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Charles J. Werth - One of the best experts on this subject based on the ideXlab platform.
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surface and interfacial properties of Nonaqueous Phase Liquid mixtures released to the subsurface at the hanford site
Vadose Zone Journal, 2009Co-Authors: Scott R Nellis, Hongkyu Yoon, Charles J. Werth, Mart Oostrom, Albert J. ValocchiAbstract: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 mixturesmore » 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.« less
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evaluation of simplified mass transfer models to simulate the impacts of source zone architecture on Nonaqueous Phase Liquid dissolution in heterogeneous porous media
Journal of Contaminant Hydrology, 2008Co-Authors: Changyong Zhang, Nandita B. Basu, Albert J. Valocchi, Charles J. Werth, Hongkyu Yoon, James W. JawitzAbstract:Abstract Nonaqueous Phase Liquid (NAPL) dissolution was studied in three-dimensional (3D) heterogeneous experimental aquifers (25.5 cm × 9 cm × 8.5 cm) with two different longitudinal correlation lengths (2.1 cm and 1.1 cm) and initial spill volumes (22.5 ml and 10.5 ml). Spatial and temporal distributions of NAPL during dissolution were measured using magnetic resonance imaging (MRI). At high NAPL spill volume, average effluent concentrations initially increased during dissolution, as NAPL pools transitioned to NAPL ganglia, and then decreased as the total NAPL–water interfacial area decreased over time. Experimental results were used to test six dissolution models: (i and ii) a one-dimensional (1D) model using either specific NAPL–water interfacial area values estimated from MR images at each time step (i.e., 1D quasi-steady state model), or an empirical mass transfer ( Sh ′) correlation (i.e., 1D transient model), (iii and iv) a multiple analytical source superposition technique (MASST) using either the NAPL distribution determined from MR images at each time step (i.e., MASST steady state model), or the NAPL distribution determined from mass balance calculations (i.e., MASST transient model), (v) an equilibrium streamtube model, and (vi) a 3D grid-scale pool dissolution model (PDM) with a dispersive mass flux term. The 1D quasi-steady state model and 3D PDM captured effluent concentration values most closely, including some concentration fluctuations due to changes in the extent of flow reduction. The 1D transient, MASST steady state and transient, and streamtube models all showed a monotonic decrease in effluent concentration values over time, and the streamtube model was the most computationally efficient. Changes during dissolution of the effective NAPL–water interfacial area estimated from imaging data are similar to changes in effluent concentration values. The 1D steady state model incorporates estimates of the effective NAPL–water interfacial area directly at each time point; the 3D PDM does so indirectly through mass balance and a relative permeability function, which causes reduced water flow through high saturation NAPL regions. Hence, when model accuracy is required, the results indicate that a surrogate of this effective interfacial area is required. Approaches to include this surrogate in the MASST and streamtube models are recommended.
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impact of spatially distributed Nonaqueous Phase Liquid saturation and water content on soil vapor extraction in heterogeneous porous media
Developments in water science, 2004Co-Authors: Hongkyu Yoon, Albert J. Valocchi, Charles J. WerthAbstract:The spatial distribution of Nonaqueous Phase Liquid (NAPL) saturation, water content, and soil permeability determines the pore-scale processes that control soil vapor extraction (SVE) and the time scales for cleanup. It is now understood that there are three forms of NAPL Phase in the vadose zone: free NAPL that is mobile and in contact with gas, residual NAPL that is immobile and in contact with gas, and trapped NAPL that is immobile and surrounded by water. We use a new k-S-P constitutive model that considers all three NAPL forms. The multiPhase flow simulator (STOMP), which includes this new constitutive model, is used to distribute NAPL in heterogeneous porous media. For soil vapor extraction, we describe a conceptual model that distinguishes rate-limited mass transfer of trapped NAPL from equilibrium partitioning of free NAPL. The current STOMP, which does not include this new conceptual model for SVE, is first used to distribute NAPL in a stratified system, and then to simulate NAPL removal during soil vapor extraction (SVE). Simulation results show that the amount of trapped NAPL depends on water saturation and soil permeability, and the formation of residual NAPL has an influence on the amount of NAPL retained in the unsaturated zone due to the reduced NAPL relative permeability. SVE results show that it is necessary to consider the change of water saturation and NAPL forms over time in order to simulate mass transfer mechanisms from various forms of NAPL.
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impact of spatially distributed Nonaqueous Phase Liquid saturation and water content on soil vapor extraction in heterogeneous porous media
Developments in water science, 2004Co-Authors: Hongkyu Yoon, Albert J. Valocchi, Charles J. WerthAbstract:The spatial distribution of Nonaqueous Phase Liquid (NAPL) saturation, water content, and soil permeability determines the pore-scale processes that control soil vapor extraction (SVE) and the time scales for cleanup. It is now understood that there are three forms of NAPL Phase in the vadose zone: free NAPL that is mobile and in contact with gas, residual NAPL that is immobile and in contact with gas, and trapped NAPL that is immobile and surrounded by water. We use a new k-S-P constitutive model that considers all three NAPL forms. The multiPhase flow simulator (STOMP), which includes this new constitutive model, is used to distribute NAPL in heterogeneous porous media. For soil vapor extraction, we describe a conceptual model that distinguishes rate-limited mass transfer of trapped NAPL from equilibrium partitioning of free NAPL. The current STOMP, which does not include this new conceptual model for SVE, is first used to distribute NAPL in a stratified system, and then to simulate NAPL removal during soil vapor extraction (SVE). Simulation results show that the amount of trapped NAPL depends on water saturation and soil permeability, and the formation of residual NAPL has an influence on the amount of NAPL retained in the unsaturated zone due to the reduced NAPL relative permeability. SVE results show that it is necessary to consider the change of water saturation and NAPL forms over time in order to simulate mass transfer mechanisms from various forms of NAPL.
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analysis of pore scale Nonaqueous Phase Liquid dissolution in etched silicon pore networks
Water Resources Research, 2003Co-Authors: Cheema Chomsurin, Charles J. WerthAbstract:[1] Predicting the dissolution rate of Nonaqueous Phase Liquids (NAPLs) in groundwater is difficult, as the effects of variable pore and NAPL blob geometry are poorly understood. To elucidate these effects, fluorescence microscopy and digital image analysis were used to quantify the size and location of variably distributed NAPL blobs during dissolution in homogeneous and heterogeneous pore networks etched into silicon wafers. Results show that the dissolution rate constant (expressed as the Sherwood number, Sh) is relatively constant regardless of pore and NAPL blob geometry when the average mass transfer length scale remains constant during dissolution. Results also show that Sh increases with Peclet (Pe) between 2 and 26 and then levels off. The limiting value of Sh reached depends on the average diffusion length scale; this length scale was directly calculated and found to vary depending on the pore and NAPL blob geometry. For example, the average diffusion length scale decreases (and Sh increases) as the pore throat width to grain diameter increases. Last, results show that the volumetric NAPL content (θn) is linearly related to the specific NAPL-water interfacial area (ait) over much of the dissolution process. However, this relationship depends on the pore and blob size distribution. For example, when multipore blobs control dissolution, the relationship between these parameters will change as smaller blobs dominate dissolution at low θn. These results are important because existing mass transfer correlations do not account for limiting values of Sh that can be obtained at high Pe for the effect of blob or pore geometry on the average diffusion length scale (and therefore on Sh) or for the effect of pore geometry and transient blob size distribution on the relationship between ait and θn.
Bruce Hobbs - One of the best experts on this subject based on the ideXlab platform.
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effects of domain shapes on the morphological evolution of Nonaqueous Phase Liquid dissolution fronts in fluid saturated porous media
Journal of Contaminant Hydrology, 2012Co-Authors: Chongbin Zhao, Bruce HobbsAbstract:Abstract The main purpose of this paper is to investigate the effects of different domain shapes in general and trapezoidal domain shape in particular on the morphological evolution of Nonaqueous Phase Liquid (NAPL) dissolution fronts in two-dimensional fluid-saturated porous media. After the governing equations of NAPL dissolution problems are briefly described, the numerical procedure consisting of a combination of the finite element and finite difference methods is used to solve these equations. The related numerical simulation results have demonstrated that: (1) domain shapes have a significant effect on both the propagating speed and the morphological evolution pattern of a NAPL dissolution front in the fluid-saturated porous medium; (2) an increase in the divergent angle of a trapezoidal domain can lead to a decrease in the propagating speed of the NAPL dissolution front; (3) the morphological evolution pattern of the NAPL dissolution front in a rectangular domain is remarkably different from that in a trapezoidal domain of a large divergent angle; (4) for a rectangular domain, the simplified dispersion model, which is commonly used in the theoretical analysis and numerical simulation, is valid for solving NAPL dissolution instability problems in fluid-saturated porous media; and (5) compared with diverging flow (when the trapezoidal domain is inclined outward), converging flow (when the trapezoidal domain is inclined inward) can enhance the growth of NAPL fingers, indicating that pump-and-treat systems by extracting contaminated groundwater might enhance NAPL dissolution fingering and lead to less uniform dissolution fronts.
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computational simulation for the morphological evolution of Nonaqueous Phase Liquid dissolution fronts in two dimensional fluid saturated porous media
Computational Geosciences, 2011Co-Authors: Chongbin Zhao, Bruce Hobbs, Klaus RegenauerliebAbstract:This paper deals with the computational aspects of Nonaqueous Phase Liquid (NAPL) dissolution front instability in two-dimensional fluid-saturated porous media of finite domains. After the governing equations of an NAPL dissolution system are briefly described, a combination of the finite element and finite difference methods is proposed to solve these equations. In the proposed numerical procedure, the finite difference method is used to discretize time, while the finite element method is used to discretize space. Two benchmark problems, for which either analytical results or previous solutions are available, are used to verify the proposed numerical procedure. The related simulation results from these two benchmark problems have demonstrated that the proposed numerical procedure is useful and applicable for simulating the morphological evolution of NAPL dissolution fronts in two-dimensional fluid-saturated porous media of finite domains. As an application, the proposed numerical procedure has been used to simulate morphological evolution processes for three kinds of NAPL dissolution fronts in supercritical NAPL dissolution systems. It has been recognized that: (1) if the Zhao number of an NAPL dissolution system is in the lower range of the supercritical Zhao numbers, the fundamental mode is predominant; (2) if the Zhao number is in the middle range of the supercritical Zhao numbers, the (normal) fingering mode is the predominant pattern of the NAPL dissolution front; and (3) if the Zhao number is in the higher range of the supercritical Zhao numbers, the fractal mode is predominant for the NAPL dissolution front.
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theoretical analyses of Nonaqueous Phase Liquid dissolution induced instability in two dimensional fluid saturated porous media
International Journal for Numerical and Analytical Methods in Geomechanics, 2010Co-Authors: Chongbin Zhao, Bruce HobbsAbstract:This paper deals with the theoretical aspects of Nonaqueous Phase Liquid (NAPL)-dissolution-induced instability in two-dimensional fluid-saturated porous media including solute dispersion effects.After some weaknesses associated with the previous work are analyzed and overcome, a comprehensive dimensionless number, known as the Zhao number, is proposed to represent the main driving force and three controlling mechanisms of an NAPL-dissolution system that has a finite domain. The linear stability analysis is carried out to derive the critical value of the comprehensive dimensionless number of the NAPL-dissolution system in a limit case as the ratio of the equilibrium concentration to the density of the NAPL approaches zero. As a result, a theoretical criterion that can be used to assess the instability of planar NAPL-dissolution fronts in two-dimensional fluid-saturated porous media of finite domains has been established. Not only can the present theoretical results be used for the theoretical understanding of the effect of solute dispersion on the instability of an NAPL-dissolution front in the fluid-saturated porous medium of either a finite domain or an infinite domain, but also they can be used as benchmark solutions for verifying numerical methods employed to simulate detailed morphological evolution processes of NAPL-dissolution fronts in two-dimensional fluid-saturated porous media. Copyright © 2009 John Wiley & Sons, Ltd.
Linda M Abriola - One of the best experts on this subject based on the ideXlab platform.
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estimating mass discharge from dense Nonaqueous Phase Liquid source zones using upscaled mass transfer coefficients an evaluation using multiPhase numerical simulations
Water Resources Research, 2006Co-Authors: John A Christ, Andrew C Ramsburg, Kurt D Pennell, Linda M AbriolaAbstract:[1] Difficulties associated with identifying the dense Nonaqueous Phase Liquid (DNAPL) source zone architecture at the field scale, combined with the computational costs of field-scale DNAPL dissolution simulations, have motivated the development of a number of simplified models that rely upon upscaled (i.e., domain-averaged) mass transfer coefficients to approximate field-scale dissolution processes. While conceptually attractive, these upscaled models have yet to be fully evaluated for prediction of mass recovery from a range of nonuniform, three-dimensional DNAPL source zones. This study compares upscaled model predictions of flux-weighted downstream concentrations and source longevity to predictions derived from three-dimensional multiPhase numerical simulation of tetrachloroethene (PCE)-NAPL dissolution for realizations of a statistically homogeneous, nonuniform aquifer. Although the functional forms of the upscaled models are generally shown to be mathematically equivalent, upscaled model flux-weighted concentration predictions varied by over one order of magnitude, with variations attributed to the dependence of the upscaled model parameters on the specific source zone scenario used for model calibration. Replacement of upscaled model calibration parameters with source zone parameters that can be obtained from site characterization information (specifically, the initial flux-weighted concentration and source zone ganglia-to-pool (GTP) mass ratio) reduced the root-mean-square error between upscaled and numerical model predictions by approximately 80%. Application of this modified model to a range of source zone scenarios (0.4 < GTP < ∞) demonstrates the efficacy of the model for use as a screening tool to relate DNAPL mass removal and flux-weighted concentrations when mass removal is less than 80%.
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Modeling dense Nonaqueous Phase Liquid mass removal in nonuniform formations: Linking source-zone architecture and system response
Geosphere, 2006Co-Authors: Lawrence D. Lemke, Linda M AbriolaAbstract:Dense Nonaqueous Phase Liquid (DNAPL) source zones comprise persistent sources of groundwater contamination that are recalcitrant to complete remediation using conventional (e.g., pump and treat) or emerging (e.g., surfactant flushing) technologies. Increased attention to the assessment of the benefits of partial mass removal from such contaminant source zones has intensified efforts to model multiPhase flow and transport behavior. This paper describes the simulated recovery of a tetrachloroethene (PCE) spill in a statistically homogeneous but nonuniform aquifer, incorporating nonuniformity in both Nonaqueous Phase Liquid saturation and pore velocities. We developed a ganglia-to-pool metric to quantify DNAPL source-zone architecture, and explored the correlation of this metric with dissolved mass flux behavior in response to partial DNAPL mass removal. Dissolution of 20%‐70% of PCE mass from models exhibiting low ganglia-to-pool ratios resulted in a larger predicted reduction of dissolved contaminant mass flux than models with high ganglia-to-pool ratios. Results of this study suggest that DNAPL source-zone characterization at field sites with homogeneous, nonuniform aquifers would benefit from inclusion of an estimate of the overall ganglia-to-pool ratio. Simulations demonstrate that flux reduction behavior depends on the source-zone architecture, which is not readily predictable using a priori assumptions about the spatial correlation of physical aquifer parameters. Model results further suggest that stochastic investigations of DNAPL source remediation at field sites should avoid reliance upon Leverett scaling of capillary entry pressures to permeability fields, which can artificially narrow the range of simulated behaviors.
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dense Nonaqueous Phase Liquid dnapl source zone characterization influence of hydraulic property correlation on predictions of dnapl infiltration and entrapment
Water Resources Research, 2005Co-Authors: Linda M Abriola, Lawrence D. Lemke, Pierre GoovaertsAbstract:[1] The influence of aquifer property correlation on multiPhase fluid migration and entrapment was explored through the use of correlated and uncorrelated porosity, permeability, and capillary pressure-saturation (Pc-Sat) parameter fields in a cross-sectional numerical multiPhase flow model. Data collected from core samples in a nonuniform sandy aquifer were used to generate three-dimensional aquifer parameter fields. Porosity was assumed to be uniform or simulated using sequential Gaussian simulation (SGS). Permeability (k) was modeled independently of porosity using SGS as well as simulated geostatistical indicator classes derived from measured grain size distribution curves. Retention characteristics were assigned employing Leverett scaling of a representative Pc-Sat curve to the geostatistical k fields or, alternatively, on the basis of simulated indicator classes and porosity values. Ensemble dense Nonaqueous Phase Liquid (DNAPL) infiltration and entrapment behavior for a hypothetical tetrachloroethylene (PCE) spill was simulated in four sets of two-dimensional profiles extracted from these realizations. Comparisons of saturation profiles and spatial moments from point source DNAPL infiltration simulations suggest that choices involving the geostatistical algorithm used to model k and the incorporation of variable versus uniform porosity have a smaller influence than choices involving the scaling of capillary retention properties to k. From these simulations it is apparent that the degree of spatial correlation in Pc-Sat parameters exerts a controlling influence on predicted DNAPL spreading and redistribution in saturated aquifers. The resultant distribution of mass within a DNAPL source zone will have implications for DNAPL recovery and subsequent mass fluxes in remediation operations.
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Volatilization of Binary Nonaqueous Phase Liquid Mixtures in Unsaturated Porous Media
Vadose Zone Journal, 2004Co-Authors: Linda M Abriola, John Lang, Scott A Bradford, Charles L. GaitherAbstract:This study examines the volatilization behavior of binary Nonaqueous Phase Liquid (NAPL) mixtures consisting of styrene, and toluene or tetrachloroethylene (PCE). Residual NAPL saturations were emplaced in unsaturated (residual water saturation) soil columns packed with Wagner 50-80 sand. Initial column effluent concentrations were measured for the NAPL mixtures at several pore gas Phase velocities. Rate-limited volatilization occurred at higher gas Phase pore velocities, and mass transfer coefficients could be reasonably predicted with a correlation developed from single component NAPL volatilization data. Long-term volatilization studies for the binary NAPL mixtures were also conducted. The effluent concentrations for both NAPL components were observed to be initially proportional to their mole fractions. After the more volatile component became depleted, a rapid drop in the effluent concentration of this component was accompanied by an increase in the mole fraction and effluent concentration of the remaining constituent to near saturated values until the free Phase NAPL was volatilized. The final stage of removal was associated with a dramatic decrease in effluent concentration, attributed to reduction in the gas-NAPL interfacial area, followed by low concentration tailing. The tailing and subsequent flow interruption behavior are likely a consequence of rate-limited desorption. Equilibrium and rate-limited simulations of the long-term volatilization experiments did not provide a satisfactory description of the data. A simulation that included a fixed concentration gradient and fitted activity coefficients provided a better characterization of the volatilization data. Various potential explanations for this “fixed gradient” volatilization behavior were considered, but additional research is needed to test these hypotheses.
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solubilization of Nonaqueous Phase Liquid hydrocarbons in micellar solutions of dodecyl alcohol ethoxylates
Environmental Science & Technology, 1994Co-Authors: Mamadou S Diallo, Linda M Abriola, Walter J. WeberAbstract:Results of an experimental investigation of hydrocarbon solubilization in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 o C are presented. The effects of surfactant hydrophile-lipophile balance (HLB) and hydrocarbon molar volume and polarity on the molar solubilization ratios (MSRs) and micelle-water partition coefficients of 11 Nonaqueous Phase Liquid (NAPL) hydrocarbons are examined. The MSRs of the alkanes (n-dodecane, n-decane, n-hexane, and cyclohexane), which sharply decrease and asymptotically approach zero with increasing HLB, are shown to depend on micellar core volume and hydrocarbon molar volume and affinity for the micellar core
Kenneth Y. Lee - One of the best experts on this subject based on the ideXlab platform.
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modeling the transport of dissolved contaminants originating from a Nonaqueous Phase Liquid source containing polycyclic aromatic hydrocarbon compounds in groundwater
Journal of Environmental Engineering and Science, 2004Co-Authors: Kenneth Y. Lee, Konstantinos Kostarelos, Donna E FennellAbstract:A three-dimensional mathematical model is developed for simulating the transport of dissolved contaminants originating from a well-defined, multicomponent Nonaqueous Phase Liquid (NAPL) source containing polycyclic aromatic hydrocarbon (PAH) compounds in groundwater. The dissolution process for each NAPL component is envisioned to occur in a successive series of small dissolution intervals (or pulses). The equilibrium aqueous Phase concentration of each component and the source dimensions are assumed to remain constant for the duration of each dissolution interval. The model accounts for possible solidification of PAH components as the NAPL source dissolves. Aqueous Phase contaminant concentrations resulting from each dissolution interval are calculated using an existing analytical solution. A synthetic PAH-rich NAPL mixture consisting of benzene and eight PAH components is used for model simulations. The model is useful for evaluation and prediction of downstream aqueous Phase contaminant concentrations ...
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UNIFAC Modeling of Cosolvent Phase Partitioning in Nonaqueous Phase Liquid-Water Systems
Journal of Environmental Engineering, 2004Co-Authors: Kenneth Y. Lee, Catherine Anne PetersAbstract:In this study, an existing thermodynamic model was used to predict equilibrium Phase partitioning behavior of a cosolvent in a two-Phase Nonaqueous Phase Liquid ~NAPL!-water system. The activity coefficients are calculated using the universal quasichemical functional group activity coefficient ~UNIFAC! method. We examined an assortment of cosolvent-NAPL pairs of environmental interest and compared the UNIFAC-predicted ternary Phase diagrams against published experimentally derived ternary Phase diagrams. Results show that the UNIFAC model is a promising method for predicting equilibrium cosolvent partitioning behavior in NAPL-water systems, and thus can be useful in estimating the potential for NAPL solubilization and mobilization in remediation processes. The cosolvent partitioning behavior is interpreted with regard to changes in the physical properties of the NAPL-water system. Changes in interfacial tension between the two Phases were estimated using an existing correlation. A viscosity experiment was conducted for selected mixtures of ethanol, toluene, and water; and the viscosity was found to increase with increasing amounts of the cosolvent.
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mass transfer coefficient and concentration boundary layer thickness for a dissolving napl pool in porous media
Journal of Hazardous Materials, 2003Co-Authors: Constantinos V. Chrysikopoulos, Pin-yi Hsuan, Marios M. Fyrillas, Kenneth Y. LeeAbstract:Analytical expressions for the time invariant, average mass transfer coefficient and the concentration boundary layer thickness applicable to dissolving single-component Nonaqueous Phase Liquid (NAPL) pools in two-dimensional, saturated, homogeneous and isotropic porous formations are derived. Good agreement between predicted and experimentally determined time invariant average mass transfer coefficients is observed.
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transport of dissolved contaminants originating from a rectangular prism shaped multicomponent Nonaqueous Phase Liquid source in saturated porous media
Environmental Earth Sciences, 2002Co-Authors: Kenneth Y. LeeAbstract:In this study, a three-dimensional semi-analytical model for simulating the transport of contaminants originating from a dissolving, rectangular prism-shaped, multicomponent Nonaqueous-Phase Liquid (NAPL) source in homogeneous, saturated porous media is presented. The NAPL dissolution process for each component is envisioned to occur in a series of consecutive short intervals (pulses). The equilibrium aqueous concentration and the source dimensions are assumed to remain constant for the duration of each pulse. Individual component aqueous Phase concentrations resulting from each dissolution interval are determined by an existing, single-component analytical solution. A synthetic NAPL mixture consisting of tetrachloroethylene and trichloroethylene was used for model simulation. Results show that for a dissolving multicomponent NAPL mixture, downstream aqueous-Phase concentrations may not always reflect source characteristics. The model is useful for an understanding of field data in homogeneous subsurface formations and for interpretation of bench-scale laboratory experiments.
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contaminant transport resulting from multicomponent Nonaqueous Phase Liquid pool dissolution in three dimensional subsurface formations
Journal of Contaminant Hydrology, 1998Co-Authors: Constantinos V. Chrysikopoulos, Kenneth Y. LeeAbstract:A semi-analytical method for simulating transient contaminant transport originating from the dissolution of multicomponent Nonaqueous Phase Liquid (NAPL) pools in three-dimensional, saturated, homogeneous porous media is presented. Each dissolved component may undergo first-order decay and may sorb under local equilibrium conditions. The NAPL pool dissolution process is envisioned to occur in a series of consecutive short intervals (pulses). The mole fraction, Nonaqueous Phase activity coefficient and aqueous solubility of every pool constituent are estimated before the initiation of each pulse, and they are assumed to remain constant for the duration of each interval. Individual component aqueous Phase concentrations resulting from each dissolution interval are estimated by existing analytical solutions applicable to single component NAPL pools, and total concentration distributions of the same component are obtained by direct superposition. The semi-analytical method is more efficient and less computationally demanding than a finite-difference approximation. Furthermore, it is shown that neglecting the changes in Nonaqueous Phase activity coefficients that occur during multicomponent NAPL pool dissolution may result in erroneous predictions. The model presented in this work is useful for the design and interpretation of experiments in laboratory bench scale aquifers, certain homogeneous subsurface formations, and for the verification of complex numerical codes.
Chongbin Zhao - One of the best experts on this subject based on the ideXlab platform.
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replacement of annular domain with trapezoidal domain in computational modeling of Nonaqueous Phase Liquid dissolution front propagation problems
Journal of Central South University, 2015Co-Authors: Chongbin Zhao, Thomas Poulet, Klaus RegenauerliebAbstract:In order to simulate the instability phenomenon of a Nonaqueous Phase Liquid (NAPL) dissolution front in a computational model, the intrinsic characteristic length is commonly used to determine the length scale at which the instability of the NAPL dissolution front can be initiated. This will require a huge number of finite elements if a whole NAPL dissolution system is simulated in the computational model. Even though modern supercomputers might be used to tackle this kind of NAPL dissolution problem, it can become prohibitive for commonly-used personal computers to do so. The main purpose of this work is to investigate whether or not the whole NAPL dissolution system of an annular domain can be replaced by a trapezoidal domain, so as to greatly reduce the requirements for computer efforts. The related simulation results have demonstrated that when the NAPL dissolution system under consideration is in a subcritical state, if the dissolution pattern around the entrance of an annulus domain is of interest, then a trapezoidal domain cannot be used to replace an annular domain in the computational simulation of the NAPL dissolution system. However, if the dissolution pattern away from the vicinity of the entrance of an annulus domain is of interest, then a trapezoidal domain can be used to replace an annular domain in the computational simulation of the NAPL dissolution system. When the NAPL dissolution system under consideration is in a supercritical state, a trapezoidal domain cannot be used to replace an annular domain in the computational simulation of the NAPL dissolution system.
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numerical modeling of toxic Nonaqueous Phase Liquid removal from contaminated groundwater systems mesh effect and discretization error estimation
International Journal for Numerical and Analytical Methods in Geomechanics, 2015Co-Authors: Chongbin Zhao, Thomas Poulet, Klaus RegenauerliebAbstract:Summary Numerical modeling has now become an indispensable tool for investigating the fundamental mechanisms of toxic Nonaqueous Phase Liquid (NAPL) removal from contaminated groundwater systems. Because the domain of a contaminated groundwater system may involve irregular shapes in geometry, it is necessary to use general quadrilateral elements, in which two neighbor sides are no longer perpendicular to each other. This can cause numerical errors on the computational simulation results due to mesh discretization effect. After the dimensionless governing equations of NAPL dissolution problems are briefly described, the propagation theory of the mesh discretization error associated with a NAPL dissolution system is first presented for a rectangular domain and then extended to a trapezoidal domain. This leads to the establishment of the finger-amplitude growing theory that is associated with both the corner effect that takes place just at the entrance of the flow in a trapezoidal domain and the mesh discretization effect that occurs in the whole NAPL dissolution system of the trapezoidal domain. This theory can be used to make the approximate error estimation of the corresponding computational simulation results. The related theoretical analysis and numerical results have demonstrated the following: (1) both the corner effect and the mesh discretization effect can be quantitatively viewed as a kind of small perturbation, which can grow in unstable NAPL dissolution systems, so that they can have some considerable effects on the computational results of such systems; (2) the proposed finger-amplitude growing theory associated with the corner effect at the entrance of a trapezoidal domain is useful for correctly explaining why the finger at either the top or bottom boundary grows much faster than that within the interior of the trapezoidal domain; (3) the proposed finger-amplitude growing theory associated with the mesh discretization error in the NAPL dissolution system of a trapezoidal domain can be used for quantitatively assessing the correctness of computational simulations of NAPL dissolution front instability problems in trapezoidal domains, so that we can ensure that the computational simulation results are controlled by the physics of the NAPL dissolution system, rather than by the numerical artifacts. Copyright © 2014 John Wiley & Sons, Ltd.
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effects of domain shapes on the morphological evolution of Nonaqueous Phase Liquid dissolution fronts in fluid saturated porous media
Journal of Contaminant Hydrology, 2012Co-Authors: Chongbin Zhao, Bruce HobbsAbstract:Abstract The main purpose of this paper is to investigate the effects of different domain shapes in general and trapezoidal domain shape in particular on the morphological evolution of Nonaqueous Phase Liquid (NAPL) dissolution fronts in two-dimensional fluid-saturated porous media. After the governing equations of NAPL dissolution problems are briefly described, the numerical procedure consisting of a combination of the finite element and finite difference methods is used to solve these equations. The related numerical simulation results have demonstrated that: (1) domain shapes have a significant effect on both the propagating speed and the morphological evolution pattern of a NAPL dissolution front in the fluid-saturated porous medium; (2) an increase in the divergent angle of a trapezoidal domain can lead to a decrease in the propagating speed of the NAPL dissolution front; (3) the morphological evolution pattern of the NAPL dissolution front in a rectangular domain is remarkably different from that in a trapezoidal domain of a large divergent angle; (4) for a rectangular domain, the simplified dispersion model, which is commonly used in the theoretical analysis and numerical simulation, is valid for solving NAPL dissolution instability problems in fluid-saturated porous media; and (5) compared with diverging flow (when the trapezoidal domain is inclined outward), converging flow (when the trapezoidal domain is inclined inward) can enhance the growth of NAPL fingers, indicating that pump-and-treat systems by extracting contaminated groundwater might enhance NAPL dissolution fingering and lead to less uniform dissolution fronts.
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computational simulation for the morphological evolution of Nonaqueous Phase Liquid dissolution fronts in two dimensional fluid saturated porous media
Computational Geosciences, 2011Co-Authors: Chongbin Zhao, Bruce Hobbs, Klaus RegenauerliebAbstract:This paper deals with the computational aspects of Nonaqueous Phase Liquid (NAPL) dissolution front instability in two-dimensional fluid-saturated porous media of finite domains. After the governing equations of an NAPL dissolution system are briefly described, a combination of the finite element and finite difference methods is proposed to solve these equations. In the proposed numerical procedure, the finite difference method is used to discretize time, while the finite element method is used to discretize space. Two benchmark problems, for which either analytical results or previous solutions are available, are used to verify the proposed numerical procedure. The related simulation results from these two benchmark problems have demonstrated that the proposed numerical procedure is useful and applicable for simulating the morphological evolution of NAPL dissolution fronts in two-dimensional fluid-saturated porous media of finite domains. As an application, the proposed numerical procedure has been used to simulate morphological evolution processes for three kinds of NAPL dissolution fronts in supercritical NAPL dissolution systems. It has been recognized that: (1) if the Zhao number of an NAPL dissolution system is in the lower range of the supercritical Zhao numbers, the fundamental mode is predominant; (2) if the Zhao number is in the middle range of the supercritical Zhao numbers, the (normal) fingering mode is the predominant pattern of the NAPL dissolution front; and (3) if the Zhao number is in the higher range of the supercritical Zhao numbers, the fractal mode is predominant for the NAPL dissolution front.
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theoretical analyses of Nonaqueous Phase Liquid dissolution induced instability in two dimensional fluid saturated porous media
International Journal for Numerical and Analytical Methods in Geomechanics, 2010Co-Authors: Chongbin Zhao, Bruce HobbsAbstract:This paper deals with the theoretical aspects of Nonaqueous Phase Liquid (NAPL)-dissolution-induced instability in two-dimensional fluid-saturated porous media including solute dispersion effects.After some weaknesses associated with the previous work are analyzed and overcome, a comprehensive dimensionless number, known as the Zhao number, is proposed to represent the main driving force and three controlling mechanisms of an NAPL-dissolution system that has a finite domain. The linear stability analysis is carried out to derive the critical value of the comprehensive dimensionless number of the NAPL-dissolution system in a limit case as the ratio of the equilibrium concentration to the density of the NAPL approaches zero. As a result, a theoretical criterion that can be used to assess the instability of planar NAPL-dissolution fronts in two-dimensional fluid-saturated porous media of finite domains has been established. Not only can the present theoretical results be used for the theoretical understanding of the effect of solute dispersion on the instability of an NAPL-dissolution front in the fluid-saturated porous medium of either a finite domain or an infinite domain, but also they can be used as benchmark solutions for verifying numerical methods employed to simulate detailed morphological evolution processes of NAPL-dissolution fronts in two-dimensional fluid-saturated porous media. Copyright © 2009 John Wiley & Sons, Ltd.