Dense Nonaqueous Phase Liquid Remediation Research Articles

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.

Density-modification displacement using colloidal biliquid aphron for entrapped DNAPL contaminated aquifer remediation

Colloidal biliquid aphron (CBLA) is a strong density modifier for dense nonaqueous phase liquids (DNAPLs). However, the underlying mechanisms responsible for density modification and displacement is not yet clear. Here, a series of batch column and sandbox experiments were conducted to achieve substantial removal and irreversible density reduction of tetrachloroethylene (PCE). The mass of PCE retained in the column and sandbox was less than 1% under suitable injection conditions, and the density of PCE in the effluent was less than that of water (fluctuated in the range of 0.74–0.96 g/cm3). The displacement process was controlled by the high viscosity ratio of CBLA to PCE (52.3). The emulsified and dissolved phase of PCE formed after reaction with CBLA, and the light nonaqueous phase liquid (LNAPL) phase formed after injecting demulsifier solution. The phase analysis played a significant role in monitoring the changes in concentration and density of PCE. The density-modification displacement technique using CBLA reduced the mass of residual PCE by a factor of 165 compared to surfactant flushing, and there was no risk of downward migration of PCE. This study contributes to a better remediation of entrapped DNAPL in contaminated aquifer.

Deep learning based optimization under uncertainty for surfactant-enhanced DNAPL remediation in highly heterogeneous aquifers

The optimization for surfactant-enhanced aquifer remediation (SEAR) of dense non-aqueous phase liquid (DNAPL)-contaminated aquifers is usually accompanied by uncertainties, which may arise from the characterization of complex aquifer heterogeneity and DNAPL source zone architecture (SZA) due to measurement sparsity. Optimization under uncertainty is computationally expensive as it involves an enormous number of model runs. The huge computational burden can be alleviated by utilizing a surrogate model for repeated model evaluations. However, most of the developed surrogates are often limited to low-dimensional optimization problems that only consider simplified aquifer heterogeneity. In this study, we developed a multi-objective simulation-optimization framework to optimize the SEAR schemes considering the characterization uncertainties from both highly heterogeneous aquifer permeability and complex SZA. A fast-to-run convolutional neural network (CNN)-based surrogate model was developed to approximate the high-dimensional and highly complex input-output mapping of the DNAPL multiphase flow simulation model. We first used the rejection sampling strategy to generate random realizations of permeability and SZA conditioning on their limited measurements and then formulated a multi-objective optimization under uncertainty problem based on these realizations. The developed 3-D CNN was trained and used as the surrogate for repeated model runs in optimization to identify the optimal SEAR schemes under uncertainty. A 3-D numerical experiment was used to test the performance of the CNN-based simulation-optimization framework. Comprehensive analysis on the obtained Pareto fronts demonstrates that the proposed framework can efficiently identify reliable Pareto-optimal solutions with a 99.8% speedup compared to the traditional optimization coupled with the forward model. Moreover, the optimization considering multiple realizations enables us to perform the risk assessment to locate the risk zone where the NAPL phase possibly exists after remediation, which provides useful information for decision-making.

Pore network and Darcy scale modelling of DNAPL remediation using ethanol flushing: Study of physical properties in DNAPL remediation

Co-solvent flushing into contaminated soils is one of the most effective techniques for Dense Non-Aqueous Phase Liquid (DNAPL) remediation. In addition to the increase of DNAPL solubility, co-solvents (e.g. ethanol) can alter the viscosity and density of aqueous phase and diffusion coefficient of solute. Any changes in these parameters can change the flow behaviour and alter the upscaled DNAPL mass transfer coefficient which is a key parameter controlling soil and groundwater remediation at Darcy-scale. While numerous studies have investigated DNAPL remediation using co-solvents at the Darcy scale, pore-scale modelling of co-solvent enhanced DNAPL remediation has not been well investigated. In this work, a three-dimensional pore-network model was developed to simulate the 1,2-dichlorobenzene (DCB) remediation experiments using ethanol-water flushing solution. The model simulates the effect of changes in solubility, viscosity, density, and diffusion coefficient during co-solvent flushing of the DNAPL. The results of pore network modelling for ethanol-water flushing for the DCB remediation were also validated using the experimental data. In addition to pore-scale modelling, a continuum scale modelling (Darcy-scale) was used for the DCB remediation using ethanol-water flushing. The results of both pore network and continuum scale modelling demonstrated that the ethanol content and flushing velocity influence the interphase mass transfer and DNAPL dissolution process. The results indicated while the mass transfer coefficient decreased in the presence of ethanol, the process of NAPL remediation was improved due to the substantial increase of solubility in the presence of co-solvent. The large scale modelling showed that NAPL bank can be formed in the front of ethanol-water mixture flushing.

Integrating deep learning-based data assimilation and hydrogeophysical data for improved monitoring of DNAPL source zones during remediation

Data assimilation techniques allow the integration of multi-source data with physical modeling of dense nonaqueous phase liquid (DNAPL) source zones to monitor their evolution in time and thereby improve remediation efforts. The DNAPL source zone is governed by multiphase physics and its architecture is usually highly irregular and nonstationary. Traditional data assimilation methods use stationary models to describe these nonstationary DNAPL saturation fields, leading to significant errors in the estimation of unknown source zone architectures (SZAs). In this study, we propose a geostatistical data assimilation method to improve performance monitoring of DNAPL remediation. We estimate the DNAPL SZA after parameterizing the DNAPL saturation field using a deep learning method instead of a stationary statistical model. First, we train a convolutional variational autoencoder (CVAE) network to capture the physics of DNAPL infiltration and depletion. Then, we incorporate the CVAE network within the Ensemble Kalman filter (EnKF) to resolve the evolving DNAPL SZA by assimilating time-lapse hydrogeophysical measurements (i.e., oscillatory hydraulic tomography, downgradient dissolved DNAPL concentration, and electrical resistivity tomography). To assess the proposed CVAE-EnKF framework's performance, we conducted numerical experiments with evolving DNAPL SZAs in a 2D heterogeneous aquifer. Results show that the proposed framework significantly improved DNAPL saturation estimates throughout the source zone over time, compared to the standard EnKF method. CVAE-EnKF reduced the estimation error of DNAPL mass remediated by 51% from EnKF, along with better predictions of source zone longevity. It also better captured the morphology and center/spread of mass of the evolving remediated portions of the source zone and provided better estimates of pooled-DNAPL mass. Furthermore, the CVAE-EnKF method is more stable and converges faster than EnKF alone. Overall, the proposed data assimilation framework can improve real-time monitoring of DNAPL remediation and risk analysis.

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.

Experiments and sensitivity coefficients analysis for multiphase flow model calibration of enhanced DNAPL dissolution

Multiphase flow modeling is often used for the comparison and optimization of subsurface nonaqueous phase liquid (NAPL) remediation schemes. The calibration of such models is a challenging task due to the lack of detailed data describing the initial NAPL spatial distribution and the processes governing the fate and transport of NAPLs in porous media. In this study laboratory scale experiments were conducted to evaluate reagent-enhanced dense nonaqueous phase liquid (DNAPL) solubilization in saturated heterogeneous media. The DNAPL consisted of both pooled and residual saturation forms. To gain insight into the influence of various input parameters on effluent concentrations, the multiphase flow program was used to compute the sensitivity coefficients of key parameters, relating to the flow, flushing solution properties, soil parameters, NAPL distribution and mass transfer coefficient. The sensitivity coefficients were, in turn, used to aid in the model calibration and to underline the difficulties associated with the calibration of multiphase flow models, most notably the non-uniqueness of the calibration process when complete information is lacking. To alleviate this uncertainty and provide additional constraints, the conducted flushing experiments were jointly used to calibrate the multiphase flow model. The results of the model calibration suggest that the interphase mass transfer coefficient is dependent on the properties of the reagent aqueous solution used for DNAPL remediation, most notably the viscosity and interfacial tension.

Correlation between DNAPL distribution area and dissolved concentration in surfactant enhanced aquifer remediation effluent: A two-dimensional flow cell study.

During the process of surfactant enhanced aquifer remediation (SEAR), free phase dense non-aqueous phase liquid (DNAPL) may be mobilized and spread. The understanding of the impact of DNAPL spreading on the SEAR remediation is not sufficient with its positive effect infrequently mentioned. To evaluate the correlation between DNAPL spreading and remediation efficiency, a two-dimensional sandbox apparatus was used to simulate the migration and dissolution process of 1,2-DCA (1,2-dichloroethane) DNAPL in SEAR. Distribution area of DNAPL in the sandbox was determined by digital image analysis and correlated with effluent DNAPL concentration. The results showed that the effluent DNAPL concentration has significant positive linear correlation with the DNAPL distribution area, indicating the mobilization of DNAPL could improve remediation efficiency by enlarging total NAPL-water interfacial area for mass transfer. Meanwhile, the vertical migration of 1,2-DCA was limited within the boundary of aquifer in all experiments, implying that by manipulating injection parameters in SEAR, optimal remediation efficiency can be reached while the risk of DNAPL vertical migration is minimized. This study provides a convenient visible and quantitative method for the optimization of parameters for SEAR project, and an approach of rapid predicting the extent of DNAPL contaminant distribution based on the dissolved DNAPL concentration in the extraction well.

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.

Stochastic cost optimization of DNAPL remediation – Field application

A stochastic remediation design optimization methodology implemented in the program Stochastic Cost Optimization Toolkit (SCOToolkit) was successfully applied to evaluate remediation options at the East Gate Disposal Yard (EGDY) at the former Fort Lewis, now Joint Base Lewis-McChord (JBLM), Washington. Non-optimized forward simulations based on calibrated parameters and their uncertainty inferred from data prior to actual thermal source remediation system implementation at the site indicated a low probability of the actual thermal system design meeting remediation criteria in a reasonable time frame. Calibration using additional data collected during thermal treatment reduced prediction uncertainty, but still predicted a high probability of taking more than 100 years to reach compliance criteria using the actual thermal treatment design with no additional remedial action. Stochastic optimization of the thermal treatment design indicated larger treatment areas were needed to capture source mass due to uncertainty in source delineation. The expected cost for the enlarged thermal treatment system was estimated to be $22M, which is nearly twice that of the actual system, suggesting that additional characterization to reduce source delineation uncertainty or consideration of an alternative strategy that is less sensitive to delineation uncertainty may be warranted. Stochastic optimization of whey injection was investigated to accelerate source zone dense nonaqueous phase liquid (DNAPL) dissolution and enhance dissolved plume biodecay. The optimized design indicated a 93% probability of meeting compliance criteria by 2100 with an expected net present value cost of $4.7M. Whey injection substantially shortened the remediation time compared to no whey injection. The results indicate that the proposed stochastic cost optimization approach is able to reduce expected remediation costs, increase the probability of achieving remediation objectives, and identify data characterization needs.

Cost Optimization of DNAPL Remediation at Dover Air Force Base Site

This study investigates stochastic optimization of dense nonaqueous phase liquid (DNAPL) remediation design at Dover Air Force Base Area 5 using emulsified vegetable oil (EVO) injection. The Stochastic Cost Optimization Toolkit (SCOToolkit) is used for the study, which couples semianalytical DNAPL source depletion and transport models with parameter estimation, error propagation, and stochastic optimization modules that can consider multiple sources and remediation strategies. Model parameters are calibrated to field data conditions on prior estimates of parameters and their uncertainty. Monte Carlo simulations are then performed to identify optimal remediation decisions that minimize the expected net present value (NPV) cleanup cost while maintaining concentrations at compliance wells under the maximum contaminant level (MCL). The results show that annual operating costs could be reduced by approximately 50% by implementing the identified optimal remediation strategy. We also show that recalibration and reoptimization after 50 years using additional monitoring data could lead to a further 60% reduction in annual operating cost increases the reliability of the proposed remediation actions.

Experimental investigation of the applications of natural freezing and curdlan biopolymer for permeability modification to remediate DNAPL contaminated aquifers in Alaska

Dense Non-aqueous Phase Liquids (DNAPL) contamination is one of the major environmental concerns today. DNAPL can remain in significant quantities as residual contaminants in the low permeability zones even after the bulk phase has been removed. As the drive fluid sweeps through the aquifer it follows the path of least resistance, which is the high permeability zone. Thus the contaminants trapped in the low permeability zones remain as residuals and serve as a source for prolonged contamination. Conventional remediation techniques are ill-equipped to deal with the heterogeneities of the aquifers. Various techniques to enhance the efficiency of the conventional methods have been tried without significant success. In this study we have tested a hypothesis, “A combination of natural freezing/thawing and microbial gel sealant in layered heterogeneous porous media along with a conventional remediation technique can be used effectively to reduce the volume of residual DNAPL in cold regions” to assess an alternative for remediation of residual DNAPL from soils and aquifers in cold regions such as Alaska. Reducing the temperature of soil formations can modify aquifer flow paths. The natural freezing of soils in winter may be used effectively to modify the flow paths. In summer, permeability modification can be accomplished by emplacement of microbial polymer gels. We have investigated a cost-effective and efficient technique to reduce the volume of residual DNAPL using a combination of natural freezing in winter and biopolymer application in summer through a series of one-dimensional laboratory scale column experiments. Specifically, we have investigated the amount of residual DNAPL volume reduction by utilizing the permeability modification approach and thus the modification of preferential flow path using freezing alone and by introducing curdlan biopolymer as a gelant to block the high conductive soil and sweep the DNAPL from residual zones. Our results suggest that a combination of natural freezing of soils in winter and a microbial biopolymer gelant in summer can be used effectively to reduce the volume of residual DNAPL by up to 90% of the total volume of residual DNAPL.

RETRACTED: Micellar solubilization of TCE and PCE by mixed surfactant: Effects of surfactant partitioning and DNAPL mixing

Efforts to remove the dense nonaqueous phase liquids (DNAPLs) in subsurface by mobilizing them face with risks of driving the contaminants deeper into aquifer zones. This spurs research for modifying the approach for in situ remediation. In this paper, micellar solubilization of trichloroethene (TCE), tetrachloroethene (PCE) and their mixture (TCE-PCE) by mixed nonionic and anionic surfactant, Triton X-100 (TX100) and sodium dodecylbenzene sulfonate (SDBS) in DNAPL/water systems was presented. Given 1:40 phase ratio of DNAPL/water (v/v) and the total surfactant concentration from 200 to 10,000mg/L, mixed TX100-SDBS at the total mass ratios of 3:1, 1:1 and 1:3 exhibited significant solubilization for TCE, PCE and TCE-PCE mixture (1:1, v/v). The solubilization extent of sole TCE by TX100-SDBS was much larger than by single TX100 and even larger than by single SDBS at the ratios of 1:1 and 1:3. The solubilization extent of sole PCE by TX100-SDBS was between those by single TX100 and single SDBS. In mixed TCE-PCE system, the solubilization extent of TCE or PCE by TX100-SDBS was larger than by single SDBS at the ratios of 1:1 and 1:3 while less than by single TX100 at large surfactant concentration. TX100 partitioning into the organic phase dictated the solubilization extent. The TX100 losses into TCE and TCE-PCE phases were great when single TX100 was used, while those into PCE phase were much less. No partitioning of SDBS into DNAPLs was observed. In mixed surfactant systems, SDBS decreased greatly the partition loss of TX100 into DNAPLs. The extent of TX100 partition decreased with the amount of SDBS increasing and the polarity of DNAPL decreasing. The mechanism for reduction of TX100 partition was discussed. TX100 and SDBS formed mixed micelles in the solution phase. The inability of SDBS to partition into DNAPLs and the mutual affinity of SDBS and TX100 in the mixed micelle controlled the partitioning of TX100 into DNAPL phase. The work presented here demonstrates that mixed nonionic-anionic surfactants may be probably potentially better systems than the corresponding single ones for remediation of polar DNAPLs, which could decrease risks of driving the contaminants deeper into aquifers.

Solubilization of DNAPLs by mixed surfactant: Reduction in partitioning losses of nonionic surfactant

Efforts to remediate the dense nonaqueous phase liquids (DNAPLs) by mobilizing them face with risks of driving the contaminants deeper into aquifer zones. This spurs research for modifying the approach for in situ remediation. In this paper, a novel solubilization of DNAPLs by mixed nonionic and anionic surfactant, Triton X-100 (TX100) and sodium dodecylbenzene sulfonate (SDBS), was presented and compared with those by single ones. Given 1:40 phase ratio of DNAPL:water (v/v) and the total surfactant concentration from 0.2 to 10 g l −1, mixed TX100–SDBS at the total mass ratios of 3:1, 1:1 and 1:3 exhibited significant solubilization for the DNAPLs, trichloroethene (TCE), chlorobenzene (CB) and 1,2-dichlorobenzene (1,2-DCB). The solubilization extent by mixed TX100–SDBS was much larger than by single TX100 and even larger than by single SDBS at the ratios of 1:1 and 1:3, respectively. TX100 partitioning into the organic phase dictated the solubilization extent. The TX100 losses into TCE, CB and 1,2-DCB phases were more than 99%, 97% and 97% when single TX100 was used. With SDBS alone, no SDBS partitioned into DNAPLs was observed and in mixed systems, SDBS decreased greatly the partition loss of TX100 into DNAPLs. The extent of TX100 partition decreased with increasing the amount of SDBS. The mechanism for reduction of TX100 partition was discussed. TX100 and SDBS formed mixed micelles in the solution phase. The inability of SDBS to partition into DNAPLs and the mutual affinity of SDBS and TX100 in the mixed micelle controlled the partitioning of TX100 into DNAPL phase. The work presented here demonstrates that mixed nonionic–anionic surfactants would be preferred over single surfactants for solubilization remediation of DNAPLs, which could avoid risks of driving the contaminants deeper into aquifers and decrease the surfactant loss and remediation cost.

Multi-Objective Optimization of the Setup of a Surfactant-Enhanced DNAPL Remediation

Surfactant-enhanced aquifer remediation (SEAR) is widely considered a promising technique to remediate dense nonaqueous phase liquid (DNAPL) contaminations in-situ. The costs of a SEAR remediation are important and depend mostly on the setup of the remediation. Costs can be associated with the installation of injection and extraction wells, the required time of the remediation (and thus labor costs, lease of installations, and energy), the extracted water volume (the purification of the extracted water), and the injected surfactant amount. A cost-effective design of the remediation setup allows an optimal use of resources. In this work, a SEAR remediation was simulated for a hypothetical typical DNAPL contamination. A constrained multi-objective optimization of the model was applied to obtain a Pareto set of optimal remediation strategies with different weights for the two objectives of the remediation: (i) the maximal removal of DNAPL mass (ii) with a minimal total cost. A relatively sharp Pareto front was found, showing a considerable tradeoff between DNAPL removal and total remediation costs. These Pareto curves can help decision makers select an optimal remediation strategy in terms of cost and remediation efficiency depending on external constraints such as the available budget and obligatory remediation goals.

Controlled release, blind test of DNAPL remediation by ethanol flushing

A dense nonaqueous phase liquid (DNAPL) source zone was established within a sheet-pile isolated cell through a controlled release of perchloroethylene (PCE) to evaluate DNAPL remediation by in-situ cosolvent flushing. Ethanol was used as the cosolvent, and the main remedial mechanism was enhanced dissolution based on the phase behavior of the water–ethanol–PCE system. Based on the knowledge of the actual PCE volume introduced into the cell, it was estimated that 83 L of PCE were present at the start of the test. Over a 40-day period, 64% of the PCE was removed by flushing the cell with an alcohol solution of approximately 70% ethanol and 30% water. High removal efficiencies at the end of the test indicated that more PCE could have been removed had it been possible to continue the demonstration. The ethanol solution extracted from the cell was recycled during the test using activated carbon and air stripping treatment. Both of these treatment processes were successful in removing PCE for recycling purposes, with minimal impact on the ethanol content in the treated fluids. Results from pre- and post-flushing partitioning tracer tests overestimated the treatment performance. However, both of these tracer tests missed significant amounts of the PCE present, likely due to inaccessibility of the PCE. The tracer results suggest that some PCE was inaccessible to the ethanol solution which led to the inefficient PCE removal rates observed. The flux-averaged aqueous PCE concentrations measured in the post-flushing tracer test were reduced by a factor of 3 to 4 in the extraction wells that showed the highest PCE removal compared to those concentrations in the pre-flushing tracer test.

DNAPL remediation with in situ chemical oxidation using potassium permanganate: Part I. Mineralogy of Mn oxide and its dissolution in organic acids

Previous studies on in situ chemical oxidation of trichloroethylene (TCE) with potassium permanganate indicated that the solid reaction product, Mn oxide, could reduce the permeability of the porous medium and impact the success of dense non-aqueous phase liquid (DNAPL) removal. In order to address the issue of permeability reduction caused by precipitation, this study investigated the mineralogy of Mn oxides and the possibilities of removing the solid precipitates by dissolution. The solid reaction product from the oxidation of TCE by permanganate is semi-amorphous potassium-rich birnessite, which has a layered mineral structure with an interlayer spacing of 7.3 Å. The chemical formula is K 0.854Mn 1.786O 4·1.55H 2O. It has a relatively small specific surface area at 23.6±0.82 m 2/g. Its point of zero charge (pzc) was measured as 3.7±0.4. This birnessite is a relatively active species and could participate in various reactions with existing organic and inorganic matter. The dissolution kinetics of Mn oxide was evaluated in batch experiments using solutions of citric acid, oxalic acid, and ethylenediaminetetraacetic acid (EDTA). Initial dissolution rates were determined to be 0.126 mM/m 2/h for citric acid, 1.35 mM/m 2/h for oxalic acid, and 5.176 mM/m 2/h for EDTA. These rates compare with 0.0025 mM/m 2/h for nitric acid at pH=2. Organic acids dissolve Mn oxide quickly. Reaction rates increase with acid concentration, as tested with citric acid. The dissolution mechanism likely involves proton and ligand-promoted dissolution and reductive dissolution. Citric and oxalic acid can induce ligand-promoted dissolution, while EDTA can induce ligand-promoted and reductive dissolutions. At low pH, proton-promoted dissolution seems to occur with all the acids tested, but this process is not dominant. Reductive dissolution appears to be the most effective process in dissolving the solid, followed by ligand-promoted dissolution. These experiments indicate the significant potential in using these organic acids to remove precipitates formed during the oxidation reaction.

Geochemical effects on metals following permanganate oxidation of DNAPLs.

The application of in situ chemical oxidation for dense, nonaqueous phase liquid (DNAPL) remediation requires delivery of substantial levels of oxidant chemicals into the subsurface to degrade target DNAPLs and to satisfy natural oxidant demand. This practice can raise questions regarding changes in subsurface conditions, yet information regarding potential effects, especially at the field scale, has been lacking. This paper describes an evaluation of the effects on metals associated with in situ chemical oxidation using potassium permanganate at Launch Complex 34 (LC34), Cape Canaveral Air Station, Florida. At LC34, high concentrations of permanganate (1 to 2 wt%) were injected into the subsurface as part of a demonstration of DNAPL remediation technologies. In a companion experimental effort at the Colorado School of Mines, field samples were characterized and laboratory batch and mini-column studies were completed to assess effects of permanganate oxidation on metals in the subsurface one year after completion of the field demonstration. Results indicated there was potential for long-term immobilization of a portion of introduced manganese and no treatment-induced loss in subsurface permeability due to deposition of manganese oxides particles, which are a product of the oxidation reactions. Permanganate treatment did cause elevated manganese, chromium, and nickel concentrations in site ground water within the treated region. Some of these metals effects can be attenuated during downgradient flow through uncontaminated and untreated aquifer sediments.

Comparison between donor substrates for biologically enhanced tetrachloroethene DNAPL dissolution.

Tetrachloroethene (PCE) dense nonaqueous-phase liquid (DNAPL) can act as a persistent groundwater contamination source for decades. Biologically enhanced dissolution of pure PCE DNAPL has potential for reducing DNAPL longevity as indicated previously (Environ. Sci. Technol. 2000, 34, 2979). Reported here are expanded studies to evaluate donor substrates that offer different remediation strategies for bioenhanced DNAPL dissolution, including pentanol (soluble substrate, fed continuously), calcium oleate (insoluble substrate, placed in column initially by alternate pumping of sodium oleate and calcium chloride), and olive oil (mixed with PCE and placed in column initially). Compared with a no-substrate column control, the DNAPL dissolution rate was enhanced about three times when directly coupled with biological transformation. The major degradation product formed was cDCE, but significant amounts of VC and ethene were also found with some columns. Extensive methanogenesis, which reduced PCE transformation, occurred in both the pentanol-fed and oleate-amended columns, but not in the olive-oil-amended column, suggesting that methanogens managed to colonize column niches where PCE DNAPL was not present. Detrimental methane production in the pentanol-fed column was nearly eliminated by presaturating the feed solution with PCE. These results suggest potential DNAPL remediation strategies to enhance dehalogenation while controlling competitive methanogenic utilization of donor substrates.

Implications of alcohol partitioning behavior for in situ density modification of entrapped dense nonaqueous phase liquids.

Surfactant-based remediation techniques have the potential to be very effective for removing dense nonaqueous-phase liquids (DNAPLs) from contaminated sites. However, a risk associated with surfactant-based remediation of DNAPLs is the potential for unwanted downward mobilization of the DNAPL contaminants, making them more difficult to remove from the subsurface. The work described here examines the use of hydrophobic alcohol solutions to reduce the densities of entrapped DNAPLs, converting them to light nonaqueous-phase liquids (LNAPLs). Results of partitioning studies are presented for alcohol-DNAPL systems, in the absence and presence of surfactants. Results indicate that alcohol concentrations near saturation are necessary for conversion of DNAPLs to LNAPLs--particularly for high-density DNAPLs such as trichloroethylene (TCE) and tetrachloroethylene (PCE). Although surfactants can increase the mass of alcohol that can be delivered to a contaminated zone, they appear to change the partitioning equilibrium such that higher alcohol concentrations are required to achieve the same result. Results of this work indicate the importance of minimizing dilution during density modification applications and suggest the concept of using an alcohol macroemulsion flood for density conversion. Implications of the results of this work for remediation system design are discussed.