2009. "Numerical and experimental investigation of DNAPL removal mechanisms in a layered porous medium by means of soil vapor extraction." Journal of Contaminant Hydrology 109(1-4):1-13. doi:10.1016/j.jconhyd.2009.07.001 Abstract The purpose of this work is to identify the mechanisms that govern the removal of carbon tetrachloride (CT) during soil vapor extraction (SVE) by comparing multiphase flow simulations with a detailed data set from a well-defined two-dimensional flow cell experiment. The flow cell was packed with two sandy soils including an embedded fine-grained sand layer. Gas concentrations at the outlet of the flow cell and 15 sampling ports inside the flow cell were measured during SVE. A dual-energy gamma radiation system was used to measure an initial NAPL saturation profile in a fine-grained sand layer. Imaging result from a dual-energy gamma radiation system with dyed CT mark along CT migration was used to construct the distribution of initial NAPL saturation in the flow cell for input to numerical simulations. Gas concentration results and photographs during SVE were compared to simulation results using a continuum-based multiphase flow simulator, STOMP (Subsurface Transport Over Multiple Phases). The measured effluent gas concentration decreased quickly at first, and then started to decrease gradually, resulting in long-term tailing. CT mass was removed quickly in coarse sand, followed by a slow removal from the fine-grained sand layer. An analytical solution for a one-dimensional advection and first-order volatilization model matched the tailing well with two fitting parameters. However, given detailed knowledge of the permeability field and initial NAPL distribution, we can predict the tailing and gas concentration profiles at sampling ports using equilibrium NAPL volatilization. NAPL flow occurs in the presence of free NAPL, and must be accounted for to accurately predict NAPL removal during the SVE experiment. The model prediction was accurate within the uncertainty of the measured or literature derived parameters (i.e., dispersivity and soil parameters). This study provides insights into the physical mechanisms of NAPL removal from a low permeability zone, and use of the local equilibrium assumption for NAPL volatilization during SVE. In addition, this study demonstrates that lack of detailed information regarding NAPL distribution and heterogeneity pattern lead overall NAPL removal to a kinetically controlled system at a 2-D flow cell scale.
2009. "Estimation of Interfacial Tension between Organic Liquid Mixtures and Water." Environmental Science & Technology 43(20):7754-7761. doi:10.1021/es901061k Abstract Knowledge of IFT values for chemical mixtures helps guide the design and analysis of various processes, including NAPL remediation with surfactants or alcohol flushing, enhanced oil recovery, and chemical separation technologies, yet available literature values are sparse. A comprehensive comparison of thermodynamic and empirical models for estimating interfacial tension (IFT) of organic chemical mixtures with water is conducted, mainly focusing on chlorinated organic compounds for 14 ternary, three quaternary, and one quinary systems. Emphasis is placed on novel results for systems with three and four organic chemical compounds, and for systems with composite organic compounds like lard oil and mineral oil. Seven models are evaluated: the ideal and nonideal monolayer models (MLID and MLNID), the ideal and nonideal mutual solubility models (MSID and MSNID), an empirical model for ternary systems (EM), a linear mixing model based on mole fractions (LMMM), and a newly developed linear mixing model based on volume fractions of organic mixtures (LMMV) for higher order systems. The two ideal models (MLID and MSID) fit ternary systems of chlorinated organic compounds without surface active compounds relatively well. However, both ideal models did not perform well for the mixtures containing a surface active compound. However, for these systems, both the MLNID and MSNID models matched the IFT data well. It is shown that the MLNID model with a surface coverage value (0.00341 mmol/m2) obtained in this study can practically be used for chlorinated organic compounds. The LMMM results in poorer estimates of the IFT as the difference in IFT values of individual organic compounds in a mixture increases. The EM, with two fitting parameters, provided accurate results for all 14 ternary systems including composite organic compounds. The new LMMV method for quaternary and higher component systems was successfully tested. This study shows that the LMMV may be able to be used for higher component systems and it can be easily incorporated into compositional multiphase flow models using only parameters from ternary systems.
2009. "An automated tool for three types of saturated hydraulic conductivity laboratory measurements." Soil Science Society of America Journal 73(2):466-470. Abstract Acquisition of porous medium hydraulic conductivity in the laboratory is usually time-consuming and costly because of the manual labor associated with the currently available techniques. Lately, there has been increased interest in automating hydraulic conductivity laboratory techniques to reduce analysis time and improve data consistency. A new apparatus is presented that is able to determine hydraulic conductivity values with the falling head, constant head, and constant flux methods in an automated fashion. In addition, the columns are designed forcing water to flow in a nominally one-dimensional manner throughout the porous medium. In this paper, hydraulic conductivity data for standard laboratory sands are presented and compared to results obtained using a standard Tempe cell configuration. Hydraulic conductivity values obtained with the new tool for the laboratory sands are consistent with literature data. For highly permeable sands, the newly obtained hydraulic conductivity values are considerable larger then values acquired using a Tempe cell configuration. The lower conductivity values for the Tempe Cell configuration are primarily the result of insufficient spreading of water in the inlet and outlet reservoirs.
2009. "Estimating Persistent Mass Flux of Volatile Contaminants from the Vadose Zone to Ground Water ." Ground Water Monitoring and Remediation 29(2):63-72. Abstract Contaminants may persist for long time periods within low permeability portions of the vadose zone where they cannot be effectively treated and are a potential continuing source of contamination to groundwater. Setting appropriate vadose zone remediation goals requires evaluating these persistent sources in terms of their impact on meeting groundwater remediation goals. One-dimensional approaches for estimating transport of volatile contaminants in the vadose zone are considered and compared to a one-dimensional flux-continuity-based assessment of vapor-phase contaminant movement from the vadose zone to the groundwater. The flux-continuity-based assessment demonstrates that the ability of the groundwater to move contaminant away from the water table controls the vapor-phase mass flux from the vadose zone across the water table. Limitations of the one-dimensional approaches are then discussed with respect to the need for further method development and application of two- or three-dimensional numerical modeling. The carbon tetrachloride (CT) plume at the U.S. Department of Energy Hanford Site is used as an example of a site where persistent vadose zone contamination needs to be considered in the context of groundwater remediation.
2009. "Desiccation of unsaturated porous media: Intermediate-scale experiments and numerical simulation." Vadose Zone Journal 8(3):643-650. Abstract Soil desiccation (drying) is recognized as a potentially robust vadose zone remediation process involving water evaporation induced by air injection and extraction. Desiccation has the potential to immobilize contaminants and could potentially improve access for other gas-phase treatments by reducing water saturation and therefore increasing sediment gas-phase permeability. Before this technology could be deployed in the field, concerns related to energy limitations, osmotic effects, and potential contaminant remobilization after rewetting need to be addressed. A series of detailed wedge-shaped, intermediate-scale laboratory experiments in unsaturated homogeneous and simple heterogeneous systems was conducted to improve the understanding of the impact of energy balance issues on soil desiccation. The experiments were simulated with the multifluid flow simulator STOMP, using independently obtained hydraulic and thermal porous medium properties. In all the experiments, the injection of dry air proved to be an effective means for removing essentially all moisture from the test media. Evaporative cooling was observed which generally decreased with increased distance from the gas inlet chamber. Observations of temperature in fine-grained sands in the heterogeneous systems show two local temperature minima associated with the cooling. The first one occurs because of evaporation in the adjacent medium-grained sand whereas the second minimum is attributed to evaporative cooling in the fine-grained sand itself. Results of the laboratory tests were simulated accurately when thermal properties of the flow cell walls and insulation material were taken into account, indicating that the proper physics were incorporated into the simulator.
2009. "Surface and Interfacial Properties of Nonaqueous-Phase Liquid Mixtures Released to the Subsurface at the Hanford Site ." Vadose Zone Journal 8(2):344-351. Abstract Surface and interfacial tensions that arise at the interface between different phases are key parameters affecting Nonaqueous Phase Liquid (NAPL) movement and redistribution in the vadose zone after spill events. In this study, the impact of major additive components on surface and interfacial tensions for organic mixtures and wastewater was investigated. Organic mixture and wastewater compositions are based upon carbon tetrachloride (CT) mixtures released at the Hanford site, where CT was discharged simultaneously with dibutyl butyl phosphonate (DBBP), tributyl phosphate (TBP), dibutyl phosphate (DBP), and a machining lard oil (LO). A considerable amount of wastewater consisting primarily of nitrates and metal salts was also discharged. The tension values measured in this study revealed that the addition of these additive components caused a significant lowering of the interfacial tension with water or wastewater and the surface tension of the wastewater phase in equilibrium with the organic mixtures, compared to pure CT, but had minimal effect on the surface tension of the NAPL itself. These results lead to large differences in spreading coefficients for several mixtures, where the additives caused both a higher (more spreading) initial spreading coefficient and a lower (less spreading) equilibrium spreading coefficient. This indicates that if these mixtures migrate into uncontaminated areas, they will tend to spread quickly, but form a higher residual NAPL saturation after equilibrium, as compared to pure CT. Over time, CT likely volatilizes more rapidly than other components in the originally disposed mixtures and the lard oil and phosphates would become more concentrated in the remaining NAPL, resulting in a lower interfacial tension for the mixture. Spreading coefficients are expected to increase and perhaps change the equilibrated organic mixtures from nonspreading to spreading in water-wetting porous media. These results show that the behavior of organic chemical mixtures should be accounted for in numerical flow and transport models.
2008. "Enhanced Remedial Amendment Delivery through Fluid Viscosity Modifications: Experiments and numerical simulations." Journal of Contaminant Hydrology 101(1-4):29-41. doi:10.1016/j.jconhyd.2008.07.007 Abstract Abstract Heterogeneity is often encountered in subsurface contamination characterization and remediation. Low-permeability zones are typically bypassed when remedial fluids are injected into subsurface heterogeneous aquifer systems. Therefore, contaminants in the bypassed areas may not be contacted by the amendments in the remedial fluid, which may significantly prolong the remediation operations. Laboratory experiments and numerical studies have been conducted to develop the Mobility-Controlled Flood (MCF) technology for subsurface remediation and to demonstrate the capability of this technology in enhancing the remedial amendments delivery to the lower permeability zones in heterogeneous systems. Xanthan gum, a bio-polymer, was used to modify the viscosity of the amendment-containing remedial solutions. Sodium mono-phosphate and surfactant were the remedial amendment used in this work. The enhanced delivery of the amendments was demonstrated in two-dimensional (2-D) flow cell experiments, packed with heterogeneous systems. The impact of polymer concentration, fluid injection rate, and permeability contract in the heterogeneous systems has been studied. The Subsurface Transport over Multiple Phases (STOMP) simulator was modified to include polymer-induced shear thinning effects. Shear rates of polymer solutions were computed from pore-water velocities using a relationship proposed in the literature. Viscosity data were subsequently obtained from empirical viscosity-shear rate relationships derived from laboratory data. The experimental and simulation results clearly show that the MCF technology is capable of enhancing the delivery of remedial amendments to subsurface lower permeability zones. The enhanced delivery significantly improved the NAPL removal from these zones and the sweeping efficiency on a heterogeneous system was remarkably increased when a polymer fluid was applied. MCF technology is also able to stabilize the fluid displacing front when there is a density difference between the fluids. The modified STOMP simulator was able to predict the experimental observed fluid displacing behavior. The simulator may be used to predict the subsurface remediation performance when a shear thinning fluid is used to remediate a heterogeneous system.
2008. "Impact of nonaqueous phase liquid (NAPL) source zone architecture on mass removal mechanisms in strongly layered heterogeneous porous media during soil vapor extraction ." Journal of Contaminant Hydrology 100(1-2):58-71. doi:10.1016/j.jconhyd.2008.05.006 Abstract An existing multiphase flow simulator was modified in order to determine the effects of four mechanisms on NAPL mass removal in a strongly layered heterogeneous vadose zone during soil vapor extraction (SVE): a) NAPL flow, b) diffusion and dispersion from low permeability zones, c) slow desorption from sediment grains, and d) rate-limited dissolution of trapped NAPL. The impact of water and NAPL saturation distribution, NAPL type (i.e., free, residual, or trapped) distribution, and spatial heterogeneity of the permeability field on these mechanisms were evaluated. Two different initial source zone architectures (one with and one without trapped NAPL) were considered and these architectures were used to evaluate seven different SVE scenarios. For all runs, slow diffusion from low permeability zones that gas flow bypassed was a dominant factor for diminished SVE effectiveness at later times. This effect was more significant at high water saturation due to the decrease of gas-phase relative permeability. Transverse dispersion contributed to fast NAPL mass removal from the low permeability layer in both source zone architectures, but longitudinal dispersion did not affect overall mass removal time. Both slow desorption from sediment grains and rate-limited mass transfer from trapped NAPL only marginally affected removal times. However, mass transfer from trapped NAPL did affect mass removal at late time, as well as the NAPL distribution. NAPL flow from low to high permeability zones contributed to faster mass removal from the low permeability layer, and this effect increased when water infiltration was eliminated. These simulations indicate that if trapped NAPL exists in heterogeneous porous media, mass transfer can be improved by delivering gas directly to zones with trapped NAPL and by lowering the water content, which increases the gas relative permeability and changes trapped NAPL to free NAPL.
2008. "Scalable Modeling of Carbon Tetrachloride Migration at the Hanford Site Using the STOMP Simulator." Vadose Zone Journal 7(2):654-666. doi:10.2136/vzj2007.0070 Abstract Numerical simulation has been applied in support of the U.S. Department of Energy’s (DOE’s) efforts to characterize the nature and distribution of carbon tetrachloride in the deep vadose zone at the Hanford site, near Richland, Washington. Three-dimensional computational domains were used, with layered and heterogeneous distributions of soil properties, in this numerical investigation into the vertical and lateral distribution of carbon tetrachloride beneath it release point (216-Z-9 trench) and the effects of soil vapor extraction process. The complexity of the modeled physical processes, namely, the nonlinearities associated with multifluid subsurface flow, including phase transitions and hysteresis in the relative permeability-saturation-capillary pressure functions, limits the grid resolution when executed using single processor computers. To achieve higher grid resolutions and acceptable detail in the subsurface distribution and remediation of carbon tetrachloride, execution on multiple processors was required. This paper describes and demonstrates a scalable implementation of a multifluid subsurface flow and transport with capabilities for volatile organic compounds, residual nonaqueous phase liquid formation in the vadose zone, and soil vapor extraction, using multiple wells. Developing scientific software for execution on parallel computers has unique challenges. The guiding objectives for developing this scalable code were to keep the source coding readable and modifiable by subsurface scientists, allow for both sequential and scalable processing, depend on domain scientists for code parallelization and scalable linear system solvers.
2008. "Thermodynamic Model for Fluid-Fluid Interfacial Areas in Porous Media for Arbitrary Drainage-Imbibition Sequences." Vadose Zone Journal 7(3):966-971. doi:10.2136/vzj2007.0185 Abstract Fluid/fluid interfacial areas are important in controlling the rate of mass and energy transfer between fluid phases in porous media. We present a modified thermodynamically based model (TBM) to predict fluid/fluid interfacial areas in porous media for arbitrary drainage/imbibition sequences. The TBM explicitly distinguishes between interfacial areas associated with continuous (free) and isolated (entrapped) nonwetting fluids. The model is restricted to two-fluid systems in which (1) no significant conversion of mechanical work into heat occurs, (2) the wetting fluid completely wets the porous medium’s solid surfaces, and (3) no changes in interfacial area due to mass transfer between phases occur. We show example calculations for two different drainage/imbibition sequences in two porous media: a highly uniform silica sand and a well-graded silt. The TBM’s predictions for interfacial area associated with free nonwetting-fluid are identical to those of a previously published geometry-based model (GBM). However, predictions for interfacial area associated with entrapped nonwetting-fluid are consistently larger in the TBM than in the GBM. Although a comparison of model predictions with experimental data is currently only possible to a limited extent, good general agreement was found for the TBM. As required model parameters are commonly used as inputs for or tracked during multifluid-flow simulations, the modified TBM may be easily incorporated in numerical codes.
2008. "MASS-REMOVAL AND MASS-FLUX-REDUCTION BEHAVIOR FOR IDEALIZED SOURCE ZONES WITH HYDRAULICALLY POORLY-ACCESSIBLE IMMISCIBLE LIQUID." Chemosphere 71(8):1511-1521. Abstract A series of flow-cell experiments was conducted to investigate aqueous dissolution and mass-removal behavior for systems wherein immiscible liquid was non-uniformly distributed in physically heterogeneous source zones. The study focused specifically on characterizing the relationship between mass flux reduction and mass removal for systems for which immiscible liquid is poorly accessible to flowing water. Two idealized scenarios were examined, one wherein immiscible liquid at residual saturation exists within a lower-permeability unit residing in a higher-permeability matrix, and one wherein immiscible liquid at higher saturation (a pool) exists within a higher-permeability unit adjacent to a lower-permeability unit. The results showed that significant reductions in mass flux occurred at relatively moderate mass-removal fractions for all systems. Conversely, minimalmass flux reduction occurred until a relatively large fraction of mass (>80%) was removed for the control experiment, which was designed to exhibit ideal mass removal. In general, mass flux reduction was observed to follow an approximately one-to-one relationship with mass removal. Two methods for estimating mass-flux-reduction/mass-removal behavior, one based on system-indicator parameters (ganglia-to-pool ratio) and the other a simple mass-removal function, were used to evaluate the measured data. The results of this study illustrate the impact of poorly accessible immiscible liquid on mass-removal and mass-flux processes, and the difficulties posed for estimating mass-flux-reduction/mass-removal behavior.
2007. "Saturation-Dependent Hydraulic Conductivity Anisotropy for Multifluid Systems in Porous Media." Vadose Zone Journal 6(4):925-934. doi:10.2136/vzj2006.0141 Abstract The hydraulic conductivity of unsaturated anisotropic soils has recently been described with a tensorial connectivity-tortuosity (TCT) concept. We extend this concept to unsaturated porous media with two or three immiscible fluids. Mathematical expressions to describe the conductivity of each fluid in anisotropic porous media under unsaturated condition are derived in the form of symmetric second order tensors. The theory is applicable to the combination of any type of saturation-pressure formulation and a generalized hydraulic conductivity model. The extended model shows that the anisotropic coefficient of a fluid is independent of the saturation of other fluids. Synthetic Miller-similar soils having hypothetical anisotropy were defined by allowing the saturated hydraulic conductivity to have different correlation ranges for different directions of flow. The extended TCT concept was tested using synthetic soils with four levels of heterogeneity and four levels of anisotropy. Numerical experiments of infiltration of two liquid phases, i.e., water and the nonaqueous phase liquid (NAPL) carbon tetrachloride, were carried out to test the extended model. The results show that, similar to water in a two-fluid (air-water) system, NAPL retention curves in a three-fluid (air-NAPL-water) system were independent of flow direction but dependent on soil heterogeneity, while the connectivity-tortuosity coefficients are functions of both soil heterogeneity and anisotropy. The extended TCT model accurately describes unsaturated hydraulic functions of anisotropic soils and can be combined into commonly used relative permeability functions for use in multifluid flow and transport numerical simulations.
2007. "Experimental and Theoretical Assessment of the Lifetime of a Gaseous-Reduced Vadose Zone Permeable Reactive Barrier." Vadose Zone Journal 6(4):1050-1056. doi:10.2136/vzj2006.0157 Abstract The feasibility of using gaseous reduction to establish a vadose zone permeable reactive barrier was evaluated through a combination of laboratory testing activities and consideration of fundamental vadose zone transport concepts. For the experimental evaluation, a series of laboratory column tests were conducted in which sediment was first treated with diluted hydrogen sulfide. Water containing dissolved oxygen was then pumped through the columns at different flow rates to determine the reoxidation rate and the reductive capacity of the treated sediment. The results indicated that the treated sediment has a significant reductive capacity consistent with the basic reactions associated with the treatment and reoxidation processes. The observed reductive capacity was found to be dependent on the flow rate of water during the reoxidation phase of the tests. At lower flow rates, the reductive capacity approached the maximum value predicted on the basis of the treatment reaction. Thus, laboratory treatment tests should reliably predict the reductive capacity of the barrier under field conditions. A theoretical approach was undertaken to estimate the lifetime of the vadose zone barrier. An initial model assumed that the barrier lifetime is determined by the reoxidation of the barrier owing to the transport of oxygen through a vadose zone interval in which all sediment is unsaturated. The results of this evaluation suggest that barrier reoxidation is primarily related to diffusion of oxygen through the gas-filled portion of the sediment pore space. If so, the barrier lifetime could be fairly short (several years). However, the presence of finer grained strata with higher moisture content could potentially increase the barrier lifetime to 100 years or more owing to a decrease in the effective diffusion coefficient for oxygen. Thus, detailed stratagraphic characterization and modeling is needed to provide an accurate assessment of barrier lifetime at specific sites.
2007. "Zero-valent Iron Emplacement in Permeable Porous Media Using Polymer Additions." Ground Water Monitoring and Remediation 27(1):122-130. Abstract At the Hanford Site in Washington, an extensive In Situ Redox Manipulation (ISRM) permeable reactive barrier was installed to prevent chromate from reaching the Columbia River. However, chromium has been detected in several wells, indicating a premature loss of the reductive capacity in the aquifer. One possible cause for premature chromate breakthrough is associated with the presence of high-permeability zones in the aquifer. The potential emplacement of zero-valent iron (Fe0) into high-permeability Hanford sediments to enhance the barrier’s reductive capacity using shear-thinning fluids containing polymers was investigated in three-dimensional wedge-shaped aquifer models. Porous media were packed in the wedge-shaped flow cell to create either a heterogeneous layered system with a high-permeability zone between two low-permeability zones or a high-permeability channel surrounded by low-permeability materials. The injection flow rate, polymer type, polymer concentration, and injected pore volumes were determined based on preliminary short- and long-column experiments. The flow cell experiments indicated that iron concentration enhancements of at least 0.6% (w/w) could be obtained using moderate flow rates and injection of 30 pore volumes. The aqueous pressure increased by a maximum of 25 KPa during infiltration, but a decrease in permeability was not observed. Under optimal conditions, the 0.6% amended Fe0 concentration would provide approximately 20 times the average reductive capacity that is provided by the dithionite-reduced Fe (II) in the ISRM barrier.
2007. "Three-Dimensional Multifluid Flow and Transport at the Brooklawn Site near Baton Rouge, LA: A Case Study." Soil & Sediment Contamination 16(2):109-141. Abstract Disposal quantities of organic wastes at the Brooklawn Site in Louisiana are suspected to equal nearly 160 Ktons, making this site one of the most contaminated DNAPL sites in the world. Remedial activities at the site include groundwater and dense nonaqueous phase liquid (DNAPL) extraction from recovery wells. DNAPL recovery has markedly declined in recent years, with many of the peripheral wells showing negligible recovery of organic liquids. Three-dimensional simulations of DNAPL movement in the subsurface were conducted using the STOMP simulator, including a new coupled well model. The objectives of this modeling effort were to (1) determine the fate and transport of infiltrated DNAPL, and (2) measure the effects of active recovery through DNAPL pumping. A detailed three-dimensional geologic model of the Brooklawn primary DNAPL disposal area was developed and used as the framework for DNAPL simulations. Additionally, site-specific data were obtained to obtain the most important hydraulic properties of the subsurface related to DNAPL movement and formation of entrapped DNAPL in the laboratory. Besides a simulation using the best available subsurface information, several sensitivity simulations were conducted to assess the effects on DNAPL migration. These simulations include DNAPL pumping, well screen extension, an alternative geology, increased DNAPL density, lower DNAPL viscosity, and more-permeable sand and silt deposits. Results of the simulations were compared to field data that define the extent of DNAPL movement based on where DNAPL has been extracted in the site recovery wells. The model simulations predict no significant reduction in the extent of the DNAPL as a result of pumping. Pumping returns diminish rapidly due to the limited radius of influence of the wells and movement of the DNAPL out of the zone of influence of the wells with a maximum radius of influence of about 6 m. The numerical analysis also demonstrates that it is impractical to extend existing wells or install new wells to retrieve enough DNAPL to affect the overall extent of DNAPL movement.
2007. "Carbon Tetrachloride Flow and Transport in the Subsurface of the 216-Z-9 Trench at the Hanford Site." Vadose Zone Journal 6(4):971-984. doi:10.2136/vzj2006.0166 Abstract Carbon tetrachloride (CT) was discharged to waste sites at the Hanford 200 West Area. Three-dimensional modeling was conducted to enhance the conceptual model of CT distribution in the vertical and lateral direction beneath the 216-Z-9 trench and to investigate the effects of soil vapor extraction. Simulations focused on migration of dense, nonaqueous phase liquid (DNAPL) consisting of carbon tetrachloride and co-disposed organics as a function of the properties and distribution of subsurface sediments and of the properties and disposal history of the waste. The simulations of CT migration were conducted using the Subsurface Transport Over Multiple Phases (STOMP) simulator. Simulation results support a conceptual model for CT distribution where CT in the DNAPL phase is expected to have migrated primarily in a vertical direction below the disposal trench. Results also show that the Cold Creek low permeability units retain more CT DNAPL within the vadose zone than other hydrologic unit during during soil vapor extraction. Additional characterization of the Cold Creek units would provide valuable information about the quantity of CT DNAPL remaining in the vadose zone. A significant amount of the disposed CT DNAPL may have partitioned to the vapor and subsequently water and sorbed phases. Any continued migration of CT from the vadose zone to the groundwater is likely through interaction of vapor phase CT with the groundwater and not through continued DNAPL migration. Additional effort is needed to enhance the understanding of rate-limited volatilization to improve simulation of the SVE process and to provide a basis for refining the design and operation of SVE systems.
2007. "A Review of Multidimensional, Multifluid Intermediate-scale Experiments: Flow Behavior, Saturation Imaging, and Tracer Detection and Quantification ." Vadose Zone Journal 6(3):610-637. doi:10.2136/vzj2006.0178 Abstract A review is presented of original multidimensional, intermediate-scale experiments involving non-aqueous phase liquid (NAPL) flow behavior, imaging, and detection/quantification with solute tracers. In a companion paper (Oostrom, M., J.H. Dane, and T.W. Wietsma. 2006. A review of multidimensional, multifluid intermediate-scale experiments: Nonaqueous phase dissolution and enhanced remediation. Vadose Zone Journal 5:570-598) experiments related to aqueous dissolution and enhanced remediation were discussed. The experiments investigating flow behavior include infiltration and redistribution experiments with both light and dense NAPLs in homogeneous and heterogeneous porous medium systems. The techniques used for NAPL saturation mapping for intermediate-scale experiments include photon-attenuation methods such as gamma and X-ray techniques, and photographic methods such as the light reflection, light transmission, and multispectral image analysis techniques. Solute tracer methods used for detection and quantification of NAPL in the subsurface are primarily limited to variations of techniques comparing the behavior of conservative and partitioning tracers. Besides a discussion of the experimental efforts, recommendations for future research at this laboratory scale are provided.
2007. "Impact Assessment of Existing Vadose Zone Contamination at the Hanford Site SX Tank Farm." Vadose Zone Journal 6(4):935-945. doi:10.2136/2006.0176 Abstract The USDOE has initiated an impact assessment of existing vadose zone contamination at the Hanford Site SX tank farm in southeastern Washington State. The assessment followed the Resource Conservation and Recovery Act (RCRA) Corrective Action process to address the impacts of past tank waste releases to the vadose zone at the single-shell tank farm. Numerical models were developed that consider the extent of contamination presently within the vadose zone and predict contaminant movement through the vadose zone to groundwater. The transport of representative mobile (technetium-99) and immobile (cesium-137) constituents was evaluated in modeling. The model considered the accelerated movement of moisture around and beneath single-shell tanks that is attributed to bare, gravel surfaces resulting from the construction of the underground storage tanks. Infiltration, possibly nearing 100 mm yr–1, is further amplified in the tank farm because of the umbrella effect created by percolating moisture being diverted by the impermeable, sloping surface of the large, 24-m-diameter, buried tank domes. For both the base case (no-action alternative) simulation and a simulation that considered placement of an interim surface barrier to minimize infiltration, predicted groundwater concentrations for technetium-99 at the SX tank farm boundary were exceedingly high, on the order of 106 pCi L–1. The predicted concentrations are, however, somewhat conservative because of our use of two-dimensional modeling for a three-dimensional problem. A series of simulations were performed, using recharge rates of 50, 30, and 10 mm yr–1, and compared to the base case (100 mm yr–1) results. As expected, lowering meteoric recharge delayed peak arrival times and reduced peak concentrations at the tank farm boundary.
2007. "An experimental investigation of nitrogen gas produced during denitrification." Ground Water 45(4):461-467. Abstract In Situ denitrification relies on the activity of indegenous or introduced denitrifying microorganisms to reduce nitrate to N2 gas. In this study, we investigated the fate of N2 gas produced during denitrification in an intermediate-scale flow cell containing packed sediments. Denitrification was stimulated by a series of nitrate and ethanol additions. Results show limited reduction of hydraulic conductivity in the aquifer material.
2007. "Hanford Site Vadose-Zone Studies: An Overview." Vadose Zone Journal 6(4):899-905. doi:10.2136/vzj2006.0179 Abstract Large quantities of radioactive and chemical wastes, created from plutonium production for nuclear weapons, are located in a remote desert setting at the U. S. Department of Energy’s Hanford Site, north of Richland, Washington, USA. Much of the waste currently resides in the vadose zone. At Hanford, the vadose zone is characterized by glacial-fluvial sediments that are often highly stratified. The extremely heterogeneous sediments give rise to complex subsurface-flow paths that contribute to uncertainty of contaminant fate and transport. Research efforts have focused on answering questions of contaminant transport from the viewpoint of geologic, biologic, geochemical and hydrologic controls. This special section of the Vadose Zone Journal highlights key research topics that are systematically addressing vadose zone problems at the Hanford Site. Research to date indicates that some of the contaminant species (e.g., Cs-137, Co-60, Sr-90, uranium, etc.) are highly reactive with Hanford sediments, as predicted by geochemical considerations, rendering them effectively immobile, except under extremely saline or acidic conditions, while other species (e.g., Tc-99, I-129, H-3) are typically mobile and have moved deep into the vadose zone and subsequently into groundwater. In addition, large quantities of organics, including carbon tetrachloride, have moved in complex ways as both vapor and liquid, and have reached the water table at some locations where they represent a potential long-term threat to groundwater. Observed transport of mobile species is linked to liquid discharges and to elevated recharge rates that occur primarily at waste sites where land surfaces are void of vegetation and where winter rains have subsequently penetrated the subsurface wastes. A series of papers in this special issue document progress to date in understanding transport rates at Hanford, why anisotropy strongly affects the distribution of subsurface contaminants, why organic contaminants such as DNAPLs (e.g., carbon tetrachloride) are so difficult to find in the deep vadose zone, and what are the impacts of hypersaline fluids on waste form degradation and subsequent transport.
2006. "Development of a Field Design for In Situ Gaseous Treatment of Sediment Based on Laboratory Column Test Data." Journal of Environmental Engineering (ASCE) 132(12):1626-1632. Abstract A testing methodology is presented that supports the development of a field design for in situ gaseous treatment of sediments with diluted hydrogen sulfide. This approach involves the collection of column breakthrough test results at various flow rates, allowing a relationship to be developed between pore velocity of the carrier gas and velocity of the hydrogen sulfide reaction front that permits sizing to the field scale. A regression fit of a set of laboratory column breakthrough test data collected in this study is utilized to illustrate the development of a field design based on a 2D radial flow analytical model. Information regarding treatment time and hydrogen sulfide consumption characteristics associated with in situ gaseous treatment can then be obtained from this model and used as a basis for estimation of treatment schedule and costs. The regression relationship can also be utilized in numerical models in more complex geometries to support the field design of in situ gaseous treatment operations.
2006. "Behavior of a Viscous LNAPL under Variable Water Table Conditions." Soil & Sediment Contamination 15(6):543 - 564 . doi:10.1080/15320380600958976 Abstract An intermediate-scale experiment in a 1.02-m-long, 0.75-m-high, and 0.05-m-wide flow cell was conducted to investigate the behavior of a viscous LNAPL under variable water table conditions. Two viscous LNAPL volumes (0.4 L) were released, one week apart, from a small source zone on top of the flow cell into a partly saturated, homogenously packed porous medium. Following a redistribution period of 30 days after the second release, the water table was increased 0.5 m in 50 minutes. After the water table rise, viscous LNAPL behavior was monitored for an additional 45 days. Fluid saturation scans were obtained periodically with a fully automated dual-energy gamma radiation system. Results show that both spills follow similar paths downwards. Within two hours after the first LNAPL arrival, the capillary fringe was reduced across the cell by approximately 0.04 m (22%). This reduction is directly related to the decrease in the air-water surface tension from 0.072 to 0.057 N/m. LNAPL drainage from the unsaturated zone was relatively slow and a considerable residual LNAPL saturation was observed after 30 days of drainage. Most of the mobile LNAPL moved into the capillary fringe during this period. After a rapid 0.5 m water table rise, the LNAPL moved up in a delayed fashion. The LNAPL used the same path upwards as it used coming down during the infiltration phase. After 45 days, the LNAPL has moved up only approximately 0.2 m. Since the LNAPL has only moved up a limited amount, nonwetting fluid entrapment was limited. The experiment was simulated using the STOMP multifluid flow simulator, which includes entrapped and residual LNAPL saturation formation. A comparison indicates that the simulator is able to predict the observed phenomena well, including residual saturation formation in the vadose zone, and limited upward LNAPL movement after the water table rise. The results of this experiment show that viscous mobile LNAPL, subject to variable water table conditions, does not necessarily float on the water table and may not appear in an observation well.
2006. "A Review of Multidimensional, Multifluid Intermediate-scale Experiments: Nonaqueous Phase Liquid Dissolution and Enhanced Remediation." Vadose Zone Journal 5:570-598. doi:10.2136/vzj2005.0125 Abstract A review is presented of original multidimensional, intermediate-scale experiments involving nonaqueous phase liquids. The experimental approach at this scale can be viewed as an important intermediary between column studies and field trials. The primary advantage of intermediate-scale flow cell experiments is that field-scale processes can be simulated under controlled conditions. The experiments are frequently conducted to provide data sets to test and verify numerical and analytical flow and transport models. The controlled setting and laboratory instrumentation reduces the uncertainty in parameter estimation, allowing comparisons between simulation and experimental results to focus on flow and transport processes. A total of about 120 original contributions were identified and reviewed. Depending on the main topic of NAPL experimental research, the papers were divided into the following sections: (1) Dissolution, (2) Enhanced Remediation, (3) Flow behavior, (4) Quantification, and, (5) Imaging. In this paper, the categories Dissolution and Enhanced Remediation are discussed and suggestions for future research are provided. In a companion paper, experimental work related to the other three categories is reviewed. The Dissolution category includes experiments in which NAPL removal occurs due to water flushing. The Enhanced Remediation section contains experimental contributions investigating surfactant flushing, alcohol flushing, surfactant/alcohol combinations, dense brine barrier strategies, oil recovery through pumping, soil vapor extraction, air sparging, steam injection, bioremediation, and other techniques.
2006. "Determination of NAPL-Water Interfacial Areas in Well-Characterized Porous Media." Environmental Science and Technology 40(3):815-822. Abstract The nonaqueous-phase liquid (NAPL)-water interfacial area is an important parameter which influences the rate of NAPL dissolution in porous media. The aim of this study was to generate a set of baseline data for specific interfacial area for a two-phase entrapped NAPL-water system in well-characterised porous media, and subsequently use these data to evaluate two current theoretical models. The first model tested distributes entrapped NAPL over the pore classes based on Land’s algorithm and assumes the resulting blobs to be spherical. The other model is thermodynamically based, assuming that reversible work done on the system results in an increase in interfacial area, such that the area between drainage and imbibition retention curves can be related to the interfacial area. Interfacial tracer tests (IFTT) were used to measure specific entrapped NAPL (hexadecane)-water interfacial areas in columns packed with four grades (12/20, 20/30, 30/40, 40/50) of silica sand. Using the anionic surfactant dihexylsulfosuccinate (Aerosol® MA80), IFTT gave specific interfacial areas between 58 cm-1 for the finest sand and 16 cm-1 for the coarsest, compared to values of between 33 cm-1 and 7 cm-1 for the first model and between 19 cm-1 and 5 cm-1 for the thermodynamic model. Results from the literature suggest that non-spherical blobs shapes occur relatively frequently; hence it is reasonable to suggest that the assumption of spherical NAPL blobs may explain the underprediction by the first model. The thermodynamic model underestimates the interfacial area because it assumes that entrapment occurs only within the largest pores. A modified version of the latter model, allowing entrapment across all pore classes, yielded values between 58 cm-1 and 13 cm-1. Of the models tested the modified thermodynamic model best predicts the interfacial area.
2005. "Remediation." Chapter 5 in Soil and Groundwater Contamination: Nonaqueous Phase Liquids, Water Resources Monograph, vol. 17, ed. Alex S. Mayer and S. Majid Hassanizadeh, pp. 141-215. American Geophyscial Union, Washington, DC. Abstract The three most frequently used remediation technolgies are discussed: 1. NAPL removal, 2. Pump-and-Treat, 3. Soil Vapor Extraction.
2005. "Site Characterization and Monitoring." Chapter 4 in Soil and Groundwater Contamination: Nonaqueous Phase Liquids, Water Resources Monograph, vol. 17, ed. Alex S. Mayer and S. Majid Hassanizadeh, pp. 97- 139. American Geophysical Union, Washington, DC. Abstract Common observations associated with monitoring and assessment of NAPL-contaminated sites are discussed. The focus is on observations in monitoring wells and soil samples and observations of NAPLs dissolved in groundwater.
2005. "Removal of Carbon Tetrachloride from a Layered Porous Medium by Means of Soil Vapor Extraction Enhanced by Desiccation and Water Table Reduction." Vadose Zone Journal 4(4):1170-1182. doi:10.2136/vzj2004.0173 Abstract A two-dimensional flow cell experiment was conducted to study the removal of the carbon tetrachloride component of a DNAPL mixture from a layered porous medium through soil vapor extraction (SVE) with moist and dry air. A dual-energy gamma radiation system was used at various times to non-intrusively determine fluid saturations. The mixture, which contained the volatile organic carbon tetrachloride, mimics the DNAPL disposed at the Hanford Site in Washington State. The flow cell, which is 100 cm long, 75 cm high and 5.5 cm wide, was packed with two sloped coarse sand and two sloped silt layers in an otherwise uniform matrix of medium-grained sand. A V-shaped fine sand layer was placed at the bottom of the flow cell to prevent DNAPL from exiting the flow cell. The water table was located 2 cm from the bottom, creating variably saturated conditions. A 500-mL spill was introduced at the top of the flow cell from a small source area. It was observed that the DNAPL largely by-passed the silt layers but easily moved into the coarse sand layers. Residual DNAPL was formed in the medium-grained sand matrix. The DNAPL caused a distinct reduction of the capillary fringe. Most of the DNAPL ended up in a pool on top of the V-shaped fine sand. Through four treatments with moist air soil vapor extraction, most residual carbon tetrachloride was removed from the medium-grained matrix and the coarse sand layers. However, soil vapor extraction with moist air was not able to remove the carbon tetrachloride from the silt layers and the pool. Through a water table reduction and subsequent soil vapor extraction with dry air, the carbon tetrachloride in the silt layers and the pool was effectively removed. Based on gamma measurements and carbon tetrachloride vapor concentration data, it was estimated that after the final remediation treatment, almost 90% of the total mass was removed.
2005. "Removal of carbon tetrachloride from a layered porous medium by means of soil vapor extraction enhanced by desiccation and water table reduction." Vadose Zone Journal 4:1170-1182. doi:10.2136/vzj2004.0173 Abstract A two-dimensional flow cell experiment was conducted to study the removal of the carbon tetrachloride component of a DNAPL mixture from a layered porous medium through soil vapor extraction (SVE) with moist and dry air. A dual-energy gamma radiation system was used at various times to non-intrusively determine fluid saturations. The mixture, which contained the volatile organic carbon tetrachloride, mimics the DNAPL disposed at the Hanford Site in Washington State. The flow cell, which is 100 cm long, 75 cm high and 5.5 cm wide, was packed with two sloped coarse sand and two sloped silt layers in an otherwise uniform matrix of medium-grained sand. A V-shaped fine sand layer was placed at the bottom of the flow cell to prevent DNAPL from exiting the flow cell. The water table was located 2 cm from the bottom, creating variably saturated conditions. A 500-mL spill was introduced at the top of the flow cell from a small source area. It was observed that the DNAPL largely by-passed the silt layers but easily moved into the coarse sand layers. Residual DNAPL was formed in the medium-grained sand matrix. The DNAPL caused a distinct reduction of the capillary fringe. Most of the DNAPL ended up in a pool on top of the V-shaped fine sand. Through four treatments with moist air soil vapor extraction, most residual carbon tetrachloride was removed from the medium-grained matrix and the coarse sand layers. However, soil vapor extraction with moist air was not able to remove the carbon tetrachloride from the silt layers and the pool. Through a water table reduction and subsequent soil vapor extraction with dry air, the carbon tetrachloride in the silt layers and the pool was effectively removed. Based on gamma measurements and carbon tetrachloride vapor concentration data, it was estimated that after the final remediation treatment, almost 90% of the total mass was removed. Key Words: DNAPL; soil vapor extraction; desiccation; remediation
2005. "A comparison of models describing residual NAPL formation in the vadose zone." Vadose Zone Journal 4(1):163-174. Abstract A major shortcoming of multifluid flow simulators is the inability to predict the retention of nonaqueous phase liquid (NAPL) in the vadose zone after long drainage periods. Recently, three theoretical models, Wipfler and Van Der Zee [J. Contam. Hydrol. 50 (2001); WVDZ model], Van Geel and Roy [J. Contam. Hydrol. 58 (2002); VGR model], and Lenhard et al. [J. Contam. Hydrol. (2004) In Press; LOD model] have been proposed for describing residual NAPL formation. The WVDZ model assumes a critical total liquid saturation below which all NAPL becomes residual. The VGR and LOD models are extensions of an existing hysteretic relative permeability – saturation – capillary pressure model and assume formation of residual NAPL during NAPL drainage and imbibition, respectively. In this paper, we compare model predictions against results of a series of static pressure cell experiments. We found no experimental evidence supporting the WVDZ concept of a critical total liquid saturation. The other two models yielded reasonable predictions. The VGR and LOD models were then incorporated into a multifluid flow simulator and simulations of two transient column experiments were conducted. Both models performed considerably better than simulations without considering the formation of residual NAPL, underwriting the importance of incorporating this process in simulators. Although the VGR and LOD models are based on different conceptual models, no clear performance differences could be observed when simulation results were compared against the transient experimental data.
2004. "Immiscible Fluids." In Encyclopedia of Soils in the Environment, ed. D. Hillel, pp. 239-247. Elsevier Ltd., London, United Kingdom. Abstract This chapter discusses the fundamentals of immiscible fluid flow in subsurface porous media.
2004. "A Practical Model for Mobile, Residual, and Entrapped NAPL in Water-Wet Porous Media." Ground Water 42(5):734-746. Abstract Flow of nonvolatile nonaqueous phase liquid (NAPL) and aqueous phases that account for mobile, entrapped, and residual NAPL in variably saturated water-wet porous media is modeled and compared against results from detailed laboratory experiments. Residual saturation formation in the vadose zone is a process that is often ignored in multifluid flow simulators, which might cause an overestimation of the volume of NAPL that reaches the ground water. Mobile NAPL is defined as being continuous in the pore space and flows under a pressure gradient or gravitational body force. Entrapped NAPL is defined as being occluded by the aqueous phase, occurring as immobile ganglia surrounded by aqueous phase in the pore space and formed when NAPL is replaced by the aqueous phase. Residual NAPL is defined as immobile, nonwater entrapped NAPL that does not drain from the pore spaces and is conceptualized as being either continuous or discontinuous. Free NAPL comprises mobile and residual NAPL. The numerical model is formulated on mass conservation equations for oil and water, transported via NAPL and aqueous phases through variably saturated porous media. To account for phase transitions, a primary variable switching scheme is implemented for the oil-mass conservation equation over three phase conditions: (1) aqueous or aqueous-gas with dissolved oil, (2) aqueous or aqueous-gas with entrapped NAPL, and (3) aqueous or aqueous gas with free NAPL. Two laboratory-scale column experiments are modeled to verify the numerical model. Comparisons between the numerical simulations and experiments demonstrate the necessity to include the residual NAPL formation process in multifluid flow simulators.
2004. "A constitutive model for air-NAPL-water flow in the vadose zone accounting for immobile, non-occluded (residual) NAPL in strongly water-wet porous media ." Journal of Contaminant Hydrology 71(1-4):261-282. Abstract A major shortcoming of multifluid flow simulators is the inability to predict the retention of nonaqueous phase liquid (NAPL)in the vadose zone after long drainage periods. Recently, three theoretical models, Wipfler and Van Der Zee [J. Contam. Hydrol. 50 (2001); WVDZ model], Van Geel and Roy [J. Contam. Hydrol. 58 (2002); VGR model], and Lenhard et al. [J. Contam. Hydrol. (2004) In Press; LOD model] have been proposed for describing residual NAPL formation. The WVDZ model assumes a critical total liquid saturation below which all NAPL becomes residual. The VGR and LOD models are extensions of an existing hysteretic relative permeability – saturation – capillary pressure model and assume formation of residual NAPL during NAPL drainage and imbibition, respectively. In this paper, we compare model predictions against results of a series of static pressure cell experiments. We found no experimental evidence supporting the WVDZ concept of a critical total liquid saturation. The other two models yielded reasonable predictions. The VGR and LOD models were then incorporated into a multifluid flow simulator and simulations of two transient column experiments were conducted. Both models performed considerably better than simulations without considering the formation of residual NAPL, underwriting the importance of incorporating this process in simulators. Although the VGR and LOD models are based on different conceptual models, no clear performance differences could be observed when simulation results were compared against the transient experimental data.
2004. "A constitutive model for air-NAPL-water flow in the vadose zone accounting for immobile, non-occluded (residual) NAPL in strongly water-wet porous media." Journal of Contaminant Hydrology 73(1-4):283-304. Abstract A hysteretic constitutive model describing relations among relative permeabilities, saturations, and pressures in fluid systems consisting of air, nonaqueous-phase liquid (NAPL), and water is modified to account for NAPL that is postulated to be immobile in small pores and pore wedges and as films or lenses on water surfaces. A direct outcome of the model is prediction of the NAPL saturation that remains in the vadose zone after long drainage periods (residual NAPL). Using the modified model, water and NAPL (free, entrapped by water, and residual) saturations can be predicted from the capillary pressures and the water and total-liquid saturation-path histories. Relations between relative permeabilities and saturations are modified to account for the residual NAPL by adjusting the limits of integration in the integral expression used for predicting the NAPL relative permeability. When all of the NAPL is either residual or entrapped (i.e., no free NAPL), then the NAPL relative permeability will be zero. We model residual NAPL using concepts similar to those used to model residual water. As an initial test of the constitutive model, we compare predictions to published measurements of residual NAPL. Furthermore, we present results using the modified constitutive theory for a scenario involving NAPL imbibition and drainage.
2004. "A constitutive model for air-NAPL-water flow in the vadose zone accounting for immobile, non-occluded (residual) NAPL in strongly water-wet porous media." Journal of Contaminant Hydrology 71(1-4):261-282. Abstract A hysteretic constitutive model describing relations among relative permeabilities, saturations, and pressures in fluid systems consisting of air, nonaqueous-phase liquid (NAPL), and water is modified to account for NAPL that is postulated to be immobile in small pores and pore wedges and as films or lenses on water surfaces. A direct outcome of the model is prediction of the NAPL saturation that remains in the vadose zone after long drainage periods (residual NAPL). Using the modified model, water and NAPL (free, trapped by water, and residual) saturations can be predicted from the capillary pressures and the water and total-liquid saturation-path histories. Relations between relative permeabilities and saturations are modified to account for the residual NAPL by adjusting the limits of integration in the integral expression used for predicting the NAPL relative permeability. When all of the NAPL is either residual or trapped (i.e., no free NAPL), then the NAPL relative permeability will be zero. We model residual NAPL using concepts similar to those used to model residual water. As an initial test of the constitutive model, we compare predictions to published measurements of residual NAPL. Furthermore, we present results using the modified constitutive theory for a scenario involving NAPL imbibition and drainage.
2003. "Numerical Modeling to Assess DNAPL Movement and Removal at the Scenic Site Operable Unit Near Baton Rouge, Louisiana: A Case Study." Soil & Sediment Contamination 12(6):901 - 926. Abstract Detailed three-dimensional multifluid flow modeling was conducted to assess movement and removal of dense nonaqueous phase liquid (DNAPL) movement at a waste site in Louisiana. The site’s subsurface consists of several permeable zones separated by (semi) confining clays. In the upper subsurface, the two major permeable zones are, starting with the uppermost zone, the +40- and +20-MSL (mean sea level) zones. At the site, a total of 23,000 m3 of DNAPL was emplaced in an open waste pit between 1962 and 1974. In this period, considerable amounts of DNAPL moved into the subsurface. By 1974 a portion of the DNAPL was removed and the waste site was filled with low-permeability materials and closed. During this process, some of the DNAPL was mixed with the fill material and remained at the site. Between 1974 and 2000, no additional DNAPL recovery activities were implemented. In an effort to reduce the DNAPL source, organic liquid has been pumped through a timed-pumping scheme from a total of 7 wells starting in calendar year 2000. The recovery wells are screened in the lower part of the waste fill material. In site investigations, DNAPL has been encountered in the +40-MSL but not in the +20-MSL zone. The following questions are addressed: (1) Where has the DNAPL migrated vertically and laterally? (2) How much further is DNAPL expected to move in the next century? (3) How effective is the current DNAPL pumping in reducing the DNAPL source? The computational domains for the simulations were derived from 3-D interpolations of borehole logs using a geologic interpretation software (EarthvisionTM ) . The simulation results show that DNAPL primarily entered the subsurface in the period 1962 – 1974, when the waste site was operational. After 1974, the infiltration rates dropped dramatically as a result of the infilling of the waste pit. The simulation results indicate that DNAPL moved from the pit into the underlying +40-MSL zone through two contact zones at the west side of the pit. Lateral movement of the DNAPL body has been relatively slow as a result of the high viscosity and the rapidly decreasing driving force after the waste pit was filled in. For all simulations, lateral movement of DNAPL in the period 1962 - 2001 is predicted to be less than 60 m from the two contact areas, while additional movement in the next century is expected to be less than 30 m. No DNAPL is predicted to enter the +20-MSL zone, which agrees with site information. The simulations also clearly demonstrate the minimal effect of the current pumping scheme on source reduction and DNAPL movement.
2003. "Flow Behavior and Residual Saturation Formation of Liquid Carbon Tetrachloride in Unsaturated Heterogeneous Porous Media." Journal of Contaminant Hydrology 64(1-2):93-112. Abstract A side effect of in situ groundwater remediation techniques that operate by establishing reducing conditions within an aquifer is that anoxic water exits these zones, posing a potential risk to aquatic organisms inhabiting areas of groundwater discharge downgradient from the site. A number of processes have been identified that can attenuate an anoxic plume in an unconfined aquifer with a fluctuating water table. The hypothesis that water table fluctuations increase oxygen transfer from air to water, through enhanced exchange from entrapped air, is tested in an intermediate-scale, fluctuating water table experiment. A dual-energy gamma radiation system was used to measure water saturations while dissolved oxygen (DO) concentrations were measured with flow-through oxygen microelectrodes. A hysteretic multifluid simulator was used to test whether the experimentally obtained water and entrapped air saturations, as well as DO concentrations, could be predicted using the assumptions of two-phase flow and equilibrium partitioning between the gas and the aqueous phases. The experimental results show that zones with entrapped air, formed during the imbibition portions of the experiment, were instrumental in re-oxygenation of the water in the zone of fluctuation. The fluctuating water table system also caused significant amounts of dissolved oxygen to be transported deeper into the flow cell. The simulator was able to predict water saturations, entrapped air saturations, and dissolved oxygen concentrations reasonably well.
2003. "Carbon Tetrachloride Flow Behavior in Unsaturated Hanford Caliche Material: An Investigation of Residual Nonaqueous Phase Liquids." Vadose Zone Journal 2(1):25-33. Abstract To obtain data that can be used to study the development of a residual NAPL saturation and to test corresponding models, a detailed transient experiment was conducted in a 170-cm long by 90-cm high by 5.5-cm wide flow cell. Fluid saturation measurements were obtained with a dual-energy gamma radiation system. The experimental conditions reflected those at the Hanford Site in Washington State, where an estimated 363-580 m3 of carbon tetrachloride was disposed to the subsurface. A key subsurface feature at the Hanford Site is a sloped Plio-Pleistocene caliche layer, which was reproduced in the experiment as a sloped lens in a medium-grained, uniform, sand matrix. The caliche contains considerable amounts of calcium carbonate and may have fluid wettability properties other than strongly water wet. A total of 800 ml of carbon tetrachloride was injected in the experimental domain at a rate of 0.5 ml min-1 from a small source area located at the surface. After apparent steady-state conditions were obtained with respect to carbon tetrachloride redistribution (i.e., the formation of residual DNAPL), saturation measurements indicate that all of the DNAPL that initially moved into the caliche, remained in this layer. This experimental result could not be reproduced with numerical multifluid flow simulations based on conventional constitutive relations between relative permeability, saturation, and fluid pressures. Water was subsequently applied to the surface at a constant rate over the full length of the caliche layer to study carbon tetrachloride displacement as a result of changing water saturations. Results show that as a result of this action, 29% of the DNAPL was removed from the caliche. However, the majority of the fluid remained in the caliche entrapped by water. Simulations with the multifluid flow simulator show that the current constitutive theory for relative permeability, saturation and capillary pressure does not describe displacement physics properly.
2003. "A Mixed-Wet Hysteretic Relative Permeability and Capillary Pressure Model for Reservoir Simulations." SPE Reservoir Evaluation and Engineering 6:328 - 334. Abstract Saturation-history-dependent relative permeabilty and capillary pressure relations for two-phase flow in mixed-wet rocks are derived and discussed.
2002. "Fluid Contents." Chapter 7.2 in Methods of Soil Analysis. Part 4. Physical Methods. Soil Science Society of America, Madison, WI. Abstract This section provides an overview of methods to determine multifluid contents in porous media.
2002. "Fluid Contents." Chapter 7.2 in Methods of Soil Analysis: Part 4 Physical Methods, vol. 5, ed. J.H. Dane and G.C. Topp, pp. 1539 - 1564. Soil Science Society of America , Madison, WI. Abstract Fluid contents of nonaqueous phase liquids in porous media are determined using nondestructive and destructive methods. The most widely used nondestructive methods are gamma and x-ray radiation techniques. Other nondestructive methods include light reflection, electric resistance, and NMR techniques. Destructive methods include extraction with organic solvents or hydrophobic porous materials. The nonintrusive, noninvasive gamma radiation technique has been traditionally used in subsurface flow and transport applications for the determination of water content and dry bulk density values. More recently, gamma radiation methods have been developed for laboratory environmental research in soil science and hydrology. Contributions by Dane et al. (1992), Host-Madsen and Jensen (1992), Hofstee et al. (1997), Lenhard et al. (1993), Illangasekare et al. (1995), Imhoff et al. (1994), Oostrom and Lenhard (1998), and Oostrom et al. (1999a,b) are examples in which the technique was used to determine volumetric liquid content values of immiscible nonaqueous phase liquids (NAPLs) in multifluid flow applications. For detailed information about this technique the reader is referred to Stillwater and Klute (1988) and Oostrom and Dane (1990).
2002. "Saturation-Pressure Relationships." Chapter 7.3 in Methods of Soil Analysis. Part 4. Physical Methods. Soil Science Society of America, Madison, WI. Abstract This section describes an overview of measurements to obtain saturation-pressure relationships for multifluid systems.
2002. "Prediction of capillary pressure-relative permeability relations." Chapter 7.5 in Methods of Soil Analysis. Part 4. Physical Methods, pp. 1591-1607. Soil Science Society of America, Madison, WI. Abstract This section provides an overview of capillary pressure-permeability relations for multifluid flow and transport in porous media.
2002. "Relative Permeability Measurements." Chapter 7.4 in Methods of Soil Analysis. Part 4. Physical Methods, pp. 1581-1590. Soil Science Society of America, Madison, WI. Abstract This section provides and overview of relative permeability measurements for multifluid flow in porous media.
2002. "Dissolution of nonuniformly distributed immiscible liquid: Intermediate-scale experiments and mathematical modeling." Environmental Science and Technology 36(5):1033-1041. Abstract There is no abstract currently available for this item
2001. "In-situ oxidation of trichloroethene by permanganate: effects on porous medium hydraulic properties." Journal of Contaminant Hydrology 50(1-2):79-98.
2001. "Theoretical Estimation of Free and Entrapped Nonwetting-Wetting Fluid Interfacial Areas in Porous Media ." Advances in Water Resources 24(8):887-898. Abstract Fluid-fluid interfacial areas play important roles in numerous subsurface processes such a dissolution, volatilization, and adsorption. Integral expressions have been derived to estimate both entrapped (discontinuous) and free (continuous) nonwetting fluid-wetting fluid specific interfacial areas in porous media. The expressions, compatible with widely used capillary head-saturation and entrapment models, require information on capillary head-saturation relation parameters, porosity, and fluid-pair interfacial tension. In addition, information on the maximum entrapped nonwetting fluid saturation as well as the main drainage branch reversal point for water and total liquid saturations is necessary to estimate entrapped fluid interfacial areas. Implementation of the interfacial area equations in continuum-based multifluid flow simulators is straightforward since no additional parameters are needed than those required by the simulators to complete the multifluid flow computations. A limited sensitivity analysis, based on experimentally obtained parameter values, showed that imposed variations resulted in logical and consistent changes in predicted specific interfacial areas for both entrapped and free nonwetting fluid-wetting fluid systems. A direct comparison with published experimental work to test the derived expressions was limited to free air-water systems and yielded reasonable results. Such comparisons are often not possible because of the lack of information given on retention parameters, and variables used to determine nonwetting fluid entrapment. This contribution is dedicated to John W. Cary.
2001. "Numerical simulation and homogenization of two-phase flow in heterogeneous porous media." Journal of Contaminant Hydrology 49:87-109. Abstract There is no abstract currently available for this item.
2001. "Effective parameters for two-phase flow in a porous medium with periodic heterogeneities." Journal of Contaminant Hydrology 49(1-2):87-109. Abstract The study of non-aqueous phase liquid (NAPL) transport in groundwater requires a correct description of multiphase flow in porous media. For the simulation of multiphase flow a number of material-dependent parameters have to be known. These include relationships between capillary pressure, relative permeability, and saturation. One of the major difficulties in characterising a porous medium is the presence of small-scale heterogeneities, which have distinctly different multiphase flow properties than the main medium. Such heterogeneities can considerably affect the spreading behaviour of non-aqueous liquids. They are often sources of localised pools of pollutants. For most practical purposes, the details of fluid distribution in such a medium are not of interest. It is also computationally not feasible to discretise a compositional multiphase model at such small scales. Even if a detailed numerical model is constructed, it is virtually impossible to obtain data for these heterogeneities. Thus, instead of modelling the subsurface at the scale of micro-heterogeneities, it is desirable to model it at a higher scale, as a homogenised medium, with effective properties.
2000. "Oxygenation of anoxic water in a fluctuating water table system: An experimental and numerical study." Journal of Hydrology 230((1-2)):70-85. Abstract A side effect of in situ groundwater remediation techniques that operate by establishing reducing conditions within an aquifer is that anoxic water exits these zones, posing a potential risk to aquatic organisms inhabiting areas of groundwater discharge downgradient from the site. A number of processes have been identified that can attenuate an anoxic plume in an unconfined aquifer with a fluctuating water table. The hypothesis that water table fluctuations increase oxygen transfer from air to water, through enhanced exchange from entrapped air, is tested in an intermediate-scale, fluctuating water table experiment. A dual-energy gamma radiation system was used to measure water saturations while dissolved oxygen (DO) concentrations were measured with flow-through oxygen microelectrodes. A hysteretic multifluid simulator was used to test whether the experimentally obtained water and entrapped air saturations, as well as DO concentrations, could be predicted using the assumptions of two-phase flow and equilibrium partitioning between the gas and the aqueous phases. The experimental results show that zones with entrapped air, formed during the imbibition portions of the experiment, were instrumental in re-oxygenation of the water in the zone of fluctuation. The fluctuating water table system also caused significant amounts of dissolved oxygen to be transported deeper into the flow cell. The simulator was able to predict water saturations, entrapped air saturations, and dissolved oxygen concentrations reasonably well.
2000. "Influence of Heterogeneity and Sampling Method on Aqueous Concentrations Associated with NAPL Dissolution." Environmental Science and Technology 34(17):3657-3664. Abstract Influence of Heterogeneity and Sampling Method on Aqueous Concentrations Associated with NAPL Dissolution
1999. "Movement and remediation of trichloroethylene in a saturated, heterogeneous porous medium 2. Pump-and-treat and surfactant flushing." Journal of Contaminant Hydrology 37(1-2):179-197. Abstract There is no abstract currently available for this item.
1999. "Movement and remediation of trichloroethylene in a saturated heterogeneous porous Medium. 1. Spill behavior and initial dissolution." Journal of Contaminant Hydrology 37(1-2):159-178. Abstract There is no abstract currently available for this item.
1999. "Partitioning tracer method for the in situ measurement of DNAPL saturation: influence of heterogeneity and sampling method." Environmental Science and Technology 33(22):4046-4053. Abstract An experimental investigation of the partitioning tracer method to detect and quantify dense nonaqueous phase liquids in a heterogeneous system.
1998. "Modeling surfactant-enhanced nonaqueous-phase liquid remediation of porous media." Soil Science 163(12):931-940. Abstract There is no abstract currently available for this item.
1998. "Multifluid flow in bedded porous media: laboratory experiments and numerical simulations." Advances in Water Resources 22(2):169-183. Abstract There is no abstract currently available for this item
1998. "Single-source gamma radiation for improved calibration and measurements in porous media systems." Soil Science 163:646-656. Abstract There is no abstract currently available for this item.
1998. "Comparison of relative permeability-saturation-pressure parametric models for infiltration and redistribution of a light nonaqueous-phase liquid in sandy porous media." Advances in Water Resources 21:145-157. Abstract To test and evaluate the ability of commonly used constitutive realtions to predict multi-fluid flow, predictions for a numerical flow and transport simulator are compared to experimental data. Three quantitative experiments were conducted in one meter-long vertical columns. The columns were filled with either a uniform sand, a sand with a broad particle-size distribution, or with a layered system where a layer of a coarse-textured uniform sand was placed between two layers of a finer-textured uniform sand. After establishing a variably water-saturated condition, a slug of a light nonaqueous-phase liquid (LNAPL) was injected uniformly at a constant rate. Water and LNAPL saturations were measured as a function of time and elevation with a dual energy gamma-radiation system. The infiltration and redistribution of the LNAPL were simulated with nonhysteretic and hysteretic parametric relative permeability saturation-pressure (k-S-P ) models. The models were calibrated using two-phase air water retention data and an established scaling theory. The nonhysteretic Brooks Corey k-S-P model, which utilizes the Burdine relative permeability model, yielded predictions that closely matched the experimental data. Use of the nonhysteretic and hysteretic k-S-P models, based on the van Genuchten S-P relations and k-S relations derived from the Mualem relative permeability model, did not agree as well with the experimental data as those obtained with the Brooks-Corey k-S-P model. Explanations for the differences in performance of the three tested parametric k-S-P models are proposed.
1998. "AA Parametric Model for Predicting Relative Permeability-Saturation-Capillary Pressure Relationships of Oil?Water Systems in Porous Media with Mixed Wettability." Transport in Porous Media 31((1)):109-131. Abstract A parametric two-phase, oil-water relative permeability/capillary pressure model for petroleum engineering and environmental applications is developed for porous media in which the smaller pores are strongly water-wet and the larger pores tend to be intermediate- or oil-wet. A saturation index, which can vary from 0 to 1, is used to distinguish those pores that are strongly water-wet from those that have intermediate- or oil-wet characteristics. The capillary pressure submodel is capable of describing main-drainage and hysteretic saturation-path saturations for positive and negative oil-water capillary pressures. At high oil-water capillary pressures, an asymptote is approached as the water saturation approaches the residual water saturation. At low oil-water capillary pressures (i.e., negative), another asymptote is approached as the oil saturation approaches the residual oil saturation. Hysteresis in capillary pressure relations, including water entrapment, is modeled. Relative permeabilities are predicted using parameters that describe main drainage capillary pressure relations and accounting for how water and oil are distributed throughout the pore spaces of a porous medium with mixed wettability. The capillary pressure submodel is tested against published experimental data, and an example of how to use the relative permeability/capillary pressure model for a hypothetical saturation-path scenario involving several imbibition and drainage paths is given. Features of the model are also explained. Results suggest that the proposed model is capable of predicting relative permeability/capillary pressure characteristics of porous media mixed wettability.
1998. "Infiltration and redistribution of perchloroethylene in partially saturated, stratified porous media." Journal of Contaminant Hydrology 34(4):293-313. Abstract There is no abstract currently available for this item