Nonaqueous Phase Liquid Research Articles

Performance evaluation of surrogate models for simulating multiphase NAPL transport in heterogeneous aquifers

Abstract A better understanding of the distribution of nonaqueous phase liquid (NAPL) plumes is of great importance to groundwater pollution remediation and control. However, the efficiency of surrogate models in simulating the transport is still not well addressed. Selecting a leakage problem as an example, 50 sets of random permeability distributions are generated using the Monte Carlo method, and a numerical model is used to obtain benchmark data of NAPL transport. Four machine learning methods are employed to simulate dense NAPL transport under point leakage sources across spatiotemporal scales. The validation of the models demonstrate that the random forest model can also effectively capture the spatial-temporal distribution of the plume in heterogeneous aquifers, with a maximum mean absolute error and root mean square error smaller than 5.55 × 10−4 and 5.88 × 10−5, respectively. Meanwhile, the multiple phase outcome from the random forest model fits well with the numerical results under the scenarios of linear leakage sources and light NAPL transport. The total time consumed in the computation is reduced by over 150 times after using the surrogate models. The results suggest that surrogate models can provide a promising way to understand NAPL transport in heterogeneous aquifers.

Study on numerical model of thermal conductivity of non-aqueous phase liquids contaminated soils based on mesoscale

The thermal conductivity of soils is an important factor affecting the efficiency of in-situ thermal desorption remediation of contaminated sites. Restrict the selection of in-situ thermal desorption repair methods and heating parameters. The existence of non-aqueous phase liquids (NAPLs) pollutants affects the original thermal conductivity of the soils. To obtain an accurate and efficient method to study the thermal conductivity of NAPLs-contaminated soils. In the paper, the NAPLs-contaminated soils models are established by the four-parameter random generation method, and the thermal conductivity is calculated based on the Lattice Boltzmann Method (LBM). Explore the impact of NAPLs-contaminated soil model on its thermal conductivity calculation results, and propose an optimized NAPLs-contaminated soil model. Finally, the numerical simulation results are compared with the experimental results, and a large number of calculations and statistical analyses are carried out on the thermal conductivity. The results indicate that it is feasible to combine the four-parameter random generation method with LBM to study the thermal conductivity of NAPLs-contaminated soils. The calculation accuracy of the two-dimensional model of NAPLs-contaminated soils is lower than that of the three-dimensional model, while the calculation of the three-dimensional model is too time-consuming. The optimized NAPLs-contaminated soils three-dimensional model has the characteristics of high computational accuracy and efficiency. Saturation and Nz have a great influence on the calculation time of thermal conductivity. The thermal conductivity of the two-dimensional model is more sensitive to anisotropy. With the decrease of model anisotropy, the increase of saturation, and the decrease of porosity, the calculation accuracy of thermal conductivity of two-dimensional and three-dimensional models is similar, otherwise, the calculation accuracy of two-dimensional model is lower. The influence of porosity and NAPLs content on thermal conductivity should be paid special attention to when in-situ thermal desorption.

Segmentation of two-phase flow X-ray tomography images to determine contact angle using deep autoencoders

This study improves the characterization of in situ contact angles in porous media by employing deep learning techniques (SegNet, UNet, ResNet, and UResNet) for multiphase segmentation of micro-CT images. The algorithms were tested on high-resolution X-ray images of a steady-state flow experiment where two fluid phases were simultaneously injected at different fractional flows. The models were trained to segment the images into solid, aqueous phase, and non-aqueous phase liquid (NAPL). The UResNet demonstrated the best performance with an f1-score of 0.966 for the test dataset. More importantly, the UResNet offered higher reliability than the watershed algorithm for various fractional flows based on visual inspection and phase distribution analysis. The porosity calculation error of the watershed method (7.8 %) was reduced to 5.1 % by UResNet. Furthermore, UResNet accurately depicted the consistently mixed-wet condition of the rock sample throughout the experiment, in contrast to the watershed segmentation that yielded inconsistencies in contact angle calculations at an aqueous phase fractional flow of 0.01.

Remediation of DNAPL-contaminated heterogeneous aquifers using colloidal biliquid aphron: Multiscale experiments and pore-scale simulations

The heterogeneity of the aquifer seriously affects the remediation efficiency of dense non-aqueous phase liquid (DNAPL) due to the low contact efficiency for remediation reagents and DNAPL at macro scale and the restricted moving of DNAPL droplets by capillary forces in pore at micro scale. In this work, colloidal biliquid aphron (CBLA) as a remediation reagent was proposed, it has the rheological properties of shear thinning and the ultra-low interfacial tension between DNAPL. This work investigated the mechanisms and efficiencies of remediation DNAPL-contaminated heterogeneous aquifers using CBLA by sandbox and micromodel scale. The results indicated that CBLA could improve the mass transfer efficiency by both increasing the sweeping and extraction efficiency. The remediation efficiency of tetrachloroethylene (PCE) in aquifers with different permeability was above 99 % in sandbox, with the highest effluent concentration of 1.89 × 105 mg/L. Furthermore, at micromodel scale, CBLA could raise the remediation efficiency by improving the mobility and emulsification of PCE droplets. There was no PCE residue in the fine pore throats, and the remediation efficiency was 99.49 % in micromodel experiment and simulation. This work provided a promising remediation reagent for removing DNAPL from heterogeneous aquifers and laid a theoretical foundation for its practical application.

Novel magnetic non-aqueous phase liquid with superior recyclability for efficient and sustainable removal of gaseous n-hexane using two-phase partitioning bioreactor

Two-phase partitioning biotechnology has been recognized as effective alternative in enhancing the purification of hydrophobic waste-gas. However, current available non-aqueous phases (NAPs) are either difficult to be recovered or exhibit low affinity for hydrophobic pollutants, causing secondary pollution, high operating costs, or limited removal efficiency. Herein, nano silicon oil-based ferrofluid (SOF), featured with high pollutants affinity and magnetism, was synthesized and employed as an innovative NAP liquid in airlift reactors for gaseous n-hexane removal. The removal efficiency of n-hexane (540 mg m−3) reached about 90.0% during 90-day operation, and the used SOF could be quickly recovered via magnetism with a high ratio of 95.6%. Mechanism analysis found, the microbial cell surface hydrophobicity was sustainably improved under the induction of SOF, allowing abundant bacteria adhered on SOF-water interface, thereby enhancing the n-hexane mass transfer directly from SOF to microorganisms. Furthermore, the microbial activity was accelerated, and potential n-hexane-degrading functional microorganisms might be domesticated.

LNAPL migration processes based on time-lapse electrical resistivity tomography

Contamination from light non-aqueous phase liquids (LNAPLs) and their derivatives, arising from exploration, production, and transportation, has become a prevalent pollution source. This poses direct threats to human health. However, conventional investigative methods face limitations when applied to studying the extent and migration process of LNAPL contamination, as well as the redistribution of LNAPL during groundwater level fluctuations. Conventional methods lack the ability to rapidly, efficiently, and in real-time acquire information about contaminated areas. Therefore, this study utilizes time-lapse electrical resistivity tomography to investigate the migration mechanism of LNAPL under unsaturated conditions, constant groundwater levels, and groundwater level reductions. A relationship between resistivity and water and oil contents was established and used for inverse calculation of LNAPL content via resistivity inversion. Time-lapse electrical resistivity tomography revealed LNAPL migration in a “concave” shape across three conditions. Groundwater presence notably slowed migration, hindering downward movement and leading to a floating oil band. A robust mathematical model was established to derive the relationship between resistivity and water and oil contents. Finally, LNAPL distribution under unsaturated conditions was inversely obtained from resistivity data, showing highest content at the top leak point, obstructed area, and bottom of soil column. Consequently, time-lapse electrical resistivity tomography demonstrates a notable capacity to characterize the LNAPL migration process. This technique constitutes an effective geophysical method for monitoring and describing the characteristics of LNAPL migration. Its significance lies in enhancing our understanding of remediation for LNAPL-induced groundwater and land contamination.

Fabrication of plasma-treated superhydrophobic polydimethylsiloxane (PDMS) – Coatėd melamine sponge for enhanced adhesion capability and sustainable oil/water separation

A persistent global issue is the need for developing novel absorbent materials with high absorption efficacy and rapid absorption rates for effective oil–water separation. This work presents a novel, resilient, and recyclable material with exceptional sorption capabilities, achieved through a simple surface modification of melamine sponge (MS) using Argon plasma treatment followed by Polydimethylsiloxane (PDMS) polymer coating (Ar-PT/PDMS@MS). This study also delineates the choiceful impact of atmospheric and vacuum plasma over the surface modification of MS. The material's super hydrophobicity, demonstrated by a high-water contact angle of ∼165°, is enhanced using novel plasma treatment to homogenously coat PDMS polymer. This treatment also imparts sturdiness to withstand harsh environmental conditions. The modified 3D networked structure of melamine sponge with a porous core was able to selectively absorb the oil of various densities with the capacity range of 42–139.5 mg/g. The batch and continuous O/W separation studies revealed the efficiency of the modified MS to be able to selectively separate light nonaqueous phase liquids (LNAPL) and dense nonaqueous phase liquids (DNAPL) oil with more than 99% efficiency. The continuous studies showed that using diesel oil the separation efficiency was >99% for 35 L of oil for more than 5 h. It was also discovered that it outperformed and exceeded commercial absorbent materials in terms of absorption capacity. The recyclability studies highlighted the ability of the modified MS to be used for more than 10 cycles without compromising its separation efficiency. These findings may foreshadow Ar-PT/PDMS@MS's ability to be a promising antagonist for oil–water (O/W) separation in real-world oil-spill scenarios.

Radiometric dating of Light Non-Aqueous Phase Liquids (LNAPLs) dispersed in soil: A low environmental impact tool for natural resource restorations and protection

Non-polar liquids released in environment cause a long-term soil and groundwater contamination. The costs associated with a multi-year remediation are high and the uncertainty in the allotment of legal responsibilities could defer the decontamination planning. Therefore, a reliable method to assess the residence time of spilled Non-Aqueous Phase Liquids (NAPLs) in soil is highly needed. In this paper, a method, using low environmental radioactivity and radiometric dating, is described and applied to real contaminations. After being alpha-recoiled from 232Th in soils,228Ra accumulates in light NAPLs, generating 228Th. A disequilibrium clock, based on γ-spectrometric determination of the ratio 228Th/228Ra, was conceived to measure the residence time of LNAPL pollutants in a soil. The pollutant extraction and γ-counting procedures are described. This approach inverts the role played by pollutants and contaminated matrixes, because the flux of alpha recoil from soil generates in pollutants a reliable model of “closed system”, which is preserved in LNAPLs due to the peculiar properties of non-polar liquids. All case studies are sited in Italy and have been supported by preliminary lab tests. Dating tests of 15 samples (including LNAPLs, contaminated soils and oil socks) were performed, in most cases, blindly. Then, the results were compared to historical data about contaminations. A good correlation between test results and site history was generally obtained. But an increasing error has to be considered in old contaminations due to the nonlinear nature of disequilibrium time equation. Other reasons of discrepancies, due to system opening, include the use of surfactants, the presence of significant amount of MnO2 in soil and the interactions of different spillages. The outcomes of this research evidence the possibility to precisely date the contamination both in soil and in non-polar liquids, offering a potential tool to settle legal disputes. Further studies could broad and improve the applicability of the method.

On quantifying global carbon emission from oil contaminated lands over centuries

Petroleum releases into the subsurface contribute to global soil carbon emissions. Quantifying releases and changes in releases of carbon from soils over the lifetime of a spill is complex. Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) embodies all key mechanisms for transformation to carbon gases and their release from soils including partitioning, transport and degradation of petroleum components. Quantification of the interconnected behaviours of the soil microbiome, fluid flow, multi-component transport, partitioning, and biodegradation is crucial for understanding NSZD. Volatilization from LNAPL, aerobic biodegradation, methanogenesis, and heat production all lead to release of greenhouse gases to the atmosphere. To estimate carbon emissions, using a validated computational platform, we modelled the long term NSZD of four petroleum hydrocarbon types; crude oil, diesel, jet fuel and gasoline, to span the major products used globally. For two soil types, we estimated 150 years of carbon emissions from annual minor and 25 mostly major petroleum hydrocarbon land release incidents since 1950 – with an estimated released mass of ~9 million tonnes across the circumstances considered. Up to 2100 the mass of carbon emitted to the atmosphere is estimated to range from 4 to 6 Teragrams, with nearly 60 % currently released. Nomographs generated help predict the fate of LNAPL plumes and carbon emissions due to NSZD, which is crucially important to management of soil and groundwater contamination. The method provides a basis to include additionally identified and future petroleum releases. It is noted that the petroleum mixture composition, degradation rates, volatilization, and subsurface characteristics all can influence carbon emission estimations.

Rock core VOC profiles diagnostic of aquitard occurrence and integrity in a multi-layered sedimentary rock aquifer flow system

Aquitard assessment poses a challenge for practitioners and researchers managing dense non-aqueous phase liquid (DNAPL) contaminants in sedimentary rock aquifer systems. Within these systems, DNAPL migration and bulk hydraulic properties are controlled by fracture networks which are difficult to characterize at relevant scales with conventional methods. A previous study at a DNAPL contaminated site used contrasts in high-resolution vertical gradients to delineate four aquitards and two surfaces with poor vertical connectivity within a thick (>200 m) sedimentary rock aquifer system primarily composed of layered sandstones. In this study, we corroborate that method by using the existing DNAPL contamination as a tracer to identify and characterize the aquitards in the sequence and assess aquitard integrity. Closely spaced samples of rock from four continuous cores were collected and analyzed for volatile organic compounds (VOCs). The cores were collected along a transect perpendicular to groundwater flow and 75 m downgradient of the DNAPL source zone. Dissolved phase contaminant concentrations were collected using high-resolution multilevel systems placed along the plume centerline, downgradient from the source. The rock core and groundwater VOC concentration profiles suggest a primary aquitard limits vertical migration of both DNAPL and the associated dissolved phase contaminants, despite relatively large, downward gradients across its lower boundary. A calibrated, three-dimensional numerical groundwater flow model indicates an anisotropy factor of 1,000 for this aquitard. The Kv values for all four aquitards are only slightly lower than the values for the aquifers. However, the aquitard Kv values are 1–3 orders of magnitude less than their Kh values, and the aquitard Kh values are often similar to or even greater than the Kh values for the aquifers. Therefore, large anisotropy, rather than exceptionally low Kv values, distinguish the aquitards from the aquifers in this system. Fracture network characterization of nearby outcrops showed the anisotropy of the aquitards can be attributed to laterally extensive, bedding parallel partings coupled with short, frequently terminating, and poorly connected vertical fractures. In addition, the rock VOC profiles combined with fracture network data show that fracture termination surfaces can be powerful inhibitors of downward DNAPL migration providing additional evidence for the importance of these features in sedimentary rock flow systems. Ultimately, this study reveals the important influence of anisotropic aquitards on DNAPL migration, accumulation, and persistence, as well as associated plume migration. Furthermore, it demonstrates that subtle contrasts critical to aquitard integrity may be easily missed with long screened wells and other conventional, low resolution site characterization methods.

Understanding of Co-boiling between Organic Contaminants and Water during Thermal Remediation: Effects of Nonequilibrium Heat and Mass Transport.

In situ thermal desorption (ISTD) provides an efficient solution to remediation of soil and groundwater contaminated with nonaqueous phase liquids (NAPLs). Establishing a relationship between the subsurface temperature rise and NAPL removal is significant to reduce energy consumption of ISTD. However, the co-boiling phenomenon between NAPL and water poses a great challenge in developing this relationship due to the nonequilibrium heat and mass transport effects. We performed a systematic experimental investigation into the local temperature rise patterns at different distances from a NAPL pool and under different degrees of superheat by selecting four representative NAPLs (i.e., trichloroethylene, tetrachlorethylene, n-hexane, and n-octane) according to their density and boiling point relative to water. The patterns of temperature rise indicated that the underground temperature field can be divided into three zones: the zone of local thermal equilibrium, the nonequilibrium zone affected by co-boiling, and the zone unaffected by co-boiling. We developed a pattern-recognition-based approach, which considers the effects of local heat and mass transport to establish a qualitative correlation between the temperature rise and NAPL removal. Our results give deeper insights into the understanding of subsurface temperatures in ISTD practice, which can serve as the guideline for more accurate and sustainable remediation.

Development and Application of an Integrated Site Remediation Technology Mix Method Based on Site Contaminant Distribution Characteristics

Millions of contaminated sites worldwide need to be remediated to protect the environment and human health. Although numerous remediation technologies have been developed, selecting optimal technologies is challenging. Several multiple criteria decision-making methods for screening the optimal remediation technology have been proposed, but they mostly focus on a specific area rather than the whole contaminated site. In recent years, the “contamination source control—process blocking—in situ remediation” technology mix model has gradually gained high appreciation. Nevertheless, the screening of technologies within each chain of this model relies heavily on arbitrary personal experience. To avoid such arbitrariness, a petroleum-contaminated site containing light non-aqueous phase liquids (LNAPLs) was used as an example, and a scientific screening and combination procedure was developed in this study by considering the distribution characteristics of contaminants. Through the procedure, a technology mix, which includes institutional control, risk monitoring, emergency response, multiphase extraction, interception ditch, monitoring of natural attenuation, hydrodynamic control, as well as some alternative technologies, was found, aiming at different locations and strata. The clear spatial relationship concept promises to enhance the effectiveness of contaminated site remediation. The proposed method only gave us a technical framework and should be tested and enriched in future studies.

Mass transfer vectors for nitric oxide removal through biological treatments

The reduction of nitric oxide (NO) emissions to atmosphere has been recently addressed using biological technologies. However, NO removal through bioprocesses is quite challenging due to the low solubility of NO in water. Therefore, the abatement of NO emissions might be improved by adding a chelating agent or a mass transfer vector (MTV) to increase the solubility of this pollutant into the aqueous phase where the bioprocess takes place. This research seeks to assess the performance of different non-aqueous phase liquids (NAPs): n-hexadecane (HEX), diethyl sebacate (DSE), 1,1,1,3,5,5,5-heptamethyl-trisiloxane (HTX), 2,2,4,4,6,8,8-heptamethylnonane (HNO), and high temperature silicone oil (SO) in chemical absorption–biological reduction (CABR) integrated systems. The results showed that HNO and HTX had the maximum gas-liquid mass transfer capacity, being 0.32 mol NO/kmol NAP and 0.29 mol NO/kmol NAP, respectively. When an aqueous phase was added to the system, the mass transfer gas–liquid of NO was increased, with HTX reaching a removal efficiency of 82 ± 3% NO with water, and 88 ± 6% with a phosphate buffer solution. All NAPs were tested for short-term toxicity assessment and resulted neither toxic nor inhibitory for the biological activity (denitrification). DSE was found to be biodegradable, which could limit its applicability in biological processes for gas treatment. Finally, in the CABR system tests, it was shown that NO elimination improved in a short time (30 min) when the three mass transfer vectors (HEX, HTX, HNO) were added to enriched denitrifying bacteria.

Phase behavior of sulfolane: Potential implications for transport in groundwater

Sulfolane is a toxic pollutant freely miscible in pure water. However, above benchmark concentrations of sulfate or thiosulfate ions, it behaves as a light non-aqueous phase liquid and separates into either bulk phases or kinetically stable emulsions. Sulfolane droplets bear a negative electrostatic charge, as revealed by electrophoretic measurements. With high sulfate salt concentrations, the counterions screen the negative charge of sulfolane droplets. This promotes droplet coalescence and bulk phase separation, as well as droplet adhesion onto negatively charged mineral substrates such as clay and sand. NaCl does not separate sulfolane from water but favors its partitioning into toluene, as demonstrated by attenuated total reflectance-Fourier transform infrared spectroscopy. Water activity measurements show that sodium sulfate interacts with water more strongly than sulfolane, justifying its ability to displace sulfolane and induce its separation. Both sulfolane and sulfate ions interact with water through hydrogen (H) bonds. Water is comprised of water species that donate and accept a different number of H bonds, depending on other molecules in water. Sulfolane is an H bond acceptor. Therefore, it mainly interacts with double H bond donors that either do not accept H or accept only one H (double donor-single acceptor, DD-SA). ATR-FTIR data indicate that sulfate ions compete with sulfolane more effectively than chlorides for interactions with DD-SA, thereby promoting better separation. Sulfolane phase separation can retard its migration in polluted aquifers. This study reveals the complex behaviour of sulfolane in water, with potential implications for sulfolane transport in groundwater.

Pore-scale investigation of surfactant-enhanced DNAPL mobilization and solubilization

Surfactant-enhanced aquifer remediation has been proved successful to remove dense non-aqueous phase liquids (DNAPLs) from contaminated sites. However, the underlying mechanisms of the DNAPL mobilization and solubilization at the pore scale remains to be addressed for efficient application to the field remediation system. In this work, the emerging microfluidic and imaging technologies are applied to investigate the dynamics of DNAPL remediation. Visualized experiments of the evolution of DNAPL remediation are performed to study the role of surfactant type, concentration and injection rate. The DNAPL remediation is dominated by mobilization followed by solubilization for most surfactants. Mobilization occurs as soon as surfactants and DNAPL are in contact until forming a new stable phase structure, and the solubilization continues until the end of injection. We observe the breakup behavior of long droplets and ganglia during the mobilization, which is attributed to the surfactant-reduced interfacial tension and thus expedites DNAPL mobilization and redistribution. During the solubilization, the formation of micelles incorporating DNAPL fractions increases the DNAPL concentration gradient and thus enhances the mass transfer, but the rate-limited diffusion of micelles reduces the mass transfer rate coefficient. Increasing the surfactant content and decreasing the injection rate can promote mobilization and solubilization. The DNAPL mobilization ability of the surfactants SDS and SDBS is stronger than SAOS and Tween 80 regardless of the injection rates. Tween 80 may be considered an ideal surfactant of only solubilization but not mobilization is desired. This work elucidates the pore-scale mechanisms during surfactant-enhanced DNAPL remediation, which are beneficial for upscaling studies, predictive modeling, and operation optimization of DNAPL remediation in the field.

Unsaturated Transport and Targeted Binding of Pluronic-Coated Nanoparticles: Lysimeter Experiments

Efficient delivery of engineered nanoparticles (NPs) to a non-aqueous-phase liquid (NAPL) target zone located above the water table requires an understanding of their transport and binding characteristics. In this investigation, NPs coated with a tunable amphiphilic copolymer were employed in a series of experiments using a 1.4-m-long lysimeter. A crude oil zone was emplaced in the lysimeter to evaluate NP binding capabilities to a representative NAPL. Experimental observations were supported by a reactive transport model. NPs coated with a polymer concentration that promotes enhanced binding to crude oil were successfully delivered and retained in the NAPL zone at concentrations about three times higher than elsewhere in the lysimeter and consistent with the distribution of total petroleum hydrocarbons. Model simulations were able to reproduce the observed asymmetrical NP breakthrough curves and retention profile. The estimated attachment rate coefficient was two orders of magnitude higher for the NAPL zone than elsewhere, supporting the observed preferential binding to the crude oil. Depth-dependent straining was used in the model to capture the NP retention observed near the top of the lysimeter, presumably due to film straining caused by the increased capillary pressure. In addition to the reversible attachment and straining mechanisms, model simulations also indicated that loss of aqueous NP mass was required deeper in the lysimeter to provide a reasonable fit to the observed NP data. Due to increased contact with sediments, the polymer structure that coats the NPs may be slowly removed, leading to aggregation and reduced mobility because of physical trapping. Findings of this study revealed that despite the demonstrated targeted binding capability of these NPs in unsaturated systems, their delivery to a target NAPL zone distant from an injection location may be a challenge due to possible aggregation over longer travel distances and thus an important design consideration.Practical ApplicationsSoil and groundwater contamination by petroleum hydrocarbons is a global issue due to the harm it causes to the environment and human health. Hence, developing novel in situ technologies is necessary due to limitations associated with ex situ remediation. One emerging technology called nanoremediation involves injecting nanosize particles into the impacted area to interact with contaminants effectively due to their small size and large surface area. By modifying their surface, particles can transport through soil and stick to specific contaminants, which is called targeted delivery. To better understand this technology, it should be studied under conditions similar to those in the field. Hence, we investigated targeted delivery of engineered nanoparticles in a large 1.4-m-long column packed with natural soil, representative of the unsaturated zone above a shallow aquifer. Our findings demonstrated preferential nanoparticle attachment to the crude oil–impacted sediment which provides the foundation to advance the use of these nanoparticles as a treatment agent and increases treatment efficiency. However, the noted increase in nanoparticle retention over longer travel distances is suggestive of their stability issues in unsaturated porous media, which may pose a limitation for the delivery of these NPs to a target zone distal from the injection location.

Using groundwater monitoring wells for rapid application of soil gas radon deficit technique to evaluate residual LNAPL

The application of the 222Radon (Rn) deficit technique using subsurface soil gas probes for the identification and quantification of light non-aqueous phase liquids (LNAPL) has provided positive outcomes in recent years. This study presents an alternative method for applying this technique in the headspace of groundwater monitoring wells. The developed protocol, designed for groundwater monitoring wells with a portion of their screen in the vadose zone, is based on the use of portable equipment that allows rapid measurement of the Rn soil gas activity in the vadose zone close to the water table (i.e., smear zone) where LNAPL is typically expected. The paper first describes the step-by-step procedure to be followed for the application of this method. Then, a preliminary assessment of the potential of the method was carried out at two Italian sites characterized by accidental gasoline and diesel spills into the subsurface from underground storage tanks. Although the number of tests conducted does not allow for definitive conclusions, the results obtained suggest that, from a qualitative point of view, Rn monitoring in the headspace of monitoring wells is a promising, fast, and minimally invasive screening method that could also potentially reduce the costs associated with field data acquisition. This method proves to be suitable for detecting the presence of LNAPL in both the mobile and residual phases with results consistent with the other lines of evidence available at the sites, such as groundwater and soil gas monitoring. Future efforts should be directed toward evaluating the accuracy of this method for a quantitative assessment of residual LNAPL saturations.

Remediation of multilayer soils contaminated by heavy chlorinated solvents using biopolymer-surfactant mixtures: Two-dimensional flow experiments and simulations.

To assess the efficiency of remediating dense non-aqueous phase liquids (DNAPLs), here heavy chlorinated solvents, through injection of xanthan solutions with or without surfactant (sodium dodecylbenzenesulfonate: SDBS), we conducted a comprehensive investigation involving rheological measurements, column (1D) and two-dimensional (2D) sandbox experiments, as well as numerical simulations on two-layers sand packs. Sand packs with grain sizes of 0.2-0.3mm and 0.4-0.6mm, chosen to represent the low and high permeable soil layers respectively, were selected to be representative of real polluted field. The rheological analysis of xanthan solutions showed that the addition of SDBS to the solution reduced its viscosity due to repulsive electrostatic forces and hydrophobic interactions between the molecules while preserving its shear-thinning behavior. Results of two-phase flow experimentsdepicted that adding SDBS to the polymer solution led to a reduced differential pressure along the soil and improvements of the DNAPL recovery factor of approximately 0.15 and 0.07 in 1D homogeneous and 2D layered systems, respectively. 2D experiments revealed that the displacement of DNAPL in multilayer zones was affected by permeability difference and density contrast in a heterogeneous soil. Simulation of multiphase flow in a multilayered system was performed by incorporating non-Newtonian properties and coupling the continuity equation with generalized Darcy's law. The results of modeling and experiments are very consistent. Numerical simulations showed that for an unconfined soil, the recovery of DNAPL by injection of xanthan solution can be reduced for more than 50%. In a large 2D experimental system, the combination of injecting xanthan with blocking the contaminated zone led to a promising remediation of DNAPL-contaminated layered zones, with a recovery of 0.87.

Characterization of DNAPL source zones in clay-sand media via joint inversion of DC resistivity, induced polarization and borehole data

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 (SN) and hydraulic conductivity (K). The DCIP-estimated K/SN 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/SN fields, while integrating DCIP data with sparse but accurate borehole data results in a high resolution characterization of the SZA.

The Effects of Spill Pressure on the Migration and Remediation of Dense Non-Aqueous Phase Liquids in Homogeneous and Heterogeneous Aquifers

The spill pressure of the contaminant source is an important factor affecting the amount, location, form, and behavior of the dense non-aqueous phase liquids (DNAPLs) that plume in a contaminated subsurface environment. In this study, perchloroethylene (PCE) infiltration, distribution and, remediation via a surfactant-enhanced aquifer remediation (SEAR) technique for a PCE spill event are simulated to evaluate the effects of the spill pressure of the contaminant source on the DNAPLs’ behavior in two-dimensional homogeneous and heterogeneous aquifers. Five scenarios with different spill pressures of contamination sources are considered to perform the simulations. The results indicate that the spill pressure of the contaminant source has an obvious influence on the distribution of DNAPLs and the associated efficiency of remediation in homogeneous and heterogeneous aquifers. As the spill pressure increases, more and more contaminants come into the aquifer and the spread range of contamination becomes wider and wider. Simultaneously, the remediation efficiency of contamination also decreases from 93.49% to 65.90% as the spill pressure increases from 33.0 kPa to 41.0 kPa for a heterogeneous aquifer with 200 realizations. The simulation results in both homogeneous and heterogeneous aquifers show the same influence of the spill pressure of the contaminant source on PCE behaviors in the two-dimensional model. This study indicates that the consideration of the spill pressure of the contaminant sources (such as underground petrol tanks, underground oil storage, underground pipeline, and landfill leakage) is essential for the disposal of contaminant leakage in the subsurface environment. Otherwise, it is impossible to accurately predict the migration and distribution of DNAPLs and determine the efficient scheme for the removal of contaminant spills in groundwater systems.