Comparison of thermal and chemical enhanced recovery of DNAPL in saturated porous media: 2D tank pumping experiments and two-phase flow modelling

Mapping and monitoring dense non-aqueous phase liquid source zone by fused surface and cross-borehole electrical resistivity tomography

Effective characterization of dense non-aqueous phase liquid (DNAPL) source zones is crucial for remediating polluted sites. DNAPL often reside as residuals or pools within high-permeability lenses and above impermeable layers due to soil heterogeneity, gravity, and capillary barriers. Given the high cost of drilling, electrical resistivity tomography (ERT) techniques-including surface ERT and cross-borehole ERT, are commonly used for DNAPL source zone mapping and monitoring. However, the low spatial resolution of ERT increases uncertainty in source zone investigations. This study proposes a method for improving DNAPL mapping and monitoring by fusing surface and cross-borehole ERT data. Sandbox experiments were conducted to simulate a heterogeneous DNAPL source zone, employing both ERT methods for static mapping and dynamic monitoring. Reflective light imaging (RLM) was used to visualize DNAPL migration and provide saturation data, allowing for the quantification of ERT's effectiveness in characterizing DNAPL distribution. The results indicate that individual ERT methods face significant challenges in DNAPL source zone mapping due to background interference. Surface ERT alone tends to underestimate the extent of deeper DNAPL source zones. However, fusing surface and cross-borehole ERT results in a complementary enhancement of vertical spatial resolution, thereby improving the characterization of DNAPL source zones. The fusion of static and time-lapse ERT data substantially enhances DNAPL source zone mapping and monitoring capabilities. By calculating the ratio of the ERT-monitored area to the actual area using resistivity change contours (5 %, 10 %, 15 %), it was found that fusing surface and cross-borehole ERT data improved monitoring resolution by 50.48 % compared to surface ERT alone and by 22.95 % compared to cross-borehole ERT. Principal component analysis (PCA) was effective in fusing time-lapse data, while the weighted average method (WAM) outperformed PCA for static resistivity data fusion.

Enhancing remediation of residual DNAPL in multilayer aquifers: Post-injection of alcohol-surfactant-polymer mixtures

Although polymer-surfactant injection is an effective remediation technology for multilayer aquifers contaminated by Dense Non-Aqueous Phase Liquids (DNAPL), the existence of residual DNAPL after treatment is inevitable. This study evaluates the efficiency of the post-injection of alcohol-surfactant-polymer (ASP) mixtures containing 1-propanol/1-hexanol, sodium dodecylbenzenesulfonate (SDBS), and xanthan in enhancing remediation of residual DNAPL in layered systems. A range of experimental devices, including batch, rheological measurements, centimetric 1D column, and decametric 2D tank experiments, were employed. Batch experiments revealed that the inclusion of 1-hexanol swelled the DNAPL volume due to alcohol partitioning. Conversely, with only 1-propanol present in the alcohol-surfactant (AS) mixture, DNAPL dissolved in the aqueous phase. The co-presence of 1-hexanol along with 1-propanol in AS mixture favored 1-propanol's partitioning into the DNAPL phase. Column experiments, following primary xanthan-SDBS (XS) injections, demonstrated that ASP mixtures with 1-hexanol (regardless of presence of 1-propanol) underwent a mobilization mechanism. DNAPL appeared in the effluent as an organic phase after the post-injection of 0.3 pore-volumes (PV), by a reduction trend in its density. In contrast, mixtures with solely 1-propanol exhibited a solubilization mechanism, with DNAPL dissolving in the aqueous phase and emerging in the effluent after approximately 1 PV. 2D tank experiments visualized mobilization and solubilization mechanisms in multilayered systems. Post-injection of the ASP mixture with solely 1-propanol led to DNAPL solubilization, demonstrated by a dark zone of varied DNAPL concentrations, followed by a clearer white zone indicating significant DNAPL dissolution. Injecting ASP mixture containing both 1-propanol and 1-hexanol mobilized swollen DNAPL ganglia throughout layers, with these droplets coalescing and migrating to the recovery point. The darkness of mobilized droplets was faded as more DNAPL was recovered. The solubilization ASP mixture enhanced the recovery factor by 0.02 while the mobilization ASP mixture led to a 0.08 increase in the recovery factor.

Evaluation of the impact of transition from porous to fractured rock media on 3D field-scale DNAPLs contamination

A 3D high-resolution subsurface characteristic (HSC) numerical model to assess migration and distribution of subsurface DNAPLs was developed. Diverse field data, including lithologic, hydrogeologic, petrophysical, and fracture information from both in situ observations and laboratory experiments were utilized for realistic model representation. For the first time, the model integrates hydrogeologic characteristics of both porous (unconsolidated soil (US) and weathered rock (WR)) and fractured rock (FR) media distinctly affecting DNAPLs migration. This allowed for capturing DNAPLs behavior within US, WR, and FR as well as at the boundary between the media, simultaneously. In the 3D HSC model, hypothetical 100-year DNAPLs contamination was simulated, quantitatively analyzing its spatiotemporal distributions by momentum analyses. Twelve sensitivity scenarios examined the impact of WR and FR characteristics on DNAPLs migration, delineating significant roles of WR. DNAPLs primarily resided in WR due to low permeability and limited penetration into FR through sparse inlet fractures. The permeability anisotropy in WR was most influential to determine the DNAPLs fate, surpassing the impacts of FR characteristics, including rock matrix permeability, fracture aperture size, and fracture + rock mean porosity. This study first attempted to apply the field-data-based multiple geological media concept in the DNAPLs prediction model. Consequently, the field-scale effects of WR and media transitions, which have been often overlooked in evaluating DNAPLs contamination, were underscored.

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 0.2-0.3 mm and 0.4-0.6 mm, 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 experiments depicted 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.

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.

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.

The Nanostructure of Alkyl-Sulfonate Ionic Liquids: Two 1-Alkyl-3-methylimidazolium Alkyl-Sulfonate Homologous Series

The functionalization of polymers with sulfonate groups has many important uses, ranging from biomedical applications to detergency properties used in oil-recovery processes. In this work, several ionic liquids (ILs) combining 1-alkyl-3-methylimidazolium cations [C_nC₁im]⁺ (4 ≤ n ≤ 8) with alkyl-sulfonate anions [C_mSO₃]^- (4 ≤ m ≤ 8) have been studied using molecular dynamics simulations, totalizing nine ionic liquids belonging to two homologous series. The radial distribution functions, structure factors, aggregation analyses, and spatial distribution functions reveal that the increase in aliphatic chain length induces no significant change in the structure of the polar network of the ILs. However, for imidazolium cations and sulfonate anions with shorter alkyl chains, the nonpolar organization is conditioned by the forces acting on the polar domains, namely, electrostatic interactions and hydrogen bonding.

Experimental study of DNAPL displacement by a new densified polymer solution and upscaling problems of aqueous polymer flow in porous media

The remediation of DNAPL-contaminated soil with lower-density fluids is ineffective due to the over-riding of displacing fluid. The densification of biopolymers is experimentally studied to develop a solution with the same density as a pollutant. Polymer solutions and contaminants are characterized through rheometer. A 1D column filled with monodisperse glass beads was used to measure their apparent viscosity in porous media. The displacement of pollutants by biopolymers (such as xanthan gum, guar gum, and carboxymethyl cellulose) and densified solutions based on barite are investigated in the 1D porous column. In addition, the polymer solution flow is studied using an upscaling method based on the shear viscosity measured with rheometer. The upscaling results are compared with the 1D column experimental outcomes. We found that carboxymethyl cellulose is the best for densifying polymer and showed the highest remediation yield for DNAPL remediation. The polymers' rheology was represented well through the Carreau rheological model. The discrepancy of apparent viscosity in porous media from polymers' shear viscosity measured with rheometer is explained by the adsorption of polymers on pore surfaces and deposition of barite particles in a porous medium, which led to a decrease in permeability. The upscaling results are in good agreement with experimental outcomes at low-pressure gradients. The impact of porous media geometry on polymer flow in porous media is described.

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.

Influence of the injection of densified polymer suspension on the efficiency of DNAPL displacement in contaminated saturated soils

Nowadays the remediation of DNAPL contaminated zones near groundwater has gained great prominence in environmental fields due to the high importance of water resources. In this work, we suggest injecting a densified polymer suspension by adding barite particles to displace DNAPL. To evaluate the efficiency of the densification of polymer suspensions on the displacement of DNAPL, various densities of barite-polymer suspension; lower, equal, and higher than the density of DNAPL were prepared and their rheological behavior was analyzed. Then flow experiments were performed using a decimetric-scale 2D tank. The displacement procedure was monitored with an imaging technique and the production and injection process data were recorded by mass balance interpretation. It was shown that the densification of the polymer suspension could improve the displacement efficiency of DNAPL up to four times. The clogging behavior of barite-polymer suspension was assessed in a 1D column. Generalized Darcy's law and the continuity equation were used to numerically simulate the experimental two-phase flow. To take into account the clogging behavior of the suspension, the transport equation of diluted species was implemented into the model. The simulation results show that the model can properly predicts the experimental consequences.

DNAPL flow and complex electrical resistivity evolution in saturated porous media: A coupled numerical simulation

Induced Polarization (IP) is a non-intrusive geophysical method to monitor Dense Non-Aqueous Phase Liquid (DNAPL) contamination and remediation processes underground. In this study, an advanced numerical code simulating DNAPL flow and complex electrical resistivity is presented. The model was validated against existing IP results and image measurements that were carried out previously in a series of 2D tank experiment. Multiphase flow modeling in porous media is coupled with electrical current modeling to simulate the process of DNAPL migration and the associated IP response. This brings a broader view of the contamination in space and time compared to surface and borehole measurements, especially when the results are supported by field measurements or laboratory experiments. The simulations are developed in 3D and are performed in COMSOL Multiphysics®. The simulations using petrophysical relationships for in-phase and quadrature resistivity and the results of the experiments are in complete accordance with each other in the parts of the tank where the saturation of DNAPL is relatively low (i.e. especially in the cone of depression in the pumping scenario). However, the parts associated with high saturation of DNAPL show high errors between the in-phase resistivity simulations and the results from experiments. The present work can be regarded as a preliminary study toward further applications of coupled IP-multiphase flow for more accurate detection and monitoring of DNAPLs. It is suggested that the choice of tool/approach in this study be extended to larger-scale studies for further investigation.

Experimental study of thermally enhanced recovery of high-viscosity DNAPL in saturated porous media under non-isothermal conditions

Thermal enhancement is known to be an efficient way to decrease the residual saturation of some common dense non-aqueous phase liquids (DNAPLs) after pumping. However, the effect of transient heat transfer during the recovery of a high-viscosity contaminant, such as coal tar, in highly permeable porous media is still unknown. A 2D tank experimental setup allowing monitoring of temperature and saturation fields during DNAPL pumping has been developed. Experiments were run under isothermal and non-isothermal conditions, at low and high flow rates. We investigated the presence of viscous fingering and how that influences the shape of the cone of depression, as well as the residual saturation. The saturation fields show that less viscous fingering occurs in pre-heated cases and that heating increases the recovery efficiency. Increasing the temperature increases the critical velocity and the viscosity ratio and helps to stabilize the interface between the non-wetting and wetting phase. Observations were first made on an oil and ethanol fluid pair because its properties were known, before extending the experiments to a coal tar and water fluid pair. Residual oil saturation after pumping was decreased by 6-16% in all pre-heated conditions. Pumping at low flow rate in these conditions leaves the smallest oil residual saturation (20%) after pumping. A low flow rate increases the recovery efficiency by reducing viscous fingering and by spreading the generated heat to a larger part of the tank. Finally, results on coal tar pumping show that the high thermal conductivity of water helps in keeping the temperature high during pumping. The residual coal tar saturation was reduced from 40% at 20 °C to 28% when pre-heating the tank. Operating at a low flow rate and with a uniform temperature is the key to recovering the highest amount of a viscous DNAPL such as coal tar from the soil and satisfying cleanup goals when using thermally enhanced pumping.

3D dynamic model empowering the knowledge of the decontamination mechanisms and controlling the complex remediation strategy of a contaminated industrial site

Knowledge of the geology and hydrogeology of the polluted site emblematize a key requirement for environmental remediation, through assembling and synthesizing findings from various sources of physical evidence. In an increasingly virtual era, digital and geo-referenced metadata may serve as tools for collecting, merging, matching, and understanding multi-source information. The main goal of this paper is to emphasize the significance of a 3D hydrogeochemical model to the portrayal and the understanding of contamination dynamics and decontamination mechanisms at a highly contaminated industrial site. Some remediation measures are active on-site, due to the evidence-based presence of chlorinated solvents in groundwater. These are attributable to a slow-release source of pollutants in the saturated zone associated with very low permeability sediments. Therefore, in this research, a new technique for the remediation of secondary sources of dense non-aqueous phase liquid (DNAPL) contamination was investigated for the first time on a full-scale application. The combination of groundwater circulation wells (IEG-GCW®) and a continuous electron donor production device was set up to boost in situ bioremediation (ISB). A multi-phase approach was followed handling and releasing data during various remediation stages, from site characterization via pilot testing to full-scale remediation, thus allowing users to monitor, analyze, and manipulate information in 3D space-time. Multi-source and multi-temporal scenarios reveal the impact of ongoing hydraulic dynamics and depict the decontamination mechanisms in response to the interventions implemented over time, by quantifying the overall performance of the adopted strategies in terms of removal of secondary sources of pollution still active at the site.