Dense Nonaqueous Phase Liquid Recovery Research Articles

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.

How to Predict the Efficacy of Free-Product DNAPL Pool Extraction Using 3D High-Precision Numerical Simulations: An Interdisciplinary Test Study in South-Western Sicily (Italy)

Dense nonaqueous phase liquids (DNAPLs) are known to be denser than water and immiscible with other fluids. Once released into the environment, they migrate downward through the variably saturated zone, causing severe damage. For this reason, it is essential to properly develop a rapid response strategy, including predictions of contaminant migration trajectories from numerical simulations modeling. This paper presents a series of simulations of free-product DNAPL extraction by means of a purpose-designed pumping well. The objective is to minimize the environmental impact caused by DNAPL release in the subsurface, estimating the recoverable free-product DNAPL, depending on the hydraulic properties of the aquifer medium, and estimating the leaving residual DNAPL that could act as a long-term pollution source. Coupling the numerical simulations to the bacterial community characterization (through biomolecular analyses), it was verified that (i) the DNAPL recovery (mainly PCE at the study site) through a pumping well would be almost complete and (ii) the application of other remediation techniques (such as bioremediation) would not be necessary to remove the pollution source because (iii) a natural attenuation process is provided by the autochthonous bacterial community, which is characterized by genera (such as Dechloromonas, Rhodoferax, and Desulfurivibrio) that have metabolic pathways capable of favoring the degradation of chlorinated compounds.

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.

Microscale investigation of DNAPL displacement by engineered graphene quantum dots in heterogeneous porous media

This x-ray microtomography study investigates the dynamic pore-scale displacement of a dense non-aqueous phase liquid (DNAPL) such as heavy crude oil in a heterogeneous aquifer rock using carbonaceous nanoparticles. The nanoparticles were synthesized from Wyoming coal and consisted of a mixture of graphene quantum dots (GQD) and engineered graphene quantum dots (E-GQD) in equal proportions. Synergistic interactions between GQD and E-GQD at the oil/water interface reduced the DNAPL/brine interfacial tension (IFT) from 13.4 mN/m to 5.4 mN/m. In addition, the nanofluid mixture altered the wettability of the minerals found in the rock (quartz, carbonate, and feldspar) from oil-wet to water-wet with average contact angles of 66°, 51°, and 60°, respectively. The wettability alteration was more pronounced in carbonates due to the tendency of GQD to adsorb on these surfaces. Analyses of the saturation profiles and pore-scale fluid occupancy maps revealed that the wettability alteration was not instantaneous and required a short soaking time for the fluid-rock interactions to take place. IFT reduction alone was not enough to displace the DNAPL at the early stages of nanofluid flooding because of the high density and viscosity of the oil. As a result, the nanofluid did not outperform brine after 1 pore volume (PV) of injection. On the other hand, the nanofluid was able to invade small pores that were not accessible to brine after 3 PV of injection due to both IFT and contact angle reduction, which lowered the threshold capillary pressure of these pores. Subsequently, more medium and large pores were invaded by the nanofluid as the injection continued, leading to an 11% increase in DNAPL recovery after 20 PV of nanofluid injection, as compared to waterflooding. This incremental recovery is significant considering that the experiments were performed with a heavy oil at ambient temperature, which is more representative of aquifer conditions. Thus, the GQD-based nanofluid has the potential to achieve much larger recoveries in deeper heavy oil reservoirs provided enough soaking time is allowed. The insights provided by this study could be used to validate pore-scale network models or guide in the design and selection of more effective nanomaterials for aquifer remediation or enhanced oil recovery applications.

Application of spectral induced polarization for characterizing surfactant-enhanced DNAPL remediation in laboratory column experiments

The widespread presence of entrapped dense non-aqueous phase liquid (DNAPL) in the subsurface poses a continuing challenge to groundwater remediation. Cost-effective and high-resolution subsurface characterization is a critical issue for further DNAPL recovery due to the complexity of DNAPL source zone architecture (SZA). Geophysical techniques provide a noninvasive, spatially continuous and cost-effective way for monitoring the DNAPL remediation process. In particular, the spectral induced polarization (SIP) method has shown great potential in environmental problems. In this study, we performed real-time SIP measurements on DNAPL contaminated soil in columns to quantitatively assess the ability of SIP method for monitoring surfactant-enhanced DNAPL remediation process. Chemical data was simultaneously collected during the remediation process to verify the results obtained by SIP method. Taking account into the variations of subsurface environment, we conducted a series of column flushing experiments under different flow rate, surfactant concentrations and fluid salinities. The results highlight that SIP method is able to effectively monitor the DNAPL remediation process, as well as to evaluate the remediation efficiency under different conditions. The variations in the flow rate, the concentration of surfactant and the salinity of pore water not only affect remediation effectiveness, but also have an impact on the SIP signatures. This study shows that SIP performs better for monitoring DNAPL remediation at a relatively low flow rate of ~ 0.4 m/d, low surfactant concentration of 5000 mg/L and high salinity of 1.0 S/m, with an error of saturation estimation (RMSES) <0.1.

Surfactant foam flushing for in situ removal of DNAPLs in shallow soils

An innovative process combining surfactant foam and surfactant flushing (SF) for the remediation of a shallow saturated soil contaminated with a dense non-aqueous phase liquid (DNAPL) residual was investigated at bench-scale. First, foam was formed by injecting DHSS surfactant and nitrogen in alternation in soil (SAG process) at constant pressure. The effect of pressure gradients set point (≤90kPam−1) on DNAPL recovery was investigated. Foam front mobilized DNAPL with a piston-like fast propagation (1.8md−1) despite the low pressure gradients. 34–60% of DNAPL residual was extracted with low surfactant consumption (<0.4kgkg−1 DNAPL removed). Then, a solubilizing agent (Tergitol) was injected into soil, previously treated with foam, to improve the DNAPL removal. It led a removal up to 95% whereas only 2 porous volume (PV) of solubilizing solution were injected. As a comparison, when the solubilizing agent was injected without foam pre-treatment, 40 PV would have been required to lead the removal up to 90%. Considering the overall treatment, the technology we developed requires 10-times less surfactant than the traditional SF technology. This technology is especially suitable for very shallow soils where injection pressure has to be kept at a low level to avoid soil heaving.

Steam-Enhanced Extraction Experiments, Simulations and Field Studies for Dense Non-Aqueous Phase Liquid Removal: A Review

Experiments, simulations and field studies for dense non-aqueous phase liquid (DNAPL) removal have demonstrated successful recovery through steam-enhanced extraction. Steam-enhanced extraction is an innovative technology for soil and groundwater remediation to remove as much contamination as possible. Most of researchers study the main DNAPL recovery mechanisms such as physical displacement by vaporization, evaporation and condensation, reduction in interfacial tension and DNAPL viscosity influenced by temperature. Other removal mechanism such as steam distillation and steam stripping also has been studied. The removal of DNAPL using steam-enhanced extraction shall be investigated to identify, acquire, analyze, visualize, and evaluate the effectiveness of the remediation. Several parameters can be controlled to justify the successful of the remediation. A comprehensive understanding of the subsurface environment, multiphase fluid flow and the physical processes is required to prevent remediation failure. Thus, it will avoid continuous contamination of the subsurface environment. The researcher can quantify the reduction in contamination remediation and acquire high quality data sets to validate future numerical model. Aim of this paper is to review and to summarize the existing laboratory experiment, simulations and field studies from other researchers regarding steam-enhanced extraction for dense non-aqueous phase liquid removal.

DNAPL accumulation in wells and DNAPL recovery from wells: Model development and application to a laboratory study

Dense nonaqueous phase liquid (DNAPL) accumulation and recovery from wells cannot be accurately modeled through typical pressure or flux boundary conditions due to gravity segregation of water and DNAPL in the wellbore, the effects of wellbore storage, and variations of wellbore inflow and outflow rates with depth, particularly in heterogeneous formations. A discrete wellbore formulation is presented for numerical modeling of DNAPL accumulation in observation wells and DNAPL removal from recovery wells. The formulation includes fluid segregation, changing water and DNAPL levels in the well and the corresponding changes in fluid storage in the wellbore. The method was added to a three-dimensional finite difference model (CompSim) for three phase (water, gas, DNAPL) flow. The model predictions are compared to three-dimensional pilot scale experiments of DNAPL (benzyl alcohol) infiltration, redistribution, recovery, and water flushing. Model predictions match experimental results well, indicating the appropriateness of the model formulation. Characterization of mixing in the extraction well is important for predicting removal of highly soluble organic compounds like benzyl alcohol. A sensitivity analysis shows that the incorporation of hysteresis is critical for accurate prediction. Among the multiphase flow and transport parameters required for modeling, results are most sensitive to soil intrinsic permeability.

Surfactant foam technology for in situ removal of heavy chlorinated compounds-DNAPLs

The use of surfactant foam for the remediation of a saturated soil contaminated with a dense non-aqueous phase liquid (DNAPL) was investigated at bench-scale. Despite the presence of the DNAPL, high foam stability was obtained for a mixture of cocamidopropyl betaïne and dodecylsulfate at 0.05%. Foams were assessed in different injection conditions and were compared to commonly used remediation methods. Strong foams improved significantly the DNAPL recovery yield, which amounted up to 98%, owing to the propagation of a flat foam front, with low dissolution (<0.5gl−1) and surfactant consumption (<10gkg−1 DNAPL recovered). The effects of important parameters (gas to liquid ratio, injection velocity, gas nature) and methods for foam production on pressure gradient (∇P), remediation efficiency and surfactant consumption were investigated. Even for low injection velocities (4×10−4ms−1), capillary numbers were high enough (∼8×10−3) to push the DNAPL efficiently. DNAPL lowered ∇P for foam propagation because of its destabilising effect. The use of CO2 as gas reduced the ∇Ps for foam propagation by 35%. ∇P were also decreased by 25% for gas to liquid ratios lower than 75%, whereas, DNAPL removal remained high. This technology should lower spreading risks and treatment costs.

Hydraulic Displacement of Dense Nonaqueous Phase Liquids for Source Zone Stabilization

Hydraulic displacement is a mass removal technology suitable for stabilization of a dense, nonaqueous phase liquid (DNAPL) source zone, where stabilization is defined as reducing DNAPL saturations and reducing the risk of future pool mobilization. High resolution three-dimensional multiphase flow simulations incorporating a spatially correlated, heterogeneous porous medium illustrate that hydraulic displacement results in an increase in the amount of residual DNAPL present, which in turn results in increased solute concentrations in groundwater, an increase in the rate of DNAPL dissolution, and an increase in the solute mass flux. A higher percentage of DNAPL recovery is associated with higher initial DNAPL release volumes, lower density DNAPLs, more heterogeneous porous media, and increased drawdown of groundwater at extraction wells. The fact that higher rates of recovery are associated with more heterogeneous porous media stems from the fact that larger contrasts in permeability provide for a higher proportion of capillary barriers upon which DNAPL pooling and lateral migration can occur. Across all scenarios evaluated in this study, the ganglia-to-pool (GTP) ratio generally increased from approximately 0.1 to between approximately 0.3 and 0.7 depending on the type of DNAPL, the degree of heterogeneity, and the imposed hydraulic gradient. The volume of DNAPL recovered as a result of implementing hydraulic displacement ranged from between 9.4% and 45.2% of the initial release volume, with the largest percentage recovery associated with 1,1,1 trichloroethane, the least dense of the three DNAPLs considered.

Dense, viscous brine behavior in heterogeneous porous medium systems

The behavior of dense, viscous calcium bromide brine solutions used to remediate systems contaminated with dense nonaqueous phase liquids (DNAPLs) is considered in laboratory and field porous medium systems. The density and viscosity of brine solutions are experimentally investigated and functional forms fit over a wide range of mass fractions. A density of 1.7 times, and a corresponding viscosity of 6.3 times, that of water is obtained at a calcium bromide mass fraction of 0.53. A three-dimensional laboratory cell is used to investigate the establishment, persistence, and rate of removal of a stratified dense brine layer in a controlled system. Results from a field-scale experiment performed at the Dover National Test Site are used to investigate the ability to establish and maintain a dense brine layer as a component of a DNAPL recovery strategy, and to recover the brine at sufficiently high mass fractions to support the economical reuse of the brine. The results of both laboratory and field experiments show that a dense brine layer can be established, maintained, and recovered to a significant extent. Regions of unstable density profiles are shown to develop and persist in the field-scale experiment, which we attribute to regions of low hydraulic conductivity. The saturated–unsaturated, variable-density groundwater flow simulation code SUTRA is modified to describe the system of interest, and used to compare simulations to experimental observations and to investigate certain unobserved aspects of these complex systems. The model results show that the standard model formulation is not appropriate for capturing the behavior of sharp density gradients observed during the dense brine experiments.

Wettability-dependent DNAPL migration in a rough-walled fracture

The effect of wettability on the migration of dense non-aqueous phase liquids (DNAPLs) through a rough-walled fracture was investigated. The migration characteristics of DNAPL were found to be strongly dependent on the wettability. For a fracture with a hydrophilic surface, DNAPL migrated through larger apertures as disconnected blobs when the groundwater flow regime was linear (Re = 1). However, for non hydrophilic surfaces DNAPL did not migrate in the same way as for the hydrophilic surface. The intermediate-wet surface, with a contact angle of ≈90°, makes gravity pressure dominant over the capillary pressure, resulting in the fastest DNAPL migration. DNAPL was retained on the hydrophobic fracture, where the capillary barrier of larger apertures forced the DNAPL to migrate through the smaller apertures. In the nonlinear flow regime of Re = 60, DNAPL generally migrated downward as a result of the inertial pressure of flowing water for all the wettability conditions, but the local downward migration paths were still determined by the capillary pressure, which resulted in the fastest and slowest migration on the hydrophilic and the hydrophobic fractures, respectively. This study implies that the hydrophilic and intermediate-wet surfaces will be favorable for DNAPL and oil recovery.

Effectiveness of Source-Zone Remediation of DNAPL-Contaminated Subsurface Systems

Concerted efforts to remediate subsurface systems contaminated with dense nonaqueous-phase liquids (DNAPLs) have met with limited success when measured by comparing solute concentrations to drinking water quality standards. One-dimensional and three-dimensional laboratory experiments and a field-scale experiment are used to investigate the effectiveness of source-zone remediation and to assess factors that contribute to the observed results. The three-dimensional laboratory experiment and the field-scale experiment used a surfactant flush followed by vapor extraction to reduce the DNAPL saturation, while vertical DNAPL mobilization was controlled using a brine barrier. DNAPL mobilization and recovery in the field-scale experiment was relatively ineffective due in part to the low saturation levels of the DNAPL. The results show essentially that complete removal of a DNAPL is required to reach typical cleanup standards and that details of the morphology and topology of a DNAPL distribution, in addition to the saturation, play an important role in determining the rate of mass transfer. The results are interpreted in terms of guidance for remediation approaches, realistic expectations for source-zone remediation, and elements needed for improved models of such systems.

Three-Dimensional Multifluid Flow and Transport at the Brooklawn Site near Baton Rouge, LA: A Case Study

Disposal quantities of organic wastes at the Brooklawn Site in Louisiana are suspected to equal nearly 1.45 × 108Kg, making this site one of the most contaminated dense nonaqueous phase liquid (DNAPL) sites in the world. Remedial activities at the site include groundwater and DNAPL extraction from recovery wells. DNAPL recovery has markedly declined in recent years, with many of the peripheral wells showing negligible recovery of organic liquids. Three-dimensional simulations of DNAPL movement in the subsurface were conducted using the STOMP simulator, including a new coupled-well model. The objectives of this modeling effort were to (1) determine the fate and transport of infiltrated DNAPL, and (2) measure the effects of active recovery through DNAPL pumping. A detailed three-dimensional geologic model of the Brooklawn primary DNAPL disposal area was developed and used as the framework for DNAPL simulations. Additionally, site-specific data were obtained to determine the most important hydraulic properties of the subsurface related to DNAPL movement and formation of entrapped DNAPL in the laboratory. In addition to a simulation using the best available subsurface information, several sensitivity simulations were conducted to assess the effects on DNAPL migration. These simulations include DNAPL pumping, well screen extension, an alternative geology, increased DNAPL density, lower DNAPL viscosity, and more-permeable sand and silt deposits. Results of the simulations were compared to field data that define the extent of DNAPL movement based on where DNAPL has been extracted in the site recovery wells. The model simulations show that pumping has a negligible effect on subsurface DNAPL saturations and movement. Pumped DNAPL volumes diminish rapidly due to the limited radius of influence of the wells and movement of the DNAPL out of the zone of influence of the wells with a maximum radius of influence of about 6 m. The numerical analysis also demonstrates that it is impractical to extend existing wells or install new wells to retrieve enough DNAPL to affect the overall extent of DNAPL movement.

Dip-angle influence on areal DNAPL recovery by co-solvent flooding with and without pre-flooding

A two-dimensional (2D) laboratory model was used to study effects of gravity on areal recovery of a representative dense non-aqueous phase liquid (DNAPL) contaminant by an alcohol pre-flood and co-solvent flood in dipping aquifers. Recent studies have demonstrated that injection of alcohol and co-solvent solutions can be used to reduce in-situ the density of DNAPL globules and displace the contaminant from the source zone. However, contact with aqueous alcohol reduces interfacial tension and causes DNAPL swelling, thus facilitating risk of uncontrolled downward DNAPL migration. The 2D laboratory model was operated with constant background gradient flow and a DNAPL spill was simulated using tetrachloroethene (PCE). The spill was dispersed to a trapped, immobile PCE saturation by a water flood. Areal PCE recovery was studied using a double-triangle well pattern to simulate a remediation scheme consisting of an alcohol pre-flood using aqueous isobutanol (∼ 10% vol.) followed by a co-solvent flood using a solution of ethylene glycol (65%) and 1-propanol (35%). Experiments were conducted with the 2D model oriented in the horizontal plane and compared to experiments at the 15° and 30° dip-angle orientations. Injection was applied either in the downward or upward direction of flow. Experimental results were compared to theoretical predictions for flood front stability and used to evaluate effects of gravity on areal PCE recovery. Sensitivity experiments were performed to evaluate effects of the alcohol pre-flood on PCE areal recovery. For experiments conducted with the alcohol pre-flood and the 2D model oriented in the horizontal plane, results indicate that 89–93% of source zone PCE was recovered. With injection oriented downward, results indicate that areal PCE recovery was 70–77% for a 15° dip angle and 57–59% for a 30° dip angle. With injection oriented upward, results indicate that areal PCE recovery was 57–60% at the 30° dip angle, which was similar to PCE recovery for injection in the downward flow direction. Lower areal PCE recovery at greater dip angles in either direction of flow was attributed to DNAPL swelling and migration, flood front instabilities and bypassing of the displaced fluid past the extraction wells during the alcohol pre-flood. Additional results demonstrate that the use of an alcohol pre-flood can be beneficial in improving DNAPL recovery in the horizontal orientation, but pre-flooding may reduce areal recovery efficiency in dip-angle orientations. This study also demonstrates the use of theoretical perturbation (fingering) analysis in predicting NAPL recovery efficiency for flooding processes in remediating aquifers with dip angles.

Correction to “Dense nonaqueous phase liquid (DNAPL) source zone characterization: Influence of hydraulic property correlation on predictions of DNAPL infiltration and entrapment”

[1] The influence of aquifer property correlation on multiphase fluid migration and entrapment was explored through the use of correlated and uncorrelated porosity, permeability, and capillary pressure-saturation (Pc-Sat) parameter fields in a cross-sectional numerical multiphase flow model. Data collected from core samples in a nonuniform sandy aquifer were used to generate three-dimensional aquifer parameter fields. Porosity was assumed to be uniform or simulated using sequential Gaussian simulation (SGS). Permeability (k) was modeled independently of porosity using SGS as well as simulated geostatistical indicator classes derived from measured grain size distribution curves. Retention characteristics were assigned employing Leverett scaling of a representative Pc-Sat curve to the geostatistical k fields or, alternatively, on the basis of simulated indicator classes and porosity values. Ensemble dense nonaqueous phase liquid (DNAPL) infiltration and entrapment behavior for a hypothetical tetrachloroethylene (PCE) spill was simulated in four sets of two-dimensional profiles extracted from these realizations. Comparisons of saturation profiles and spatial moments from point source DNAPL infiltration simulations suggest that choices involving the geostatistical algorithm used to model k and the incorporation of variable versus uniform porosity have a smaller influence than choices involving the scaling of capillary retention properties to k. From these simulations it is apparent that the degree of spatial correlation in Pc-Sat parameters exerts a controlling influence on predicted DNAPL spreading and redistribution in saturated aquifers. The resultant distribution of mass within a DNAPL source zone will have implications for DNAPL recovery and subsequent mass fluxes in remediation operations.

Dense nonaqueous phase liquid (DNAPL) source zone characterization: Influence of hydraulic property correlation on predictions of DNAPL infiltration and entrapment

The influence of aquifer property correlation on multiphase fluid migration and entrapment was explored through the use of correlated and uncorrelated porosity, permeability, and capillary pressure‐saturation (Pc‐Sat) parameter fields in a cross‐sectional numerical multiphase flow model. Data collected from core samples in a nonuniform sandy aquifer were used to generate three‐dimensional aquifer parameter fields. Porosity was assumed to be uniform or simulated using sequential Gaussian simulation (SGS). Permeability (k) was modeled independently of porosity using SGS as well as simulated geostatistical indicator classes derived from measured grain size distribution curves. Retention characteristics were assigned employing Leverett scaling of a representative Pc‐Sat curve to the geostatistical k fields or, alternatively, on the basis of simulated indicator classes and porosity values. Ensemble dense nonaqueous phase liquid (DNAPL) infiltration and entrapment behavior for a hypothetical tetrachloroethylene (PCE) spill was simulated in four sets of two‐dimensional profiles extracted from these realizations. Comparisons of saturation profiles and spatial moments from point source DNAPL infiltration simulations suggest that choices involving the geostatistical algorithm used to model k and the incorporation of variable versus uniform porosity have a smaller influence than choices involving the scaling of capillary retention properties to k. From these simulations it is apparent that the degree of spatial correlation in Pc‐Sat parameters exerts a controlling influence on predicted DNAPL spreading and redistribution in saturated aquifers. The resultant distribution of mass within a DNAPL source zone will have implications for DNAPL recovery and subsequent mass fluxes in remediation operations.

Predicting DNAPL entrapment and recovery: the influence of hydraulic property correlation

The influence of aquifer property correlation on multiphase fluid migration, entrapment and recovery was explored by incorporating correlated and uncorrelated porosity, permeability, and capillary pressure-saturation (Pc-Sat) parameter fields in a cross-sectional numerical multiphase flow model. Comparison of two-dimensional entrapped organic saturation distributions for a simulated tetrachloroethylene (PCE) spill in ensembles of aquifer realizations suggests that the degree of spatial correlation in Pc-Sat parameters exerts a controlling influence on dense nonaqueous phase liquid (DNAPL) spreading and redistribution in saturated aquifers. The predicted evolution of DNAPL source zones and resultant remediation efficiency under surfactant enhanced aquifer remediation (SEAR) also appear to be strongly influenced by the spatial correlation of aquifer parameters and multiphase flow constitutive relationships. Results for a limited number of realizations selected from each ensemble showed that removal of 60% to 99% of entrapped PCE could reduce dissolved contaminant concentration and mass flux by approximately two orders of magnitude under natural gradient conditions. Aqueous phase contaminant mass flux did not vary uniformly as a function of % DNAPL removed, however, and notable differences in behavior were observed for models incorporating correlated versus uncorrelated Pc-Sat and permeability fields. Although these results must be confirmed through analysis of additional realizations, it is likely that similar or larger differences between correlated and uncorrelated system behavior will be observed in aquifers with greater spatially variability than that of the nonuniform, homogeneous sand aquifer studied here.

Numerical Modeling to Assess DNAPL Movement and Removal at the Scenic Site Operable Unit Near Baton Rouge, Louisiana: A Case Study

Detailed three-dimensional multifluid flow modeling was conducted to assess movement and removal of dense nonaqueous phase liquid (DNAPL) movement at a waste site in Louisiana. The site's subsurface consists of several permeable zones separated by (semi) confining clays. In the upper subsurface, the two major permeable zones are, starting with the uppermost zone, the +40- and +20-MSL (mean sea level) zones. At the site, a total of 23,000 m3 of DNAPL was emplaced in an open waste pit between 1962 and 1974. In this period, considerable amounts of DNAPL moved into the subsurface. By 1974 a portion of the DNAPL was removed and the waste site was filled with low-permeability materials and closed. During this process, some of the DNAPL was mixed with the fill material and remained at the site. Between 1974 and 2000, no additional DNAPL recovery activities were implemented. In an effort to reduce the DNAPL source, organic liquid has been pumped through a timed-pumping scheme from a total of 7 wells starting in c...

Numerical Modeling to Assess DNAPL Movement and Removal at the Scenic Site Operable Unit Near Baton Rouge, Louisiana: A Case Study

Detailed three-dimensional multifluid flow modeling was conducted to assess movement and removal of dense nonaqueous phase liquid (DNAPL) movement at a waste site in Louisiana. The site's subsurface consists of several permeable zones separated by (semi) confining clays. In the upper subsurface, the two major permeable zones are, starting with the uppermost zone, the +40- and +20-MSL (mean sea level) zones. At the site, a total of 23,000 m3 of DNAPL was emplaced in an open waste pit between 1962 and 1974. In this period, considerable amounts of DNAPL moved into the subsurface. By 1974 a portion of the DNAPL was removed and the waste site was filled with low-permeability materials and closed. During this process, some of the DNAPL was mixed with the fill material and remained at the site. Between 1974 and 2000, no additional DNAPL recovery activities were implemented. In an effort to reduce the DNAPL source, organic liquid has been pumped through a timed-pumping scheme from a total of 7 wells starting in calendar year 2000. The recovery wells are screened in the lower part of the waste fill material. In site investigations, DNAPL has been encountered in the +40-MSL but not in the +20-MSL zone. The following questions are addressed: (1) Where has the DNAPL migrated vertically and laterally? (2) How much further is DNAPL expected to move in the next century? (3) How effective is the current DNAPL pumping in reducing the DNAPL source? The computational domains for the simulations were derived from 3-D interpolations of borehole logs using a geologic interpretation software (Earthvision™). The simulation results show that DNAPL primarily entered the subsurface in the period 1962–1974, when the waste site was operational. After 1974, the infiltration rates dropped dramatically as a result of the infilling of the waste pit. The simulation results indicate that DNAPL moved from the pit into the underlying +40-MSL zone through two contact zones at the west side of the pit. Lateral movement of the DNAPL body has been relatively slow as a result of the high viscosity and the rapidly decreasing driving force after the waste pit was filled in. For all simulations, lateral movement of DNAPL in the period 1962–2001 is predicted to be less than 60 m from the two contact areas, while additional movement in the next century is expected to be less than 30 m. No DNAPL is predicted to enter the +20-MSL zone, which agrees with site information. The simulations also clearly demonstrate the minimal effect of the current pumping scheme on source reduction and DNAPL movement.