DNAPL Pool Research Articles

Mobilization pilot test of PCE sources in the transition zone to aquitards by combining mZVI and biostimulation with lactic acid

The potential toxic and carcinogenic effects of chlorinated solvents in groundwater on human health and aquatic ecosystems require very effective remediation strategies of contaminated groundwater to achieve the low legal cleanup targets required. The transition zones between aquifers and bottom aquitards occur mainly in prograding alluvial fan geological contexts. Hence, they are very frequent from a hydrogeological point of view. The transition zone consists of numerous thin layers of fine to coarse-grained clastic fragments (e.g., medium sands and gravels), which alternate with fine-grained materials (clays and silts). When the transition zones are affected by DNAPL spills, free-phase pools accumulate on the less conductive layers. Owing to the low overall conductivity of this zone, the pools are very recalcitrant. Little field research has been done on transition zone remediation techniques. Injection of iron microparticles has the disadvantage of the limited accessibility of this reagent to reach the entire source of contamination. Biostimulation of indigenous microorganisms in the medium has the disadvantage that few of the microorganisms are capable of complete biodegradation to total mineralization of the parent contaminant and metabolites. A field pilot test was conducted at a site where a transition zone existed in which DNAPL pools of PCE had accumulated. In particular, the interface with the bottom aquitard was where PCE concentrations were the highest. In this pilot test, a combined strategy using ZVI in microparticles and biostimulation with lactate in the form of lactic acid was conducted. Throughout the test it was found that the interdependence of the coupled biotic and abiotic processes generated synergies between these processes. This resulted in a greater degradation of the PCE and its transformation products. With the combination of the two techniques, the mobilization of the contaminant source of PCE was extremely effective.

Sulfidated nano zerovalent iron (S-nZVI) for in situ treatment of chlorinated solvents: A field study

Sulfidated nano zerovalent iron (S-nZVI), stabilized with carboxymethyl cellulose (CMC), was successfully synthesized on site and injected into the subsurface at a site contaminated with a broad range of chlorinated volatile organic compounds (cVOCs). Transport of CMC-S-nZVI to the monitoring wells, both downgradient and upgradient, resulted in a significant decrease in concentrations of aqueous-phase cVOCs. Short-term (0–17 days) total boron and chloride measurements indicated dilution and displacement in these wells. Importantly however, compound specific isotope analysis (CSIA), changes in concentrations of intermediates, and increase in ethene concentrations confirmed dechlorination of cVOCs. Dissolution from the DNAPL pool into the aqueous phase at the deepest levels (4.0–4.5 m bgs) was identifiable from the increased cVOCs concentrations during long-term monitoring. However, at the uppermost levels (∼1.5 m above the source zone) a contrasting trend was observed indicating successful dechlorination. Changes in cVOCs concentrations and CSIA data suggest both sequential hydrogenolysis as well as reductive β-elimination as the possible transformation mechanisms during the short-term abiotic and long-term biotic dechlorination. One of the most positive outcomes of this CMC-S-nZVI field treatment is the non-accumulation of lower chlorinated VOCs, particularly vinyl chloride. Post-treatment soil cores also revealed significant decreases in cVOCs concentrations throughout the targeted treatment zones. Results from this field study show that sulfidation is a suitable amendment for developing more efficient nZVI-based in situ remediation technologies.

Natural attenuation of pools and plumes of carbon tetrachloride and chloroform in the transition zone to bottom aquitards and the microorganisms involved in their degradation

In the transition zone between aquifers and aquitards, DNAPL pools of carbon tetrachloride and chloroform accumulate because of heterogeneity in this zone. Natural attenuation occurs at pools and plumes, indicating that remediation might be possible. The aims of the study were: i) to assess the role of heterogeneity in the natural attenuation of these compounds, ii) determine degradation processes within this zone, and iii) identify dechlorinating microorganisms. For this, groundwater concentrations, redox-sensitive parameters, CSIA isotopic and DGGE molecular techniques were used. The main findings at depth of the transition zone were: (1) the important key control played by heterogeneity on natural attenuation of contaminants. (2) Heterogeneity caused the highly anoxic environment and dominant sulfate-reducing conditions, which accounts for more efficient natural attenuation. (3) Heterogeneity also explains that the transition zone constitutes an ecotone. (4) The bacteria size exclusion is governed by the pore throat threshold and determines the penetration of dechlorinating microorganisms into the finest sediments, which is relevant, since it implies the need to verify whether microorganisms proposed for bioremediation can penetrate these materials. (5) Reductive dechlorination caused the natural attenuation of contaminants in groundwater and porewater of fine sediments. In the case of carbon tetrachloride, it was an abiotic process biogenically mediated by A. suillum, a bacterium capable of penetrating the finest sediments. In the case of chloroform, it was a biotic process performed by a Clostridiales bacterium, which is unable to penetrate the finest materials. (6) Both microorganisms have potential to be biostimulated to dechlorinate contaminants in the source and the plume in the transition zone. These outcomes are particularly relevant given the longevity of DNAPL sources and have considerable environmental implications as many supply wells in industrial areas exploit aquifers contaminated by chlorinated solvents emerging from DNAPL pools accumulated on the low-conductivity layers in transition zones.

Simulation of DNAPL infiltration and spreading behaviour in the saturated zone at varying flow velocities and alternating subsurface geometries

The influence of varying groundwater flow velocities on DNAPL infiltration and spreading behaviour was investigated by multiphase modelling using TMVOC and PetraSim. The multiphase models were calibrated by results of previously conducted laboratory experiments for the complete spatio-temporal range of the experiments. The small scale 2D scenario modelling was applied to qualify and quantify changes in position, architecture, geometry and dissolution of a TCE body in a fully saturated homogeneous sandy medium. The applied flow velocities ranging from 0.05 up to 40.00 m/day exhibited that the DNAPL TCE is affected even at the lowest flow velocity in its position, its size and its architecture. Additionally, several impermeable lenses with simple geometry were assumed in the model, to investigate the influence of stratified subsoil. In the experimental set-ups, the DNAPL body reacts more sensitive to the applied groundwater flow velocities than to the geometrical set-up of the scenarios. A possible consequence can be the transportation and displacement of a DNAPL pool due to natural or anthropogenic induced high groundwater flow velocities, as by Pump and Treat facilities, complicating site investigation process and planning of remediation activities.

Characterization of a heterogeneous DNAPL source zone in the Borden aquifer using partitioning and interfacial tracers: Residual morphologies and background sorption

A partitioning interwell tracer test (PITT) was performed in the Borden sand aquifer to characterize an aged heterogeneous DNAPL source zone. This zone evolved during 5 years of natural groundwater flow following the infiltration of 50 L chlorinated solvents DNAPL. To assess the lateral variability of remaining DNAPL mass and morphology, four sweepzones were analyzed. The low saturation residual nature of the source zone required correction of tracer breakthrough data for natural background sorption. Corrected estimates of the DNAPL percentage remaining (total 13.2–16.6%), average saturation (0.05–0.18%) and distribution across the sweepzones were in good agreement with previous findings based on detailed transect monitoring, core analyses and ground-penetrating radar reflection. Using a newly defined metric “average spherical radius equivalent (ASRE)”, sweepzone estimates of the average size of DNAPL presence indicated the dominance of single pore DNAPL blobs and suggested the absence of DNAPL pools. Tracer tests indicated that DNAPL presence in the most DNAPL depleted sweepzone was potentially overestimated due to increased sediment sorption by residualized Sudan IV that was added to the DNAPL infiltrate. As hydrophobic compounds are normally present in spent solvent DNAPL, this suggests that additional sorption needs to be considered when using PITTs to characterize aged DNAPL source zones.

Modeling Kinetic Interphase Mass Transfer for Two-Phase Flow in Porous Media Including Fluid–Fluid Interfacial Area

Interphase mass transfer in porous media takes place across fluid–fluid interfaces. At the field scale, this is almost always a kinetic process and its rate is highly dependent on the amount of fluid–fluid interfacial area. Having no means to determine the interfacial area, modelers usually either neglect kinetics of mass transfer and assume local equilibrium between phases or they estimate interfacial area using lumped parameter approaches (in DNAPL pool dissolution) or a dual domain approach (for air sparging). However, none of these approaches include a physical determination of interfacial area or accounts for its role for interphase mass transfer. In this work, we propose a new formulation of two-phase flow with interphase mass transfer, which is based on thermodynamic principles. This approach comprises a mass balance for each component in each phase and a mass balance for specific interfacial area. The system is closed by a relationship among capillary pressure, interfacial area, and saturation. We compare our approach to an equilibrium model by showing simulation results for an air–water system. We show that the new approach is capable of modeling kinetic interphase mass exchange for a two-phase system and that mass transfer correlates with the specific interfacial area. By non-dimensionalization of the equations and variation of Peclet and Damkohler number, we make statements about when kinetic interphase mass transfer has to be taken into account by using the new physically based kinetic approach and when the equilibrium model is a reasonable simplification.

Effects of reduced contaminant loading on downgradient water quality in an idealized two-layer granular porous media

The following explores the issue of how reductions in contaminant loading to plumes will effect downgradient water quality. An idealized scenario of two adjacent layers of uniform geologic media, one transmissive and the other low permeability, is considered. A high concentration source, similar to a thin DNAPL pool, is introduced in the transmissive layer immediately above the low permeability layer. While the source is active, dissolved constituents are driven along the contact by advection and into the low permeability layer by transverse diffusion. Removing the source reverses the concentration gradient between the layers, driving back diffusion of contaminants from the low permeability layer. Laboratory studies involving four contaminants demonstrate that 15 to 44% of the introduced contaminant moves into the low permeability zone (along a distance of 87 cm in a sand tank) over a period of 25 days. The greatest movement of contaminants into the low permeability zone is seen with the contaminants with the greatest sorption coefficients. A unique two-dimensional analytical solution is developed for the two-layer scenario. Processes addressed include advection; transverse dispersion; adsorption and degradation in the transmissive zones; and diffusion, adsorption, and degradation in the low permeability layer. Laboratory data agree favorably with the analytical solutions. Collectively, the laboratory results and analytical solutions provide a basis for testing other modeling approaches that can be applied to more complex problems. A set of field-scale scenarios are considered using the analytical solutions. Results indicate that improvement in water quality associated with source removal diminish with distance downgradient of the source. Furthermore, contaminant degradation and contaminant adsorption in the stagnant zone are shown to be critical factors governing the timing and magnitude of downgradient improvements in water quality. For five of six scenarios considered, observed improvements in water quality 100 m downgradient of the source fall in the range of 1 to 2 orders of magnitude 15 years after complete source removal. The sixth scenario, involving a contaminant half-life of three years and no adsorption, shows greater than three order of magnitude improvements in downgradient water quality within one year of source removal.

Intermediate-scale 2D experimental investigation of in situ chemical oxidation using potassium permanganate for remediation of complex DNAPL source zones

In situ chemical oxidation is a technology that has been applied to speed up remediation of a contaminant source zone by inducing increased mass transfer from DNAPL sources into the aqueous phase for subsequent destruction. The DNAPL source zone can consist of one or more individual sources that may be present as an interconnected pool of high saturation, as a region of disconnected ganglia at residual saturation, or as combinations of these two morphologies. Potassium permanganate (KMnO 4) is a commonly employed oxidant that has been shown to rapidly destroy DNAPL compounds like PCE and TCE following second-order kinetics in an aqueous system. During the oxidation of a target DNAPL compound, or naturally occurring reduced species in the subsurface, manganese oxide (MnO 2) solids are produced. Research has shown that these manganese oxide solids may result in permeability reductions in the porous media thus reducing the ability for oxidant to be transported to individual DNAPL sources. It can also occur at the DNAPL–water interface, decreasing contact of the oxidant with the DNAPL. Additionally, MnO 2 formation at the DNAPL–water interface, and/or flow-bypassing as a result of permeability reductions around the source, may alter the mass transfer from the DNAPL into the aqueous phase, potentially diminishing the magnitude of any DNAPL mass depletion rate increase induced by oxidation. An experiment was performed in a two-dimensional (2D) sand-filled tank that included several discrete DNAPL source zones. Spatial and temporal monitoring of aqueous PCE, chloride, and permanganate concentrations was used to relate changes in mass depletion of, and mass flux, from DNAPL residual and pool source zones to chemical oxidation performance and MnO 2 formation. During the experiment, permeability changes were monitored throughout the 2D tank and these were related to MnO 2 deposition as measured through post-oxidation soil coring. Under the conditions of this experiment, MnO 2 formation was found to reduce permeability in and around DNAPL source zones resulting in changes to the overall flow pattern, with the effects depending on source zone configuration. A pool with little or no residual around it, in a relatively homogeneous flow field, appeared to benefit from resulting MnO 2 pore-blocking that substantially reduced mass transfer from the pool even though there was relatively little PCE mass removed from the pool. In contrast, a pool with residual around it (in a more typical heterogeneous flow field) appeared to undergo increased mass transfer as MnO 2 reduced permeability, altering the water flow and increasing the mixing at the DNAPL–water interface. Further, the magnitude of increased PCE mass depletion during oxidation appeared to depend on the PCE source configuration (pool versus ganglia) and decreased as MnO 2 was formed and deposited at the DNAPL–water interface. Overall, the oxidation of PCE mass appeared to be rate-limited by the mass transfer from the DNAPL to aqueous phase.

Comment on “Mobilization and entry of DNAPL pools into finer sand media by cosolvents: two-dimensional chamber studies” by Michael E. Van Valkenburg and Michael D. Annable

Comment on “Mobilization and entry of DNAPL pools into finer sand media by cosolvents: two-dimensional chamber studies” by Michael E. Van Valkenburg and Michael D. Annable

Modeling sub‐grid‐block‐scale dense nonaqueous phase liquid (DNAPL) pool dissolution using a dual‐domain approach

A dual‐domain approach is developed for calculating the advective and dispersive mass flux of chemical leaving one or more sub‐grid‐block‐scale DNAPL pool(s) in an integral finite difference numerical simulation. The contaminated zone in a simulation is divided into two fractions: one that contains DNAPL pools and one in which DNAPL is not present. With a dual‐permeability formulation the two regions may have the same or different flow properties depending on the conceptual model for pool formation, and fluids can freely flow through both domains. The local dispersive flux of contaminant away from the DNAPL pool is calculated using a well‐known analytical solution for steady state advection and dispersion. This expression is integrated analytically to give a first‐order mass transfer relationship for the total dissolution rate inside a grid block as a function of the horizontal water velocity, the transverse dispersivity, the DNAPL solubility, and the dissolved concentration in the media not containing the DNAPL. The calculated interphase mass transfer rate using a single dual‐domain integral finite difference grid block is shown to match experimental data and an analytical solution for DNAPL pool dissolution.

Influence of porous medium and NAPL distribution heterogeneities on partitioning inter-well tracer tests: a laboratory investigation

The inter-well partitioning tracer test seemingly provides an attractive way of investigating the presence of non-aqueous phase liquids (NAPLs) in aquifers. Although the feasibility of this rather new technique has been tested in the field, only few laboratory experiments have been performed to test its validity at scales greater than the column scale. In this study, a partitioning tracer test was conducted in a nominally two-dimensional, intermediate-scale flow cell containing a tetrachloroethene (PCE) spill. Tracer breakthrough curves were obtained at 14 sampling ports and an extraction well containing three outlets. Estimated PCE contents resulting from the tracer technique were compared to PCE saturations obtained at 800 locations by gamma radiation. An inverse procedure, based on the two-site, non-equilibrium convection–dispersion equation, was used in addition to the moment method for sampling port data analysis. Although the inverse procedure produced slightly better results than the moment method, the tracer technique generally underestimated the amount of DNAPL contained in our aquifer model. Notably, our results showed that the pooled part of the DNAPL spill was not detected. This was attributed to the low aqueous phase permeability within the DNAPL pool in combination with rate-limited partitioning of the tracers into the DNAPL. Our analysis indicate a general difficulty for the tracer technique to detect NAPL located in pools or large lenses.

Bench-scale visualization of DNAPL remediation processes in analog heterogeneous aquifers: surfactant floods and in situ oxidation using permanganate

We have conducted well-controlled DNAPL remediation experiments within a 2-D, glass-walled, sand-filled chamber using surfactants (Aerosol MA and Tween 80) to increase solubility and an oxidant (permanganate) to chemically degrade the DNAPL. Initial conditions for each remediation experiment were created by injecting DNAPL as a point source at the top of the chamber and allowing the DNAPL to migrate downward through a water-filled, heterogeneous, sand-pack designed to be evocative of a fluvial depositional environment. This migration process resulted in the DNAPL residing as a series of descending pools. Lateral advection across the chamber was used to introduce the remedial fluids. Photographs and digital image analysis illustrate interactions between the introduced fluids and the DNAPL. In the surfactant experiments, we found that DNAPL configured in a series of pools was easily mobilized. Extreme reductions in DNAPL/water interfacial tension occurred when using the Aerosol MA surfactant, resulting in mobilization into low permeability regions and thus confounding the remediation process. More modest reductions in interfacial tension occurred when using the Tween 80 surfactant resulting in modest mobilization. In this experiment, capillary forces remained sufficient to exclude DNAPL migration into low permeability regions allowing the excellent solubilizing properties of the surfactant to recover almost 90% of the DNAPL within 8.6 pore volumes. Injection of a potassium permanganate solution resulted in precipitation of MnO 2, a reaction product, creating a low-permeability rind surrounding the DNAPL pools. Formation of this rind hindered contact between the permanganate and the DNAPL, limiting the effectiveness of the remediation. From these experiments, we see the value of performing visualization experiments to evaluate the performance of proposed techniques for DNAPL remediation.

Laboratory-scale in situ chemical oxidation of a perchloroethylene pool using permanganate

In situ chemical oxidation (ISCO) is an emerging technology for the destruction of some chlorinated solvents present in subsurface environments. A laboratory investigation using a physical model was designed to assess the effectiveness of using permanganate as an oxidant to reduce the mass of a perchloroethylene (PCE) pool. The physical model was filled with silica sand overlying a silica flour base, simulating a two-dimensional saturated sand zone overlying a capillary barrier. PCE was introduced into the model so that it rested on top of the silica flour base, forming a dense nonaqueous phase liquid pool. The experimental methodology involved flushing the model with a permanganate solution for 146 days. During this period, measurements of chloride were used to assess the extent of pool oxidation. Before and after the oxidant flush, the quasi-steady state dissolution from the PCE pool was evaluated. Additionally, tracer studies were completed to assess changes in the flow field due to the oxidation process. At the termination of the experiment nine soil cores extracted from the model were used to detect the presence of MnO 2 deposits and to quantify the mass of PCE remaining in the system. Excavation of the remaining material in the model revealed that the MnO 2 distribution throughout the model was consistent with that observed in the cores. The oxidant flush was concluded before all of the pure phase PCE had been completely oxidized; however, ∼45% of the PCE mass was removed, resulting in a fourfold decrease in the quasi-steady state aqueous phase mass loading of PCE from the pool. Measurements of chloride during the oxidant flush and of PCE in the soil cores suggested that the oxidation reaction occurred primarily at the upgradient edge of the PCE pool. MnO 2 deposits within the model aquifer decreased the velocity of water directly above the pool, and the overall mass transfer from the remaining PCE pool. The results of this experimental study indicate that ISCO using permanganate is capable of removing substantial mass from a DNAPL pool; however, the performance of ISCO as a pool removal technology will be limited by the formation and precipitation of hydrous MnO 2 that occurs during the oxidation process.

Site‐Specific Design for Dual Phase Recovery and Stabilization of Pooled DNAPL

AbstractVolume reduction and lowering of capillary pressure within a large DNAPL pool are utilized as objectives in the design of a large‐scale dual phase recovery system at a chemical manufacturing facility in the United States. By reducing DNAPL pool height through mass removal, capillary pressure is lowered, resulting in a reduced potential for future vertical and horizontal mobilization of the chlorinated solvent DNAPL pool. The DNAPL pool extends over an approximately 200 m by 275 m area in low permeability fill deposits overlying a clay aquitard. A three‐dimensional multiphase flow model was employed to arrive at a final design incorporating nine horizontal drains (total length 664 m) and a pulsed pumping system. The numerical model was calibrated to the results of a 42‐day field pilot‐test involving the removal of approximately 25,000 L of DNAPL from a single, 55 m long horizontal drain. Numerical simulation revealed that gravity drainage, as opposed to hydraulic gradients in the water phase, is the dominant recovery mechanism at this site. This stems from the relatively high density and the viscosity of the DNAPL, and the relatively low permeability of the formation deposits. The use of pulsed pumping is shown to reduce the volume of contaminated ground water recovered from the 9‐drain system, without significant reduction of the total volume of DNAPL recovered.

Modeling the Transport of Contaminants Originating from the Dissolution of DNAPL Pools in Aquifers in the Presence of Dissolved Humic Substances

A two-dimensional finite difference numerical model was developed to describe the transport of dissolved organics originating from nonaqueous phase liquid (NAPL) pool dissolution in saturated porous media in the presence of dissolved humic substances. A rectangular NAPL pool was considered in a homogeneous porous medium with unidirectional interstitial groundwater velocity. It was assumed that dissolved humic substances and aqueous phase contaminants may sorb onto the solid matrix under local equilibrium conditions. The contaminant in the aqueous phase may undergo first-order decay. Also, the dissolved contaminant may sorb onto humic substances. The transport properties of dissolved humic substances are assumed to be unaffected by sorbing contaminants, because dissolved humic macromolecules are much larger than dissolved contaminants and sorp- tion of nonpolar contaminants onto humic substances do not affect the overall surface charge of humic substances. The sorption characteristics of dissolved humic substances onto clean sand were determined from column experiments. An effective local mass transfer rate coefficient accounting for the presence of dissolved humic substances was developed. Model simulations indicate that dissolved humic substances substantially increase NAPL pool dissolution, and consequently reduce the required pump-and-treat aquifer remediation time.

DNAPL Flow Behavior in a Contaminated Aquifer: Evaluation of Field Data

AbstractA pool of dense nonaqueous phase liquid (DNAPI.) containing TCE and other chlorinated solvents has been removed from the subsurface at Hill Air Force Base, Uthah. as part of an interim remedial action. The removal of the DNAPI. pool means that future off‐site migration of dissolved contaminants in the ground water is minimized, and costs for final remedial actions are reduced. A pump‐and‐treat system recovered more than 23.000) gallons of DNAPI. and one million gallons of contaminated ground water from the aquifer. The efficiency of this remedial action was evaluated on the basis of extensive field and laboratory data. The behavior of DNAPI. flow in the aquifer sands was characterized by collecting core samples from two borings in the DNAPL pool and measuring relative permeabilities and DMAPI. saturation. Core Hooding results show that approximately one‐third of the DNAPI. originally in the pool is not recovered by water displacement, but remains as a residual saturation held in place by capillary pressure. However, subsequent Hooding with two pore volumes of surfactant solution reduced the residual DNAPI. saturation in the sand by one order of magnitude. Analytical and numerical models for the DNAPI flow behavior at the site were developed. This is the first time that such models have been developed and applied to an actual DNAPI. pumping lest conducted in the field. Because measured permeabilities and residual saturations were used lo calibrate the models. the model predictions could be used lo provide valuable insights into the controlling mechanisms for DNAPL recovery. The data collection and modeling procedures outlined in this paper can be used lo enhance the efficiency and minimize the cost 10 clean up this and other DNAPI.‐contaminated sites.

A Field Study of Enhanced Recovery of DNAPL Pooled Below the Water Table

AbstractA large subsurface pool of waste solvent product, consisting primarily of 1,1,1‐tichloroethanc and carbon tetrachloride, was encountered during investigations at an industrial site in northern New Jersey. In the 1950s the product was discharged through a settling chamber directly below the shallow water table. Eventually the product accumulated within elongated depressions of erosional surface of varved clays at depths 10 to 15 feet below grade. The host sediment, line to medium sand, was overlain by line sand and silt. The delineated area of pooled DNAPLs covered 2750 feet2, and the maximum pool thickness exceeded 3 feel.The primary recovery involved pumping product from nine wells. Each recovery well was equipped with a sump extending into the clay, which enabled the system to keep the product pumping level below the bottom of the pool. A total of 3495 gallons of solvent product was recovered over two years. Nearly half of this volume was produced by two wells placed at the lowest points of the pools. Postpumping sampling of the former pools indicated that 43 to 94 percent of the pooled solvent mass was removed during the primary recovery. Average initial product salutation within the pool was estimated at 53.2 percent of the total porosity measured at 31 percent. Average residual saturation after the primary product recovery was 3.7 percent of the total pore volume.To test the feasibility of residual product recovery, an experimental secondary recovery was undertaken. Using sheet piling, a 506 feet2 lest cell was constructed inside the former DNAPL pool. The cell featured a central recovery well, six peripheral wells, and monitoring probes. I he selected sequence of secondary operations included partial dewatering. hot water injection, final dewatering, and thermally enhanced vapor extraction (TVE). During six weeks of the secondary recovery operations. 87.9 gallons of product were removed, of which 72 percent was from TVE. 25 percent from hydraulic mobilization effects. and 3 percent from dissolution of residuals. Confirmatory soil sampling showed an average reduction of residual contamination by 93.4 percent in comparison to the concentration of residuals prior to the secondary recovery. For the lest cell, a combined total solvent recovery of 99.6 percent was achieved. This high recovery exceeded DNAPL recoveries expected or achieved in other field‐scale attempts.

Soil‐Gas Signatures from Volatile Chlorinated Solvents: Borden Field Experiments

AbstractField experiments have been conducted at the Borden research site to evaluate the widely applied soil‐gas survey method. In particular, the ability of surveys to delineate DNAPL chlorinated solvent sources and associated ground‐water plumes has been investigated. Field experiments indicated that dissolved‐phase plumes from DNAPL pools or residual located about a meter or more below the water table are unlikely to be directly detected by soil‐gas surveys. Soil‐gas plumes observed at real sites are attributed to volatilization of source material in the vadose zone and consequent formation of very shallow interface zone ground‐water contamination that is able to partition to the soil gas as it transports downgradient. The distribution of DNAPL sources and dissolved‐phase plumes deeper in the ground‐water zone may often bear little resemblance to the shallow interface zone ground‐water plume above, and hence the location of this deeper contamination will remain highly difficult to identify by soil‐gas surveys. Field observations are confirmed with simple analytical 1‐D diffusion modeling and the implications of the results to the practice of soil‐gas surveys at contaminated sites indicated.

DNAPL pool dissolution in saturated porous media: procedure development and preliminary results

In order to study dissolution kinetics of dense non-aqueous-phase liquids (DNAPL's), a procedure was developed to create a DNAPL pool at the bottom of a specially designed experimental aquifer. The aquifer was housed in an all glass rectangular tank with internal dimensions 75 cm × 21.1 cm × 36.8 cm. The tank was equipped with sampling ports, inlet and outlet wells, constant-head reservoirs, and pumps for full hydraulic control. The pool was formed inside a 27.9 cm × 18 cm × 4.7 cm internal dimensions glass pan containing a gravel layer 1.6 cm thick at the bottom and the remainder filled with aquifer sand. The pools formed had a distinct, but not perfectly flat upper surface. The fraction of sand penetrated by DNAPL was larger at elevations closer to the bottom of the tank. Dissolution experiments, using TCA and mixture of TCE and TCA pools, resulted in aqueous concentrations much lower than the respective solubility. Pulsed pumping increased the respective aqueous concentrations by a factor of ≈ 6.