Dense Nonaqueous Phase Liquids Remediation Research Articles

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.

Pore-Scale Mechanisms of Solid Phase Emergence During DNAPL Remediation by Chemical Oxidation.

In situ chemical oxidation (ISCO) has proven successful in the remediation of aquifers contaminated with dense nonaqueous phase liquids (DNAPLs). However, the treatment efficiency can often be hampered by the formation of solids or gas, reducing the contact between remediation agents and residual DNAPLs. To further improve the efficiency of ISCO, fundamental knowledge is needed about the complex multiphase flow and reactive transport processes as new solid and fluid phases emerge at the microscale. Here, via microfluidic experiments, we study the pore-scale dynamics of trichloroethylene degradation by permanganate. We visualize how the remediation evolves under the influence of solid phase emergence and explore the roles of injection rate, oxidant concentration, and stabilization supplement. Combining image processing, pressure analysis, and stoichiometry calculations, we provide comprehensive descriptions of the oxidant concentration-dependent growth patterns of the solid phase and their impact on the remediation efficiency. We further corroborate the stabilization mechanism provided by phosphate supplement, which is effective in inhibiting solid phase generation and thus highly beneficial for the oxidation remediation. This work elucidates the pore-scale mechanisms during remediation of chlorinated solvents with a particular context in the solid phase production and the associated effects, which is of general significance to understanding various processes in natural and engineered systems involving solid phase emergence or aggregation phenomena, such as groundwater and soil remediation.

Mapping and Monitoring of DNAPL Source Zones With Combined Direct Current Resistivity and Induced Polarization: A Field‐Scale Numerical Investigation

AbstractDirect current (DC) resistivity has been widely investigated for non‐invasive mapping of dense non‐aqueous phase liquids (DNAPLs); however, due to its difficulty in distinguishing DNAPLs from adjacent soils, the DC method is limited for static detection of DNAPLs and more often employed for monitoring DNAPL changes over time. Time‐domain induced polarization (IP) can provide complementary information to better discriminate between DNAPL, water and surrounding soils. Since highly resistive DNAPLs tend to laterally spread and pool on polarizable (chargeable) clay lenses, combined DC and IP (DCIP) has the potential to enhance static detection, and also monitoring of DNAPL source zones (SZs). The objective of this study is to assess DCIP for characterizing and monitoring DNAPL SZs at the field‐scale. A new DNAPL‐DCIP numerical model was first developed that couples a 3D multiphase flow model, which simulates DNAPL release and remediation scenarios, with a 3D DCIP model, which calculates the corresponding resistivity and chargeability response. The sensitivity of the DCIP response to key DNAPL and soil properties was then analyzed at a single subsurface location, closely matching previous theoretical and experimental observations. Finally, a field‐scale simulation of DNAPL release and remediation was conducted, with simultaneous mapping by DCIP surveys. Results demonstrate that chargeability can provide enhanced understanding of the lithological distribution that controls the variability in the DNAPL SZ, with time‐lapse resistivity being used to monitor DNAPL mass changes during SZ remediation. This numerical study suggests that combined DCIP can be valuable for site characterization and time‐lapse monitoring performance at DNAPL‐impacted sites.

Surfactant-Enhanced Permanganate Oxidation on Mass-Flux Reduction and Mass Removal (MFR-MR) Relationship for Pool-Dominated TCE Source Zones in Heterogeneous Porous Media

Chlorinated solvents generally enter the subsurface as dense nonaqueous phase liquids (DNAPL) and accumulation mostly occurs in aquifers as pool-dominated zones that can cause long-term aqueous phase groundwater contamination. In situ remediation of DNAPL source zones in such systems is crucial for protecting and/or restoring groundwater quality in these aquifer systems having significant groundwater potential. The objective of the project was to investigate the surfactant-enhanced permanganate oxidation efficacy for pool-dominated DNAPL source zones in heterogeneous aquifer media. A complementary objective of this study was to investigate the impact of nonuniform distribution of DNAPL source zones, surfactant-enhanced dissolution and surfactant-enhanced permanganate oxidation conditions on mass-flux reduction/mass-removal behavior relationships. A series of 2-D flow-cell tank experiments using various grain sizes silica sand and natural soil were conducted as part of this study. DNAPL trichloroethene (TCE) was used as a chlorinated solvent, and SDS (sodium dodecyl sulfate) and KMnO4 (potassium permanganate) were used as anionic surfactant and oxidant (remediation agents), respectively. The results were compared with a water-flooding experiment to test the remediation effort. Although, high fractions of TCE source zones in the heterogeneous porous media were removed by sodium dodecyl sulfate (SDS)-enhanced flushing, TCE removal in this system exhibited an extended multi-step concentration elution behavior. This nonideal behavior was observed for both the water-flood and SDS-flushing experiments. The results emphasized that in the early stage, some portion of the organic liquid is hydraulically accessible (matrix) whereas the later stage of mass removal was controlled by the more poorly accessible mass (pool) associated with higher saturation zones. Our results also showed that the distribution and the emplacement of organic liquid and flow-field heterogeneity had a significant influence on remediation and removal for both flushing solution (SDS and water). It was postulated that when SDS/MnO4 was applied with sufficient dosage and provided enough contact time, pool-dominated source zones could be remediated more efficiently compared to surfactant flushing alone. As a result, the performance of technology depends on the site characteristics which are critical to characterize effective DNAPL remediation strategies for contaminated sites.

The change of representative elementary volume of DNAPL influenced by surface active agents during long-term remediation period in heterogeneous porous media

Representative elementary volume (REV) is important to characterize dense nonaqueous phase liquids (DNAPLs) during surfactant-Enhanced aquifer remediation (SEAR) period. To investigate the REVs of DNAPL in remediation, a perchloroethylene (PCE) SEAR experiment is conducted in a two dimensional (2D) heterogeneous translucent porous media. Light transmission techniques are used to quantify PCE saturation (Soil) and PCE-water interfacial area (AOW). Afterward, corresponding REVs are estimated using a criterion of relative gradient error (εgi) to reveal the change of REVs of DNAPL over the entire remediation period. Results from this work suggest the presence of surface active agents strongly affect the REVs of DNAPL. At the beginning of the SEAR experiment, the frequency of minimum Soil-REV size closely follows a Gaussian distribution in 0.0mm–11.0mm. Simultaneously, the frequency of minimum AOW-REV size is close to a Gaussian distribution in 2.0mm–9.0mm and appears a peak value in 13.0mm–14.0mm. As SEAR experiment proceeds, both the shapes of frequency and cumulative frequency of REV sizes are changed. At the end of SEAR experiment, the frequency of minimum Soil-REV and minimum AOW-REV size tend to Gaussian distributions in 0.0mm–6.0mm and 0.0mm–9.0mm, respectively, which suggest both minimum Soil-REV size and minimum AOW-REV size show decreasing tendency. Continuous quantification of the REVs of DNAPL is realized in this study to reveal the change of REVs influenced by surface active agent. The finding has important significance on improving our understanding of the characteristics of DNAPL in SEAR process, simulating DNAPL remediation and designing appropriate remediation scheme with high-resolution.

Reduction in contaminant mass flux with induced oxide mineral precipitation in quasi-multidimensional systems

Dense non-aqueous-phase liquids (DNAPLs) are difficult to be removed from the highly heterogeneity aquifer. In situ chemical oxidation (ISCO) is one of the main DNAPL remediation technologies, but when using permanganate as oxidant to remediate the DNAPL in groundwater, the permeability within the DNAPL source area can be reduced due to the MnO2 solid-phase formation. Hence, the study of the complex dynamic change in the DNAPL source zone is meaningful for improvement in the ISCO efficiency. A developed one-dimensional column experiment was used in this study. The columns used in the experiments were parallel-connected. Limited time interval and low-concentration permanganate flushing were applied in the experiments to mimic real field condition. It was found that the permeability within the DNAPL source zone was reduced only for short time due to the precipitation and flocculation of MnO2 formed. The flow flux and mass flux within the DNAPL source zone decreased during permanganate oxidation while rebounded again after the oxidation. The experiment results illustrated that low concentration of permanganate oxidation can activate the DNAPL source area. Based on this phenomenon, an effective ISCO could be developed to remediate the source zone.

Improved time-lapse electrical resistivity tomography monitoring of dense non-aqueous phase liquids with surface-to-horizontal borehole arrays

Electrical resistivity tomography (ERT) has long-standing potential to improve characterization of sites contaminated with dense non-aqueous phase liquids (DNAPLs). However, ERT is rarely used at DNAPL sites due to the complexity of the DNAPL target coupled with the inherent limitations of traditional (surface and cross-hole) ERT configurations. Horizontal boreholes are being increasingly used in remedial strategies at contaminated field sites. This paper presents a novel surface-to-horizontal borehole (S2HB) ERT configuration for DNAPL site investigations. This array configuration is combined with four-dimensional (4D) inversion methods (applied on two-dimensional (2D) time-lapse monitoring datasets) to explore the potential benefit of S2HB ERT for mapping the spatial and temporal evolution of DNAPL mass during remediation. A field scale DNAPL remediation scenario was first simulated by a coupled DNAPL-ERT model. This demonstrated that S2HB ERT may provide significant improvements over surface ERT, particularly for delineating DNAPL mass removal at depth. A laboratory experiment was then performed to validate the S2HB ERT approach in a physical system. The experiment confirmed that 4D S2HB ERT provides improved time-lapse monitoring of NAPL changes. Confidence in the ERT responses obtained from the experiment was increased by direct comparison to the actual distribution of NAPL mapped by excavation. Independent simulation of the experiment with the DNAPL-ERT model demonstrated that the model is reliable for simulating real systems. This initial study demonstrates significantly improved resistivity imaging with surface-to-horizontal borehole ERT and its potential as a non-destructive site characterization tool for mapping DNAPL mass changes during remediation.

Adaptive Kriging surrogate model for the optimization design of a dense non-aqueous phase liquid-contaminated groundwater remediation process

The optimization model needs to call the simulation model to calculate the response under different conditions for many times, and this is computationally expensive and time-consuming. To solve this problem, surrogate models can be used to yield insight into the functional relationship between the design variables and the responses, instead of simulation models in the optimization. In this paper, an integrated optimization method based on adaptive Kriging surrogate models was proposed and applied to the cost optimization of a surfactant enhanced aquifer remediation process for dense non-aqueous phase liquids (DNAPLs). First, the initial samples were created by Latin hypercube sampling, and then the responses corresponding to the initial samples were computed by a simulation model. The initial Kriging model was derived through these samples. Secondly, the adaptive Kriging surrogate model was proposed based on updating initial Kriging with new samples via infill sampling criteria. The results showed that it had improved the accuracy of the surrogate model, and the added samples had provided more information about the simulation model than the common samples. Even with the same number of samples, the adaptive Kriging surrogate model performed better than the common Kriging surrogate model, which was built only once. What's more, the integrated approach not only greatly reduced the computational burden, but also determined the actual optimal DNAPLs remediation strategy.

Evaluating four-dimensional time-lapse electrical resistivity tomography for monitoring DNAPL source zone remediation

Practical, non-invasive tools do not currently exist for mapping the remediation of dense non-aqueous phase liquids (DNAPLs). Electrical resistivity tomography (ERT) exhibits significant potential but has not yet become a practitioner's tool due to challenges in interpreting the survey results at real sites. This study explores the effectiveness of recently developed four-dimensional (4D, i.e., 3D space plus time) time-lapse surface ERT to monitor DNAPL source zone remediation. A laboratory experiment demonstrated the approach for mapping a changing NAPL distribution over time. A recently developed DNAPL-ERT numerical model was then employed to independently simulate the experiment, providing confidence that the DNAPL-ERT model is a reliable tool for simulating real systems. The numerical model was then used to evaluate the potential for this approach at the field scale. Four DNAPL source zones, exhibiting a range of complexity, were initially simulated, followed by modeled time-lapse ERT monitoring of complete DNAPL remediation by enhanced dissolution. 4D ERT inversion provided estimates of the regions of the source zone experiencing mass reduction with time. Results show that 4D time-lapse ERT has significant potential to map both the outline and the center of mass of the evolving treated portion of the source zone to within a few meters in each direction. In addition, the technique can provide a reasonable, albeit conservative, estimate of the DNAPL volume remediated with time: 25% underestimation in the upper 2m and up to 50% underestimation at late time between 2 and 4m depth. The technique is less reliable for identifying cleanup of DNAPL stringers outside the main DNAPL body. Overall, this study demonstrates that 4D time-lapse ERT has potential for mapping where and how quickly DNAPL mass changes in real time during site remediation.

Comparison of upscaled models for multistage mass discharge from DNAPL source zones

AbstractAnalytical upscaled models that can describe the depletion of dense nonaqueous phase liquids (DNAPLs) and the associated mass discharge are a practical alternative to computationally demanding and data‐intensive multiphase numerical simulators. A major shortcoming of most existing upscaled models is that they cannot reproduce the nonmonotonic, multistage effluent concentrations often observed in experiments and numerical simulations. Upscaled models that can produce multistage concentrations either require calibration, which increases the cost of applying them in the field, or use dual‐domain conceptual models that may not apply for spatially complex source zones. In this study, a new upscaled model is presented that can describe the nonmonotonic, multistage average concentrations emanating from complex DNAPL source zones. This is achieved by explicitly considering the temporal evolution of three source zone parameters, namely source zone projected area, the average of local‐scale DNAPL saturations, and the average of local‐scale aqueous relative permeability, without using empirical parameters. The model is evaluated for two real and twelve hypothetical centimeter‐scale complex source zones. The proposed model captures the temporal variations in concentrations better than an empirical model and a dual‐domain ganglia‐to‐pool ratio model. The results provide evidence that effluent concentrations downgradient of DNAPL source zones are controlled by the evolution of the aforementioned macroscopic parameters. This knowledge can be useful for the interpretation of field observations of effluent concentrations downstream of DNAPL source zones, and for the development of predictive upscaled models. Advances in DNAPL characterization techniques are needed to quantify these macroscopic parameters that can be used to guide DNAPL remediation efforts.

Use of geostatistical models in DNAPL source zone architecture and dissolution profiles assessment in spatially variable aquifer

Following the accidental subsurface release of dense nonaqueous phase liquids (DNAPLs), spatial variability of physical and chemical soil/contaminant properties can exert a controlling influence on infiltration pathways and organic entrapment. DNAPL spreading, fingering, and pooling typically result in source zones characterized by irregular contaminated regions with complex boundaries. Spatial variability in aquifer properties also influences subsequent DNAPL dissolution and aqueous transport dynamics. An increasing number of studies have investigated the effects of subsurface heterogeneity on the fate of DNAPL; however, previous work was limited to the examination of the behavior of single-component DNAPL in systems with simple and well-defined aqueous and solid surface chemistry. From a DNAPL remediation point of view, such an idealized assumption will bring a large discrepancy between the designs based on the model simulation and the reality. The research undertaken in this study seeks to stochastically explore the influence of spatially variable porous media on DNAPL entrapment and dissolution profiles in the saturated groundwater aquifer. A 3D, multicomponent, multiphase, compositional model, UTCHEM, was used to simulate natural gradient water flooding processes in spatially variable soils. Porosity was assumed to be uniform or simulated using sequential Gaussian simulation (SGS) and sequential indicator simulation (SIS). Soil permeability was treated as a spatially random variable and modeled independently of porosity, and a geostatistical method was used to generate random distributions of soil permeability using SGS and SIS (derived from measured grain size distribution curves). Equally possible 3D ensembles of aquifer realizations with spatially variable permeability accounting of physical heterogeneity could be generated. Tetrachloroethene (PCE) was selected as a DNAPL representative as it was frequently discovered at many contaminated groundwater sites worldwide, including Thailand. The randomly generated permeability fields were incorporated into UTCHEM to simulate DNAPL source zone architecture under 96-L hypothetical PCE spill in heterogeneous media and stochastic analysis was conducted based on the simulated results. Simulations revealed considerable variations in the predicted PCE source zone architecture with a similar degree of heterogeneity, and complex initial PCE source zone distribution profoundly affected PCE recovery time in heterogeneous media when subject to natural gradient water flush. The necessary time to lower PCE concentrations below Thai groundwater quality standard ranged from 39 years to more than 55 years, suggesting that spatial variability of subsurface formation significantly affected the dissolution behavior of entrapped PCE. The temporal distributions of PCE saturation were significantly altered owing to natural gradient water flush. Therefore, soil heterogeneity is a critical factor to design strategies for characterization and remediation of DNAPL contaminated sites. The systematic and comprehensive design algorithm developed and described herein perhaps serves as a template for application at other DNAPL sites in Thailand.

DNAPL mapping by ground penetrating radar examined via numerical simulation

Abstract Successful remediation of sites contaminated with dense non-aqueous phase liquids (DNAPLs) requires adequate characterisation of the volume and extent of the DNAPL source zone. Ground penetrating radar (GPR) has been proposed to characterise the subsurface distribution of DNAPLs; however, its effectiveness for real applications remains unproven. The objective of this study was to evaluate the potential of GPR to map realistic DNAPL-spill scenarios within heterogeneous subsurface environments, and to monitor the progress of subsequent remedial efforts. This was investigated by creating a novel link between two published numerical models: DNAPL3D-MT to generate a realistic DNAPL scenario, and GPRMAX to simulate a sequence of GPR surveys applied at the surface. A published volumetric mixing model permitted conversion of an evolving hydrogeological domain into a bulk permittivity domain. A field-scale, two-dimensional, surface release of a chlorinated solvent DNAPL into heterogeneous silty sand was employed as a demonstration case, including complete DNAPL remediation by dissolution and its mapping by time-lapsed 100 MHz surface GPR scans. Qualitative and quantitative interpretation of the results reveal that realistic GPR scans are simulated, generating a complex GPR response that is sensitive to both variations in DNAPL saturation and intrinsic subsurface heterogeneity. As a result, deconvoluting the response in the absence of a pristine site scan remains challenging. However, with the aid of newly developed GPR analysis tools presented here, including the combination of ‘sequential scan subtraction’ with ‘normalized radar trace and radar section sum of squares’, changes in DNAPL distribution (mass reduction and remobilization) are demonstrated to be quantifiable. Thus, it is concluded that periodically monitoring the time-lapsed remediation of the source zone with GPR is particularly promising.

Guest Editorial: Contaminant Source Zones: Remediation or Perpetual Stewardship?

Guest Editorial: Contaminant Source Zones: Remediation or Perpetual Stewardship?

Density-modified displacement for dense nonaqueous-phase liquid source-zone remediation: density conversion using a partitioning alcohol.

Entrapped and pooled dense nonaqueous-phase liquids (DNAPLs) often persist in aquifers and serve as a long-term source of groundwater contamination. To address the problematic nature of DNAPL remediation, a surfactant-enhanced aquifer remediation (SEAR) technology, density-modified displacement (DMD), has been developed which significantly reduces the risk of downward migration of displaced DNAPLs. The DMD method is designed to accomplish DNAPL density conversion through the introduction of a partitioning alcohol, n-butanol (BuOH), in a predisplacement flood using conventional horizontal flushing schemes. Subsequent displacement and recovery of the resulting LNAPL is achieved by flushing with a low-interfacial tension surfactant solution. The objective of this study was to investigate density conversion of two representative DNAPLs, chlorobenzene (CB) and trichloroethene (TCE). A series of batch experiments was performed to assess changes in NAPL composition, density, and phase behavior as a function of BuOH mole fraction. Experimental results were used to develop contaminant/BuOH/water ternary phase diagrams and to elucidate regions of contrasting NAPL density. UNIQUAC calculations are presented to support measured compositional and phase behavior data. Density conversion of CB and TCE, relative to water, occurred at NAPL BuOH mole fractions of 0.38 and 0.50, respectively. Significant incorporation of water into the organic phase was observed at relatively high BuOH mole fractions and was shown to limit changes in NAPL composition and density. Interfacial tensions between CB-NAPL and TCE-NAPL and a 6% (by wt) BuOH aqueous solution were found to decrease with increasing NAPL BuOH mole fraction, although in both cases the measured values remained above 2.5 dyn/cm. Total trapping number calculations suggest that, in most aquifer formations, density conversion can be achieved without premature NAPL displacement using a 6% (by wt) BuOH aqueous solution.

In-situ oxidation of trichloroethene by permanganate: effects on porous medium hydraulic properties

In-situ oxidation of dense nonaqueous-phase liquids (DNAPLs) by strong oxidants such as potassium permanganate (KMnO 4) has been proposed as a possible DNAPL remediation strategy. In this study, we investigated the effects of in-situ trichloroethene (TCE) oxidation by KMnO 4 on porous medium hydraulic properties. In particular, we wanted to determine the overall effects of concurrent solid phase (MnO 2) precipitation, gas (CO 2) evolution and TCE dissolution resulting from the oxidation reaction on the porous medium's aqueous-phase relative permeability, k rw. Three TCE removal experiments were conducted in a 95-cm long, 5.1-cm i.d. glass column, which was homogeneously packed with well-characterized 30/40-mesh silica sand. TCE was emplaced in the sand-pack in residual, entrapped form through a sequence of water/TCE imbibition and drainage steps. The column was then flushed under constant aqueous flux conditions for up to 104 h with either deionized water (reference experiment), deionized water containing 5 mM KMnO 4 or deionized water containing 5 mM KMnO 4 and 300 mM Na 2HPO 4. Aqueous-phase relative permeabilities were computed from measured flow rates and measurements of aqueous-phase pressure head, h obtained using pressure transducers connected to tensiometers distributed along the column length. A dual-energy gamma radiation system was used to monitor changes in fluid saturation that occurred during each experiment. In addition, column effluent samples were collected for chemical analyses. Dissolution of TCE during deionized water flushing led to an increase in k rw by ∼22% and a local reduction in h. On the other hand, vigorous CO 2 gas production and precipitation of MnO 2 was visually observed during flushing with deionized water that contained 5 mM KMnO 4. As a consequence, k rw declined by ∼96% and h increased locally by more than 1000 cm H 2O during the first 24 h of the experiment, causing sand-pack ruptures and pump failure. Conversely, less CO 2 gas production and MnO 2 precipitation was visually observed during flushing with deionized water that contained 5 mM KMnO 4 and 300 mM Na 2HPO 4. Consequently, only small increases in h (<15 cm H 2O) were observed in this experiment due to a reduction in k rw of ∼53%. While we must attribute changes in h due to variations in k rw to our specific experimental design (constant aqueous flux, one-dimensional flow experiments), these experiments nevertheless confirm that successful application of in situ chemical oxidation of TCE requires consideration of detrimental processes such as MnO 2 precipitation and CO 2 gas formation. In addition, our results indicate that utilization of a buffered oxidant solution may improve the effectiveness of in-situ oxidation of TCE by KMnO 4 in otherwise weakly buffered porous media.