsurface system. Temporal moments were computed utilizing experimentally observed and HYDRUS-simulated dissolved-phase toluene concentration data, representative of LNAPL, under stable and dynamic groundwater table fluctuation (GWTF) scenarios. Zeroth temporal moment (ZTM) showed that the hydrocarbon mass recovery varied from 1,804 to 5,190.6 mg/L × h, with the highest variation for the rapid GWTF scenario. An increase in the ZTM of hydrocarbon was observed with an increase in the rate of change of magnitude of the water table and pore velocity fluctuation as in the case of a rapid GWTF as compared to a stable GWTF case. The value of mean residence time for the stable groundwater table case was highest for the entire experimental duration, followed by slow, general, and rapid cases. Temporal moment analysis revealed that the high dissolved-phase hydrocarbon recovery could be achieved by manipulating groundwater table conditions. The present study provides a powerful technique to improve dissolved hydrocarbon remediation in mineral aquifers using hydrological restorations.

Modeling Dissolution of Soluble Compounds from Multi-Component NAPL Using a Desorption Approximation.

Groundwater professionals require methods to estimate the potential time required to achieve remedial goals, including locations within and downgradient of zones containing non-aqueous phase liquids (NAPLs). NAPLs have long been recognized as persistent contaminant sources to groundwater. Dissolution of multi-component NAPLs is particularly complex, and numerical models that explicitly simulate it are not widely available. This study introduces an equilibrium partitioning approximation to simulate the dissolution of the most soluble chemical components from multi-component NAPL containing a significant fraction of relatively insoluble mass. The effective distribution coefficient that describes depletion of a specific compound from NAPL is estimated based on the properties of the NAPL and the porous medium. This study also presents numerical modeling results that support the utility of the method, with verification using published empirical data collected during dissolution of residual coal tar in a controlled laboratory sand tank experiment. The numerical modeling method uses equilibrium partitioning as an approximation and matched the concentrations of the two most soluble NAPL components in and downgradient of the NAPL zone with reasonable accuracy. The results suggest that the method should be useful for screening level assessments and can be adapted to compare relative groundwater restoration timeframes of select NAPL components for various remedial alternatives.

Multiphase migration and transformation of BTEX on groundwater table fluctuation in riparian petrochemical sites

Light non-aqueous phase liquids (LNAPL) are considered to be a composition-based risk, containing multiple chemical ingredients that release dissolved- and vapor-phase plumes. In dissolved form, there is a saturation-based risk as the water source expands, affecting groundwater aquifers on a larger scale in the aquifer. As a typical pollutant found in petrochemical contaminated sites, the migration and transformation of benzene, toluene, ethylbenzene, and o-xylene (BTEX) between gas, aqueous, and NAPL phases are distinctly affected by groundwater table fluctuation (GTF). BTEX multiphase migration and transformation pattern in a petrochemical factory at the riverside was simulated based on the TMVOC model in differentiating pollution distribution and interphase transformation under stable or fluctuating groundwater tables conditions. TMVOC model performed an excellent simulation effect on the migration and transformation of BTEX in GTF circumstances. In comparison with the stable groundwater table condition, the BTEX pollution depth under GTF increased by 0.5m, the pollution area increased by 25%, and the total mass increased by 0.12 × 102kg. In both cases, the mass reduction of NAPL-phase pollutants was more significant than the total mass reduction of pollutants, and GTF further promoted the mass conversion of NAPL-phase pollutants to water pollutants. Prominently, as the groundwater table rises, the GTF can correct for evacuation, and the transport flux of gaseous pollutants at the atmospheric boundary decreases with increasing transport distance. Furthermore, descended groundwater table will intensify the transmission flux of gaseous pollutants at the atmospheric boundary with the transmission range expanding, which can be harmful to human health on the surface due to gaseous pollutants entering into the air.

Break-up and mobilization of DNAPL by acoustic excitation: Experimental evidence and pore network modeling

Dense non-aqueous phase liquids (DNAPLs) are long-term groundwater contaminants due to their high toxicity and slight solubility in water. The use of acoustic waves to remobilize trapped ganglia in subsurface porous systems have some advantages over pre-existing solutions including eliminating the bypassing effect and new environmental hazards. Designing an effective acoustically assisted remediation method for such purposes relies on understanding the underlying mechanisms and developing validated models. In this work, pore-scale microfluidic experiments were run to investigate the interplay between break-up and remobilization under sonication at different levels of flow rate and wettability conditions. Based on the experimental observation and pore-scale physical characteristics, a pore network model was developed and verified against the experimental results. Such a model was developed based on a two-dimensional network and scaled up to three-dimensional networks. In the experiments, processing of two-dimensional images showed that acoustic waves can remobilize trapped ganglia. The other observed effect of vibration is to break up blobs and reduce the mean ganglia size. Recovery enhancements were greater in hydrophilic micromodels as compared to hydrophobic system. A strong correlation was found between the remobilization and breakup indicating that the trapped ganglia are breaking up due to acoustic stimulation firstly and then a background viscous force may get them flowing under the new generated fluid distribution. In modeling, the simulation results of residual saturation reasonably matched with experimental observations. The differences between the prediction by the model and the experimental data at verification points is less than 2% for data before and after the acoustic excitation. The transitions from three-dimensional simulations were used to propose a modified capillary number. This study gives a better understanding of the mechanisms behind the effect of acoustic waves in porous media and provides a predictive tool for evaluating enhancement in fluid displacement.

Influence of advection on the soil gas radon deficit technique for the quantification of LNAPL

The Radon (Rn) deficit technique is a rapid, low-cost, and non-invasive method to identify and quantify light non-aqueous phase liquids (LNAPL) in the soil. LNAPL saturation is typically estimated from Rn deficit using Rn partition coefficients, assuming equilibrium conditions. This work examines the applicability of this method in the presence of local advective fluxes that can be generated by groundwater fluctuations or biodegradation processes in the source zone. To this end, a one-dimensional analytical model was developed to simulate the steady-state diffusive-advective transport of soil gas Rn in the presence of LNAPL. The analytical solution was first validated against an existing numerical model adapted to include advection. Then a series of simulations to study the effect of advection on Rn profiles were carried out. It was found that in high-permeability soils (such as sandy soils), advective phenomena can significantly affect Rn deficit curves in the subsurface compared with those expected, assuming either equilibrium conditions or a diffusion-dominated transport. Namely, in the presence of pressure gradients generated by groundwater fluctuations, applying the traditional Rn deficit technique (assuming equilibrium conditions) can lead to an underestimation of LNAPL saturation. Furthermore, in the presence of methanogenesis processes (e.g., in the case of a fresh LNAPL of petroleum hydrocarbons), local advective fluxes can be expected above the source zone. In such cases, Rn concentrations above the source zone can be higher than those above background areas without advective phenomena, resulting in Rn deficits higher than 1 (i.e., Rn excess), and thus leading to a wrong interpretation regarding the presence of LNAPL in the subsurface if advection is not considered. Overall, the results obtained suggest that advection should be considered in the presence of pressure gradients in the subsurface to ensure an effective application of the soil gas Rn-deficit technique for quantitative estimation of LNAPL saturation.

Retention effect and mode of capillary zone on the migration process of LNAPL pollutants based on experimental exploration

Two-dimensional sand tank experiments were designed to investigate the retention process of the capillary zone during the migration of light non-aqueous phase liquid (LNAPL) pollutants. The fine sand and silt media experiments simulated the LNAPL migration process given a shallow point source leakage scenario. The results indicate that the LNAPL was retained in the capillary zone. A retention factor, based on the ratio of the change in the vertical migration velocity of the LNAPL front with time, was proposed to quantitatively characterize the retention effect. The retention factor and time satisfied the function of σ=A×exp(−kt). And the retention factor increased gradually with time, indicating the enhanced retention effect of capillary zone on the vertical migration of LNAPL. The concentration change rate was then used to investigate the LANPL redistribution process, which had a relationship with time of νc=B×ln(t)+C. The capillary zone could be divided from top to bottom into a weak retention zone (B > 0, vc < 0), a strong retention zone (B < 0), and a barrier zone (B > 0, vc > 0). The retention effect of capillary zone on LNAPL migration gradually strengthened during the vertical migration of LNAPL. In addition, the coefficient B had a relationship with the environmental factors (i.e., EC, pH, and ORP) of B=a×sin(b×α×β×γ)c and the fitting coefficient R2 of the function was above 0.913 for both media, indicating a strong correlation between the LNAPL redistribution process and the key environmental factors.