Residual Dense Nonaqueous Phase Liquid 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.

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.

Feasibility evaluation of novel anionic-nonionic gemini surfactants for surfactant-enhanced aquifer remediation

Surfactant-enhanced aquifer remediation (SEAR) can effectively remove residual dense non-aqueous phase liquids. In this work, a series of environment-friendly anionic-nonionic gemini surfactants, namely, ethylene glycol succinate polyoxyethylene sulfonate (GEOnS-m, n = 3, 5 and 7; m = 10 and 12), were synthesized and evaluated for application in SEAR. The critical micelle concentrations of GEOnS-m were 2.14 × 10−3–1.62 × 10−2 mM, and the molar solubilization ratios of GEOnS-m for tetrachloroethylene were 1.93–3.44, which were 1.5–5 times higher than traditional surfactants. The GEOnS-m solutions remained stable at 5–25 °C, 50 mM salt, and pH = 5.0–9.0. The micelle size decreased as the temperature decreased, which reduced the solubilization capacity (24.9%–37.0%) of the GEOnS-m solutions. Inorganic salts could improve the solubilization capacity of the GEOnS-m solutions by promoting micelle polymerization and increasing the hydrophobic core volume for solubilizing tetrachloroethylene. This work found that 0.80%–4.00% of GEOnS-m adsorbed on the media during simulated column flushing, which was less than the traditional surfactants due to stronger electrostatic repulsion between the micelles and the media. Meanwhile, the solubilization capacity of GEOnS-m was positively correlated with the hydrophobic carbon chain length, while, the adsorption amount in the media and application cost of GEOnS-m were positively correlated with the EO number and negatively correlated with the hydrophobic carbon chain length. Comprehensive evaluation results revealed that GEO3S-12 was preferred for SEAR. This work not only developed suitable surfactants for SEAR but also attempted to further the rational design of surfactants for diverse applications by combining different functional groups into one molecule.

Experimental Study on Residual and Dissolution of DNAPL in Transparent Fracture

Dense nonaqueous phase liquid (DNAPL) is one of the main pollution sources of the underground environment. After DNAPL enters the underground environment, the migration and diffusion process has experienced a variety of media, resulting in large-scale and long-term pollution. To better understand the residual and dissolution process of DNAPL in fractures, the visual experiment of DNAPL residual and dissolution in the fracture was carried out using the transparent fracture model made of glass material, and the changes of DNAPL migration morphology and residual distribution in the crack were obtained. The results showed that the migration front of DNAPL in the fracture was finger shaped in the process of displacement of water phase by DNAPL phase, which was consistent with the state in porous media. When the process of water phase displacing DNAPL phase was over, discrete and aggregated DNAPL droplets remained in the fracture. The residual DNAPL was mainly concentrated in the area ranged from 0.3 mm to 0.8 mm. The dissolution rate of DNAPL in the fracture changed from fast to slow, and there was an obvious tailing period. The increased velocity of water phase flow significantly shortened the time of dissolution process. The DNAPL with hydrolysis reaction accounted for only 0.86% on average in the dissolution process. The findings of this study are helpful to the remediation of the underground environment.

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.

Influence of distribution characteristics of residual DNAPL on mass transfer in porous media under ethanol co-solvent flushing

The remediation of residual dense non-aqueous phase liquid (DNAPL) enhanced by co-solvents or surfactants has been investigated in many studies. Nevertheless, the influences of the residual DNAPL distribution characteristics in a porous medium, and the concentrations of the co-solvents or surfactants on the mass transfer rate of DNAPL remain relatively unexplored. For this reason, existing empirical models neglect the effects of the heterogeneity in the distribution of residual DNAPL, and the compositions of flushing floods on the DNAPL mass transfer rate. In this work, a two-dimensional micro-model is used to quantify the lumped mass transfer rate coefficients of PCE. Results show that increasing the injection rate and the ethanol concentration of flushing floods improves the mass transfer rate of PCE. Moreover, this micro-model allows for accurate quantification of variations in morphological characteristics of PCE blobs. The relative standard deviation σ of residual PCE saturation, as an indicator of morphological characteristics of PCE blobs, varies with decreasing saturation. Standard saturation S0 is used to represent the heterogeneity of residual PCE distribution. The more heterogeneous the PCE distribution, the larger the S0, and the slower the mass transfer rate. According to the variation of relative standard deviation σ with saturation, the PCE dissolution process can be divided into three stages, where the strong convection, weak convection, and diffusion within a limited distance are the primary controlling mechanisms for PCE mass transfer, respectively. Based on our study, a new empirical model, considering the distribution characteristics of residual PCE and the ethanol concentrations, is proposed to predict the PCE mass transfer process dominated by different mass transfer mechanisms in different dissolution stages.

Compatibility of nonionic and anionic surfactants with persulfate activated by alkali in the abatement of chlorinated organic compounds in aqueous phase

Surfactant Enhanced In-Situ Chemical Oxidation (S-ISCO) is an emerging technology in the remediation of sites with residual Dense Non-Aqueous Phase Liquids (DNAPLs), a ubiquitous problem in the environment and a challenge to solve. In this work, three nonionic surfactants: E-Mulse3® (E3), Tween80 (T80), and a mixture of Tween80-Span80 (TS80), and an anionic surfactant: sodium dodecyl sulfate (SDS), combined with persulfate activated by alkali (PSA) as oxidant have been investigated to remove the DNAPL generated as liquid waste in lindane production, which is composed of 28 chlorinated organic compounds (COCs).Because the compatibility between surfactants and oxidants is a key aspect in the S-ISCO effectiveness the unproductive consumption of PS by surfactants was investigated in batch (up to 864 h) varying the initial concentration of PS (84–42 mmol·L−1) and surfactants (0–12 g·L−1) and the NaOH:PS molar ratio (1 and 2). The solubilization capacity of a partially oxidized surfactant was analyzed by estimating its Equivalent Surfactant Capacity, ESC, (as mmolCOCs dissolvedgsurf−1) and comparing it to the expected value for an unoxidized surfactant, ESCo. Finally, the abatement of DNAPL with simultaneous addition of surfactant and PSA was studied.At the conditions used, a negligible unproductive consumption of PS was found by SDS; meanwhile, PS consumption at 360 h ranged between 70 and 80% using the nonionic surfactants. The highest ratios of ESC/ESCo were found with SDS and E3 and these surfactants were chosen for the S-ISCO treatment. When oxidant and surfactant were simultaneously applied for DNAPL abatement the COC conversion was more than three times higher with E3 (0.6 at 360 h) than SDS. Moreover, it was obtained that the time needed for the removal of a mass of DNAPL by PSA in the absence of surfactants was notably higher than the time required when a suitable surfactant was added.

Selective removal of chlorinated organic compounds from lindane wastes by combination of nonionic surfactant soil flushing and Fenton oxidation

The extensive use of the organochlorine pesticide lindane in the second half of the 20th century generated large volumes of wastes over the world. Among these wastes, a dense non-aqueous phase liquid (DNAPL), mainly composed of chlorobenzenes, hexachlorocyclohexanes and heptachlorocyclohexanes, was dumped in insecure landfills remaining in the subsurface and contaminating the groundwater. A coupled process, combining soil flushing (with a nonionic surfactant) and Fenton oxidation, was proposed to deal with this problem. A commercial surfactant (E-Mulse 3 ®) was used to extract most of the residual DNAPL in soil at column conditions. The resulting surfactant flushing solution (SFS) presented high concentration of chlorinated organic compounds (COCs = 3693 mg L−1). In order to recover the surfactant and abate the COCs, the SFS was treated by Fenton process using three doses of hydrogen peroxide (200%, 100% and 50% of the theoretical stoichiometric amount for the complete mineralization of COCs; maintaining a molar ratio of H2O2:Fe = 32). Conversions of COCs above 80% were obtained when H2O2 doses of 100% and 200% of the stoichiometric amount were used at 144 h and 48 h, respectively. Non-aromatic compounds resulted to be less prone to oxidation by hydroxyl radicals than chlorobenzenes. The oxidation of the surfactant was significantly lower than that of the pollutants; therefore the surfactant capacity was maintained after the oxidation treatment and it could be reused in further flushing steps, improving the economy of the process.

Integrity of Clay Till Aquitards to DNAPL Migration: Assessment Using Current and Emerging Characterization Tools

AbstractField investigations were carried out to determine the occurrence of tetrachloroethene (PCE) dense nonaqueous phase liquid (DNAPL), the source zone architecture and the aquitard integrity at a 30‐ to 50‐year old DNAPL release site. The DNAPL source zone is located in the clay till unit overlying a limestone aquifer. The DNAPL source zone architecture was investigated through a multiple‐lines‐of‐evidence approach using various characterization tools; the most favorable combination of tools for the DNAPL characterization was geophysical investigations, membrane interface probe, core subsampling with quantification of chlorinated solvents, hydrophobic dye test with Sudan IV, and Flexible Liner Underground Technologies (FLUTe) NAPL liners with activated carbon felt (FACT). While the occurrence of DNAPL was best determined by quantification of chlorinated solvents in soil samples supported by the hydrophobic dye tests (Sudan IV and NAPL FLUTe), the conceptual understanding of source zone architecture was greatly assisted by the indirect continuous characterization tools. Although mobile or high residual DNAPL (S t > 1%) only occurred in 11% of the source zone samples (intact cores), they comprised 86% of the total PCE mass. The dataset, and associated data analysis, supported vertical migration of DNAPL through fractures in the upper part of the clay till, horizontal migration along high permeability features around the redox boundary in the clay till, and to some extent vertical migration through the fractures in the reduced part of the clay till aquitard to the underlying limestone aquifer. The aquitard integrity to DNAPL migration was found to be compromised at a thickness of reduced clay till of less than 2 m.

Bioremediation Management Reduces Mass Discharge at a Chlorinated DNAPL Site

AbstractAdaptive site management and aggressive bioremediation in the source zone of a complex chlorinated dense nonaqueous phase liquid (DNAPL) site reduced total chlorinated hydrocarbon mass discharge by nearly 80%. Successful anaerobic bioremediation of chlorinated hydrocarbons can be impaired by inadequate concentrations of electron donors, competing electron acceptors, specific inhibitors such as chloroform, and potentially by high contaminant concentrations associated with residual DNAPL. At the study site, the fractured bedrock aquifer was impacted by a mixture of chlorinated solvents and associated daughter products. Concentrations of 1,1,2,2‐tetrachloroethane (1,1,2,2‐TeCA), 1,1,2‐trichloroethane (1,1,2‐TCA), and 1,2‐dichloroethane (1,2‐DCA) were on the order of 100 to 1000 mg/L. Chloroform was present as a co‐contaminant and background sulfate concentrations were approximately 400 mg/L. Following propylene glycol injections, concentrations of organohalide‐respiring bacteria including Dehalococcoides and Dehalogenimonas spp. increased by two to three orders of magnitude across most of the source area. Statistical analysis indicated that reaching volatile fatty acid concentrations greater than 1000 mg/L and depleting sulfate to concentrations less than 50 mg/L were required to achieve a Dehalococcoides concentration greater than the 104 cells/mL recommended for generally effective reductive dechlorination. In a limited area, chloroform concentrations greater than 5 mg/L inhibited growth of Dehalococcoides populations despite the availability of electron donor and otherwise appropriate geochemical conditions. After implementing a groundwater recirculation system targeting the inhibited area, chloroform concentrations decreased permitting significant increases in concentrations of Dehalococcoides and vinyl chloride reductase gene copies.

ERD of Residual TCE DNAPL Achieving Nondetect from a Single Injection of Food‐Grade Waste

Residual dense nonaqueous phase liquid (DNAPL) composed of trichloroethene (TCE) was identified in a deeper interval of an overburden groundwater system at a manufacturing facility located in northern New England. Site hydrostratigraphy is characterized by two laterally continuous and transmissive zones consisting of fully‐saturated fine sand with silt and clay. The primary DNAPL source was identified as a former dry well with secondary contributions from a proximal aboveground TCE storage tank. A single additive‐injection mobilization in 2001 utilizing a food‐grade injectate formulated with waste dairy product and inactive yeast enhanced residual TCE DNAPL destruction in situ by stimulating biotic reductive dechlorination. The baseline TCE concentration was detected up to 97,400 μg/L in the deeper interval of the overburden groundwater system, and enhanced reductive dechlorination (ERD) achieved >99 percent reduction in TCE concentrations in groundwater over nine years with no evidence of sustained rebound. TCE concentrations have remained nondetect below 2.0 μg/L for the last five consecutive sampling rounds between 2013 and 2015. ERD utilizing a food‐grade injectate is a green remediation technology that has destroyed residual DNAPL at the site and achieved similar results at other residual DNAPL sites during both pilot‐ and full‐scale applications. ©2016 Wiley Periodicals, Inc.

Dense Nonaqueous-Phase Liquid Architecture in Fractured Bedrock: Implications for Treatment and Plume Longevity.

Partitioning tracer testing was performed in discrete intervals within a fractured bedrock tetrachloroethene (PCE) dense nonaqueous-phase liquid (DNAPL) source area to assess the fracture flow field and DNAPL architecture. Results confirmed that the partitioning tracer testing was able to identify and quantify low levels of residual DNAPL along flow paths in hydraulically conductive fractures. DNAPL fracture saturations (Sn) ranged from undetectable to 0.007 (DNAPL volume/fracture volume). A comparison of the fracture flow field to the DNAPL distribution indicated that the highest value of Sn was observed in the least transmissive fracture (or fracture zone). Application of a simple ambient dissolution model showed that the DNAPL present in this low transmissivity zone would persist longer than the DNAPL present in more transmissive fractures and would persist for 200 years (in the absence of any degradation reactions). Assessment of PCE mass distribution between the rock matrix and fractures showed that, due to the presence of DNAPL, the rock matrix accounted for less than 10% of the total PCE mass. The evaluation of PCE concentration profiles in the rock matrix and the estimated diffusional flux from the rock matrix suggest that the elevated PCE groundwater concentrations observed in the fractures likely are due to the presence of the residual DNAPL sources and that removal of the residual DNAPL sources within the fractures would result in a significant decrease in dissolved PCE concentrations in the source area.

High-resolution delineation of chlorinated volatile organic compounds in a dipping, fractured mudstone: Depth- and strata-dependent spatial variability from rock-core sampling

Synthesis of rock-core sampling and chlorinated volatile organic compound (CVOC) analysis at five coreholes, with hydraulic and water-quality monitoring and a detailed hydrogeologic framework, was used to characterize the fine-scale distribution of CVOCs in dipping, fractured mudstones of the Lockatong Formation of Triassic age, of the Newark Basin in West Trenton, New Jersey. From these results, a refined conceptual model for more than 55years of migration of CVOCs and depth- and strata-dependent rock-matrix contamination was developed. Industrial use of trichloroethene (TCE) at the former Naval Air Warfare Center (NAWC) from 1953 to 1995 resulted in dense non-aqueous phase liquid (DNAPL) TCE and dissolved TCE and related breakdown products, including other CVOCs, in underlying mudstones. Shallow highly weathered and fractured strata overlie unweathered, gently dipping, fractured strata that become progressively less fractured with depth. The unweathered lithology includes black highly fractured (fissile) carbon-rich strata, gray mildly fractured thinly layered (laminated) strata, and light-gray weakly fractured massive strata. CVOC concentrations in water samples pumped from the shallow weathered and highly fractured strata remain elevated near residual DNAPL TCE, but dilution by uncontaminated recharge, and other natural and engineered attenuation processes, have substantially reduced concentrations along flow paths removed from sources and residual DNAPL. CVOCs also were detected in most rock-core samples in source areas in shallow wells. In many locations, lower aqueous concentrations, compared to rock core concentrations, suggest that CVOCs are presently back-diffusing from the rock matrix. Below the weathered and highly fractured strata, and to depths of at least 50meters (m), groundwater flow and contaminant transport is primarily in bedding-plane-oriented fractures in thin fissile high-carbon strata, and in fractured, laminated strata of the gently dipping mudstones. Despite more than 18years of pump and treat (P&T) remediation, and natural attenuation processes, CVOC concentrations in aqueous samples pumped from these deeper strata remain elevated in isolated intervals. DNAPL was detected in one borehole during coring at a depth of 27m. In contrast to core samples from the weathered zone, concentrations in core samples from deeper unweathered and unfractured strata are typically below detection. However, high CVOC concentrations were found in isolated samples from fissile black carbon-rich strata and fractured gray laminated strata. Aqueous-phase concentrations were correspondingly high in samples pumped from these strata via short-interval wells or packer-isolated zones in long boreholes. A refined conceptual site model considers that prior to P&T remediation groundwater flow was primarily subhorizontal in the higher-permeability near surface strata, and the bulk of contaminant mass was shallow. CVOCs diffused into these fractured and weathered mudstones. DNAPL and high concentrations of CVOCs migrated slowly down in deeper unweathered strata, primarily along isolated dipping bedding-plane fractures. After P&T began in 1995, using wells open to both shallow and deep strata, downward transport of dissolved CVOCs accelerated. Diffusion of TCE and other CVOCs from deeper fractures penetrated only a few centimeters into the unweathered rock matrix, likely due to sorption of CVOCs on rock organic carbon. Remediation in the deep, unweathered strata may benefit from the relatively limited migration of CVOCs into the rock matrix. Synthesis of rock core sampling from closely spaced boreholes with geophysical logging and hydraulic testing improves understanding of the controls on CVOC delineation and informs remediation design and monitoring.

Steady State DNAPL Dissolution in Three-Dimensional Fractured Sandstone Network Experiments

The distribution of residual dense nonaqueous phase liquid (DNAPL) in the subsurface plays a critical role in the DNAPL dissolution kinetics. However, measuring residual DNAPL at the field scale in fractured bedrock settings is generally impractical. This research uses a three-dimensional (3D), bench-scale, fractured-rock network comprised of low-porosity sandstone to evaluate the dissolution kinetics of tetrachloroethylene (PCE) DNAPL at residual saturation during ambient groundwater conditions. To our knowledge, this work presents the first experiments to investigate DNAPL dissolution in 3D bench-scale fractured systems. DNAPL dissolution in the relatively uniform fracture network was evaluated and described using an effective parameter, the bulk mass transfer coefficient (KL). Results from dissolution experiments revealed a positive, statistically significant correlation between KL and DNAPL-water interfacial area, and between KL and DNAPL saturation, analogous to porous media experiments. While aperture size and uniformity influenced DNAPL trapping and interfacial area in the fracture network experiments, DNAPL-altered aperture size did not have a statistically significant influence on dissolution rates. The mass transfer behavior for our 3D fracture network was more similar to one-dimensional (1D) porous media experiments than to two-dimensional (2D) and 3D porous media experiments, which primarily used nonaqueous phase liquid (NAPL) pools. This result is likely due to a combination of the coexistence of the nonwetting NAPL phase and the flowing-aqueous phase in the larger fractures or at fracture intersections, as well as better mixing due to dynamic flow pathways in the network. Finally, residual NAPL saturations could not be replicated in the networks, possibly due to the unpredictable flow-switching behavior of multiphase fluids in fractured rock.

Tracing of residual multiple DNAPL sources in the subsurface using 222Rn as a natural tracer at an industrial complex in Wonju, Korea

Depth-discrete tracing of residual dense non-aqueous phase liquid (DNAPL) sources in the subsurface is of great importance in making decisions related to contaminated groundwater remediation. Temporal variations in the natural tracer 222Rn and contaminant concentrations in groundwater contaminated with multiple chlorinated contaminants, such as trichloroethene, carbon tetrachloride, and chloroform, were examined to trace residual multiple DNAPL contaminants at an industrial complex in Wonju, Korea. The 222Rn activities and multiple DNAPL concentrations in the groundwater fluctuated irregularly according to the groundwater recharge. The natural tracer 222Rn in groundwater present in the soil layer, originating from the underlying crystalline biotite granite, had a wide range from 29,000 to 179,000 Bq/m3, and total concentrations of chlorinated solvents ranged from 0.06 to 17.77 mg/l, indicating the ambiguous results of 222Rn for tracing the residual DNAPL sources. In this paper, a method is presented to locate zones with a high probability of containing depth-discrete residual multiple DNAPL sources using 222Rn and considering relative contaminant concentrations. The results demonstrate that the combination of the 222Rn activities as a natural tracer and the relative contaminant concentrations is able to be used as a useful tool for tracing residual DNAPLs.

Dissolution of dense non-aqueous phase liquids in vertical fractures: Effect of finger residuals and dead-end pools

Understanding the dissolution behavior of dense non-aqueous phase liquids (DNAPLs) in rock fractures under different entrapment conditions is important for remediation activities and any related predictive modeling. This study investigates DNAPL dissolution in variable aperture fractures under two important entrapment configurations, namely, entrapped residual blobs from gravity fingering and pooling in a dead-end fracture. We performed a physical dissolution experiment of residual DNAPL blobs in a vertical analog fracture using light transmission techniques. A high-resolution mechanistic (physically-based) numerical model has been developed which is shown to excellently reproduce the experimentally observed DNAPL dissolution. We subsequently applied the model to simulate dissolution of the residual blobs under different water flushing velocities. The simulated relationship between the Sherwood number Sh and Peclet number Pe could be well fitted with a simple power-law function (Sh=1.43Pe⁰·⁴³). To investigate mass transfer from dead-end pools, another type of trapping in rock fractures, entrapment and dissolution of DNAPL in a vertical dead-end fracture was simulated. As the entrapped pool dissolves, the depth of the interface between the DNAPL and the flowing water increases linearly with decreasing DNAPL saturation. The interfacial area remains more or less constant as DNAPL saturation decreases, unlike in the case of residual DNAPL blobs. The decreasing depth of the contact interface changes the flow field and causes decreasing water flow velocity above the top of the DNAPL pool, suggesting the dependence of the mass transfer rate on the depth of the interface, or alternatively, the remaining mass percentage in the fracture. Simulation results show that the resultant Sherwood number Sh is significantly smaller than in the case of residual blobs for any given Peclet number, indicating slower mass transfer. The results also show that the Sh can be well fitted with a power-law function of Pe and remaining mass percentage. The obtained relationships of dimensionless groups concerning the mass transfer characteristics at the level of individual fractures can be further used in predictive modeling of dissolution at a larger (fracture network) scale.

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.

Assessment of Chemical Oxidation for Treatment of DNAPL in Fractured Sandstone Blocks

Laboratory experiments were performed in discretely-fractured sandstone blocks to evaluate the use of chemical oxidants to treat residual dense nonaqueous phase liquid (DNAPL) tetrachloroethene (PCE). Persulfate (chelated ferrous iron and alkaline activated) and permanganate oxidants were evaluated. Results showed that the maximum DNAPL mass removal rates ranged from 4,700 mg/L/day by using permanganate, to 2,600 mg/L/day by using chelated iron activated persulfate, to 480 mg/L/day by using alkaline activated persulfate. The rate of DNAPL mass removal rapidly declined over the 10 to 60 day application period, limiting DNAPL mass removal rates to only 2 to 20% of the residual DNAPL present on the fractures. The permanganate experiment was terminated after 10 days owing to plugging of the fracture. In the persulfate-treated fractures, substantial rebound in dissolved PCE concentrations were observed after persulfate addition ceased. Use of interfacial tracers in the persulfate experiments showed that the ...

Impact of seasonal variations in hydrological stresses and spatial variations in geologic conditions on a TCE plume at an industrial complex in Wonju, Korea

AbstractSpatial and temporal variations in a trichloroethylene (TCE) plume at an industrial complex in Wonju, Korea, were examined based on hydrogeological data and seven rounds of groundwater quality data collected over a year. The site has considerable vertical heterogeneities; the top layer of soil is covered by impermeable paving material at several locations, followed by a series of reclaimed or residual soil layers, and with weathered rocks to the crystalline biotite granite at the bottom. Areal heterogeneity in the surface conditions plays an important role in controlling groundwater recharge. The heterogeneity structure is influenced by complex surface conditions paved with asphalt and concrete. Owing to the presence of limited recharge area and concentrated summer precipitation events, the effects of seasonal variations on groundwater hydraulics tend to diminish with distance from the recharge area. This result was established by analysing the influence of the contrasting surface recharge conditions between the near‐source zone and the far zone, and the seasonally concentrated precipitation on the transport patterns of a TCE plume. In addition, variations in the plume's downstream contaminant flux levels were also analysed along a transect line near the source zone. The results show that the general tendency of the TCE plume contaminant concentration and mass discharges were reproducible if we account for seasonal recharge variations and the associated changes in the groundwater level. During recharge events, the TCE concentration variations appear to be influenced by leaching of the residual dense non‐aqueous‐phase liquid (DNAPL) TCE trapped in the unsaturated zone. This result shows that seasonal variations in contaminant plume near the source zone is inevitable at this site, and that these variations indicate the presence of residual DNAPL at or above the water table, at least in some isolated locations. Copyright © 2011 John Wiley & Sons, Ltd.

Bioaugmentation for Treatment of Dense Non-Aqueous Phase Liquid in Fractured Sandstone Blocks

Laboratory experiments were performed in discretely fractured sandstone blocks to evaluate the use of bioaugmentation to treat residual dense non-aqueous phase liquid (DNAPL) tetrachloroethene (PCE). Significant dechlorination of PCE and growth of Dehalococcoides spp. (DHC) occurred within the fractures. DNAPL dissolution was enhanced during bioaugmentation by up to a factor of approximately 3.5, with dissolved PCE concentrations at or near aqueous solubility. The extent of dechlorination and DNAPL dissolution enhancement were dependent upon the fracture characteristics, residence time in the fractures, and dissolved concentration of PCE. No relationship was observed between planktonic DHC concentrations exiting the fracture and the observed extents of PCE dechlorination and DNAPL dissolution. Measured planktonic DHC concentrations exiting the fracture increased with increasing flow rate and bioaugmentation dosage, suggesting that these parameters may be important for distribution of DHC to treat dissolved chlorinated ethenes migrating downgradient of the DNAPL source. Bioaugmentation dosage, for the DHC dosages and conditions studied, did not have a measurable impact on DNAPL dissolution or dechlorination within the fractures themselves. Overall, these results indicate that bioaugmentation may be a viable remedial option for treating DNAPL sources in bedrock.