Nonaqueous Phase Liquid Dissolution Research Articles

Evaluation of an upscaled model for DNAPL dissolution kinetics in heterogeneous aquifers

Estimates of contaminant fluxes from DNAPL sources as a function of time and DNAPL mass reduction are important to assess the long-term sustainability and costs of monitored natural attenuation and to determine the benefits of partial source removal. We investigate the accuracy of the upscaled mass transfer function (MTF) proposed by Parker and Park [Parker JC, Park E. Modeling field-scale dense nonaqueous phase liquid dissolution kinetics in heterogeneous aquifers. WRR 2004;40:W05109] to describe field-scale dissolved phase fluxes from DNAPL sources for a range of scenarios generated using high-resolution 3-D numerical simulations of DNAPL infiltration and long-term dissolved phase transport. The results indicate the upscaled MTF is capable of accurately describing field-scale DNAPL dissolution rates as a function of time. For finger-dominated source regions, an empirical mass depletion exponent in the MTF takes on values greater than one which results in predicted mass flux rates that decrease continuously with diminishing DNAPL mass over time. Lens-dominated regions exhibit depletion exponents less than one, which results in more step-function like mass flux versus time behavior. Mass fluxes from DNAPL sources exhibiting both lens- and finger-dominated subregions were less accurately described by the simple MTF, but were well described by a dual-continuum model of the same form for each subregion. The practicality of calibrating a dual-continuum model will likely depend on the feasibility of obtaining spatially resolved field measurements of contaminant fluxes or concentrations associated with the subregions using multilevel sampling or some other means.

Characterization of pore scale NAPL morphology in homogeneous sands as a function of grain size and NAPL dissolution

In this study, we investigate pore scale morphology of nonaqueous phase liquids (NAPLs) trapped in different pore sizes using tracer techniques. Specific interfacial area and saturation of NAPL trapped in homogeneous sands were measured using the interfacial and partitioning tracer techniques. The observed NAPL–water interfacial areas increased in a log-linear fashion with decreasing sand grain size, but showed no clear trend with residual NAPL saturation formed in the various grain sizes. The measured values were used to calculate the NAPL morphology index, which characterizes the spatial NAPL distribution within the pore space. The NAPL morphology indices, increased exponentially with decreasing grain size, indicating that the NAPL becomes smaller, but more blobs. For a fixed grain size, the specific interfacial area and saturation of the NAPL were measured following changes caused by dissolution using alcohol. The observed interfacial areas showed a decrease linearly as a function of the NAPL saturation.

An experimental study of acoustically enhanced NAPL dissolution in porous media

AbstractThe effects of acoustic waves on the dissolution of dense nonaqueous phase liquids in water saturated porous media are investigated. Experiments of trichloroethylene (TCE) ganglia dissolution within a water saturated column, packed with glass beads, are conducted. Acoustic waves with pressure amplitudes ranging from 0 to 1625 Pa and frequencies ranging from 0 to 285 Hz are employed to the interstitial fluid at the inlet of the packed column. Effluent dissolved TCE concentrations are observed to increase up to 120% in the presence of acoustic pressure waves compared to the case where TCE dissolution without acoustic waves is monitored. The observed effluent dissolved TCE concentration increase is attributed to enhanced mass flux at the TCE‐water interface, caused by acoustic waves. Highest dissolution rates occur at discrete frequencies suggesting resonance effects or the presence of standing waves. Although acoustic waves enhance TCE dissolution, they dissipate almost exponentially with distance from the acoustic source. © 2004 American Institute of Chemical Engineers AIChE J, 50: 3271–3280, 2004

Using Continuum Approach to Quantify the Remediation of Nonaqueous Phase Liquid Contaminated Fractured Permeable Formations

This study addresses the feasibility of using a continuum modeling approach to simulate pump-and-treat remediation of nonaqueous phase liquid (NAPL) contaminated fractured permeable formations. A simplified discrete fracture model, which incorporates permeable blocks with embedded parallel equidistant constant aperture fractures, was used to simulate the NAPL dissolution in an idealized fractured permeable formation. The applicability of this model is defined by the ranges of a dimensionless mobility number and interphase mass transfer coefficient. A continuum based model able to simulate phenomena predicted by the discrete fracture model has also been used. Three dimensionless parameters referring to organic solute advection and dispersion, and the continuum interphase mass transfer coefficient govern the performance of the continuum model. The nonlinear relationships between the discrete fracture and continuum model have been identified and formulated. However, the simplified conceptual models of this study may be inapplicable to many types of fractured formations. Ranges of possible use of the continuum modeling were determined in terms of dimensionless parameters. The discrete fracture and continuum approaches of this study can be useful for the preliminary evaluation of ideas concerning optimization of the remediation of NAPL contaminated fractured permeable formations.

Simple screening models of NAPL dissolution in the subsurface

Three simple screening models of nonaqueous phase liquid (NAPL) dissolution in the subsurface are proposed based on the NAPL mass conservation and the assumption of proportionality between the residual NAPL source zone concentration and the remaining residual NAPL mass. The purpose of the proposed models is to predict the solute concentration in the zone of the residual NAPL as a result of dissolution. The predicted source zone concentration decrease is used to simulate and account for the decrease of dissolution rate with time. The proposed simple NAPL dissolution models enable the pseudo-equilibrium formulation to be used and therefore the numerical simulations for field application problems can be simplified compared to the non-equilibrium counterpart. With proper choice of empirical parameters, the proposed simple screening models can work as well as more complex dissolution rate correlation models, such as that of Imhoff et al. [Water Resour. Res. 30 (1994) 307–320]. It is found that the proposed models are very good for quantifying non-equilibrium dissolution, which is characterized by tailing of breakthrough curves. The models are especially useful for situations of small residual NAPL saturation, which are typical for many field applications.

Acoustically enhanced multicomponent NAPL ganglia dissolution in water saturated packed columns.

The impact of acoustic pressure waves on multicomponent nonaqueous phase liquid (NAPL) ganglia dissolution in water saturated columns packed with glass beads was investigated. Laboratory data from dissolution experiments with two and three component NAPL mixtures suggested that acoustic waves significantly enhance ganglia dissolution due to the imposed oscillatory interstitial water velocity. The dissolution enhancement was shown to be directly proportional to the acoustic wave frequency. Furthermore, it was demonstrated that the greatest dissolution enhancement in the presence of acoustic waves is associated with the component of the NAPL mixture having the smallest equilibrium aqueous solubility. Finally, square shaped acoustic waves were shown to lead to greater NAPL dissolution enhancement compared to sinusoidal and triangular acoustic waves. The results of this study suggested that aquifer remediation using acoustic waves is a promising method particularly for aquifers contaminated with NAPLs containing components with very low equilibrium aqueous solubilities.

Flux and product distribution during biological treatment of tetrachloroethene dense non-aqueous-phase liquid.

Flux in non-aqueous-phase liquid (NAPL)-contaminated systems containing active microbial populations (including Dehalococcoides sp.) was investigated using a quantitative mass balance and phase distribution approach. Batch systems containing mixed NAPL with an initial tetrachloroethene (PCE) mole fraction ranging from 0.1 to 0.4 provided a means for comparing systems where mass transfer and aqueous concentration were controlled by the initial NAPL composition. Although the use of mixed NAPL with increasing PCE mole fractions introduced a mass-transfer variable on the abiotic dissolution rate, it was determined that biological systems produced flux rates that were similar to each other regardless of the initial PCE mole fraction. Thus, organisms appeared to be dechlorinating near their maximum conversion rates, and the result was the accumulation of cis-1,2-dichloroethene (cDCE) followed by slow conversion to vinyl chloride (VC). Increases in the initial PCE mole fractions in the NAPL had a negative impact on product distribution due to the presence of a larger concentration of a more favorable electron acceptor. Because the mass converted to cDCE was present largely in the dissolved phase in all systems, the production of this metabolite was a favorable outcome in terms of NAPL dissolution. The pH dropped as low as 4.9 in active systems, indicating that the amount of HCl released during the reductive dechlorination process was large enough to overwhelm the buffering capacity. This pH effect was more pronounced in systems that exhibited extensive dechlorination to VC, further suggesting that rapid dechlorination of PCE NAPL can alter chemical characteristics in source zone regions.

Pore network simulation of the dissolution of a single‐component wetting nonaqueous phase liquid

Soil wettability has been recently recognized as a factor that can dramatically influence the dissolution behavior of residual nonaqueous phase liquids (NAPL). A NAPL that wets the solid surface is trapped within the smaller pores and along the corners of pores invaded by water (the nonwetting phase). We present a two‐dimensional network simulator of wetting NAPL dissolution, inspired by observations of this process in transparent glass micromodels. The network model idealizes the pore space as a network of cubic pores connected by square tubes, following respective distributions. In accordance with micromodel observations, capillary equilibrium is assumed to exist between NAPL‐water interfaces along pore corners and within pores. Advection and diffusion of the organic dissolved in the aqueous phase, as well as dissolution mass transfer from residual NAPL, are explicitly accounted for in the model. Pores filled with NAPL are invaded at a rate which is controlled by mass transfer from dissolving thick NAPL films in pore corners and in order of increasing entry capillary pressure, resulting in quasi‐static drainage and fingering of the aqueous phase. Loss of NAPL continuity due to rupture of thick NAPL films and heterogeneity are found to affect profoundly the dissolution behavior, resulting in concentration tailing. The network simulator reproduces qualitatively the behavior observed in column experiments with oil‐wet media.

Modeling NAPL dissolution fingering with upscaled mass transfer rate coefficients

The dissolution of nonaqueous phase liquids (NAPLs) at residual saturation in porous media has sometimes resulted in the development of preferential dissolution pathways or NAPL dissolution fingers. While NAPL dissolution fingering may be modeled using numerical simulators with fine discretization, this approach is computational intensive. We derived an expression for an upscaled mass transfer rate coefficient that accounts for the growth of dissolution fingers within porous media contaminated uniformly with residual NAPL. This expression was closely related to the lengthening of the dissolution front. Data from physical experiments and numerical simulations in two dimensions were used to examine the growth of the dissolution front and the corresponding upscaled mass transfer rate coefficient. Using this upscaled mass transfer rate coefficient, the time when dissolution fingering results in a reduction in the overall mass transfer rate and thus controls the rate of NAPL dissolution was determined. This crossover time is a convenient parameter for assessing the influence of dissolution fingering on NAPL removal. For the physical experiments and numerical simulations analyzed in this study, the crossover time to dissolution fingering control always occurred before the dissolution front had moved 14 cm within NAPL-contaminated porous media, which is small compared to the scale of typical systems of concern. To verify the utility of this approach, data from a three-dimensional physical experiment were predicted reasonably well using an upscaled mass transfer rate coefficient that was determined independently from this experiment.

Analysis of pore‐scale nonaqueous phase liquid dissolution in etched silicon pore networks

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.

The effect of multicomponent diffusion on NAPL dissolution from spherical ternary mixtures

This paper investigates the dissolution characteristics of ternary nonaqueous phase liquid (NAPL) mixtures with the goal of comparing the relative contributions of multicomponent (intra-NAPL) diffusion, film transfer and thermodynamic nonideality. These contributions are compared at the pore scale and intermediate scale (several centimeters downstream from the source). Trichloroethene (TCE), tetrachloroethene (PCE) and 1,1,1-trichloroethane (TCA) were selected to model a reasonably ideal mixture; TCE, PCE and octanol were selected as a relevant nonideal mixture. A multicomponent diffusion-based dissolution model incorporating hydrodynamic theory was formulated to estimate intra-NAPL concentration gradients and associated aqueous interfacial concentrations for ideally shaped (spherical) NAPL blobs. Pore scale dissolution times for this model were compared to those generated using the conventional well-mixed NAPL dissolution model, applying the same film transfer boundary condition in both cases. Activity coefficients (spatially and temporally variable for the diffusion model, temporally variable for the well-mixed model) were estimated using UNIFAC. NAPL interfacial concentration histories generated using the pore scale models were used as input in a three-dimensional groundwater transport model (MT3DMS) to compare downstream concentration distributions. For the relatively large NAPL bodies simulated ( r=0.6 cm), intra-NAPL diffusion effects were found to be significant at the pore scale and were strongly impacted by the mixture's thermodynamic ideality. At the intermediate scale, and for the conditions tested, modest differences in the simulations suggested that intra-NAPL diffusion effects would be negligible compared to those associated with mixture composition uncertainty, dissolution rate processes related to NAPL-induced permeability effects and hydrodynamic issues associated with flow field heterogeneity.

Nonaqueous liquid pool dissolution in three-dimensional heterogeneous subsurface formations

A three-dimensional numerical flow and contaminant transport model is developed to investigate the effect of variable hydraulic conductivity on average mass transfer coefficients associated with the dissolution of dense nonaqueous phase liquid (DNAPL) pools in heterogeneous, water-saturated subsurface formations. Randomly generated, three-dimensional hydraulic conductivity fields are used to represent a heterogeneous confined aquifer. Model simulations indicate that the average mass transfer coefficient is inversely proportional to the variance of the log-transformed hydraulic conductivity. A power law correlation relating the Sherwood number to the variance of the log-transformed hydraulic conductivity and appropriate Peclet numbers is developed. A reasonable fit between predicted and numerically determined mass transfer coefficients is observed.

Mass transfer correlations for nonaqueous phase liquid dissolution from regions with high initial saturations

The application of existing correlations for nonaqueous phase liquid (NAPL) dissolution, which were developed in small, one‐dimensional columns, to larger‐scale, heterogeneous or multidimensional systems has shown the predicted dissolution behavior depends greatly on the correlation used. Variation among existing correlations is due to the system scale, NAPL‐water interfacial area, and the nature of mass transfer or hydrodynamic mechanisms that are lumped in the correlation. In this paper, new mass transfer correlation is developed using NAPL dissolution data from a small 2‐D experimental cell that contained a well‐characterized heterogeneous distribution of grain sizes. The new correlation can be used for quantifying NAPL dissolution rates over a wide range of NAPL saturations and aqueous phase velocities within the NAPL source zone. When incorporated in a finite difference transport model, the correlation provides reasonably good predictions for systems with initially high NAPL saturations that are then reduced through the dissolution process. It is shown that NAPL dissolution is slower in this case due to the larger amorphous blobs that result from preferential flow and dissolution pathways. These large blobs have significantly less surface area in comparison with small discrete blobs that result from capillary entrapment. In comparison with other published dissolution correlations, the slower mass transfer rate is characterized with a significantly higher exponent on the NAPL saturation term.

Transport of dissolved contaminants originating from a rectangular prism-shaped, multicomponent nonaqueous-phase liquid source in saturated porous media

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.

Nonaqueous Phase Liquid Dissolution and Soil Organic Matter Sorption in Porous Media: Review of System Similarities

We examine similarities in constraints to mass transfer of hydrophobic organic compounds (HOCs) between the aqueous and various organic phases in porous media at the grain scale. Published research and data are reviewed regarding equilibrium coefficients and first-order rate constants for mass transfer of HOCs between water and natural organic matter present in various geosorbents (e.g., soils, sediments, and aquifer solids), and several oils (petroleum products, decane, and coal tar). We propose how these equilibrium and mass-transfer coefficients can be estimated by methods common to all organic phases. Equilibrium coefficients can be reliably estimated using Raoult's law. First-order rate constants obtained from extensive literature data appear to be dependent on aqueous-phase diffusion and duration of exposure (or contact). The aqueous diffusion domain may be either an interfacial film (for low viscosity oil ganglia) or a retarded immobile water zone (for geosorbents).

Effects of source zone heterogeneity on surfactant-enhanced NAPL dissolution and resulting remediation end-points

The effectiveness of removal of nonaqueous phase liquids (NAPLs) from the entrapment source zone of the subsurface has been limited by soil heterogeneity and the inability to locate all entrapped sources. The goal of this study was to demonstrate the uncertainty of degree of source removal associated with aquifer heterogeneity. In this demonstration, source zone NAPL removal using surfactant-enhanced dissolution was considered. Model components that simulate the processes of natural dissolution in aqueous phase and surfactant-enhanced dissolution were incorporated into an existing code of contaminant transport. The dissolution modules of the simulator used previously developed Gilland–Sherwood type phenomenological models of NAPL dissolution to estimate mass transfer coefficients that are upscaleable to multidimensional flow conditions found at field sites. The model was used to simulate the mass removal from 10 NAPL entrapment zone configurations based on previously conducted two-dimensional tank experiments. These entrapment zones represent the NAPL distribution in spatially correlated random fields of aquifer hydraulic conductivity. The numerical simulations representing two-dimensional conditions show that effectiveness of mass removal depends on the aquifer heterogeneity that controls the NAPL entrapment and delivery of the surfactant to the locations of entrapped NAPLs. Flow bypassing resulting from heterogeneity and the reduction of relative permeability due to NAPL entrapment reduces the delivery efficiency of the surfactant, thus prolonging the remediation time to achieve desired end-point NAPL saturations and downstream dissolved concentrations. In some extreme cases, the injected surfactant completely bypassed the NAPL source zones. It was also found that mass depletion rates for different NAPL source configurations vary significantly. The study shows that heterogeneity result in uncertainties in the mass removal and achievable end-points that are directly related to dissolved contaminant plume development downstream of the NAPL entrapment zone.

Modelling the fate of oxidisable organic contaminants in groundwater

Subsurface contamination by organic chemicals is a pervasive environmental problem, susceptible to remediation by natural or enhanced attenuation approaches or more highly engineered methods such as pump-and-treat, amongst others. Such remediation approaches, along with risk assessment or the pressing need to address complex scientific questions, have driven the development of integrated modelling tools that incorporate physical, biological and geochemical processes. We provide a comprehensive modelling framework, including geochemical reactions and interphase mass transfer processes such as sorption/desorption, non-aqueous phase liquid dissolution and mineral precipitatation/dissolution, all of which can be in equilibrium or kinetically controlled. This framework is used to simulate microbially mediated transformation/degradation processes and the attendant microbial population growth and decay. Solution algorithms, particularly the split-operator (SO) approach, are described, along with a brief résumé of numerical solution methods. Some of the available numerical models are described, mainly those constructed using available flow, transport and geochemical reaction packages. The general modelling framework is illustrated by pertinent examples, showing the degradation of dissolved organics by microbial activity limited by the availability of nutrients or electron acceptors (i.e., changing redox states), as well as concomitant secondary reactions. Two field-scale modelling examples are discussed, the Vejen landfill (Denmark) and an example where metal contamination is remediated by redox changes wrought by injection of a dissolved organic compound. A summary is provided of current and likely future challenges to modelling of oxidisable organics in the subsurface.

Measuring and modeling the dissolution of nonideally shaped dense nonaqueous phase liquid pools in saturated porous media

A three‐dimensional physical aquifer model was used to study the dissolution of a dense nonaqueous phase liquid (DNAPL) pool. The model aquifer comprised a packing of homogeneous, medium‐sized sand and conveyed steady, unidirectional flow. Tetrachloroethene (PCE) pools were introduced within model aquifers atop glass‐ and clay‐lined aquifer bottoms. Transient breakthrough at an interstitial velocity of 7.2 cm/h, and three‐dimensional steady state concentration distributions at velocities ranging from 0.4 to 7.2 cm/h were monitored over periods of 59 and 71 days for the glass‐ and clay‐bottom experiments, respectively. Pool‐averaged mass transfer coefficients were obtained from the observations via a single‐parameter fit using an analytical model formulated with a second type boundary condition to describe pool dissolution [Chrysikopoulos, 1995]. Other model parameters (interstitial velocity, longitudinal and transverse dispersion coefficients, and pool geometry) were estimated independently. Simulated and observed dissolution behavior agreed well, except for locations relatively close to the pool or the glass‐bottom plate. Estimated mass transfer coefficients ranged from 0.15 to 0.22 cm/h, increasing weakly with velocity toward a limiting value. Pool mass depletions of 31 and 43% for the glass‐ and clay‐bottom experiments failed to produce observable changes in the plumes and suggested that changes in pool interfacial area over the period of the experiment were negligible. Dimensionless mass transfer behavior was quantified using a modified Sherwood number (Sh*). Observed Sh* values were found to be about 2–3 times greater than values predicted by an existing theoretical mass transfer correlation, and 3–4 times greater than those estimated previously for an ideally configured trichloroethene (TCE) pool (circular and smooth). It appeared that the analytical model's failure to account for pore‐scale pool‐water interfacial characteristics and larger scale pool shape irregularities biased the Sh* estimates toward greater values.

Nonideal equilibrium dissolution of trichloroethene from a decane‐based nonaqueous phase liquid mixture: Experimental and modeling investigation

Batch equilibrium solubility studies were conducted to examine the solubilization behavior of a chlorinated solvent, trichloroethene (TCE), from a fuel‐based nonaqueous phase liquid (NAPL) mixture. An alkane (n–decane) was used as a model compound because it is often a primary compound in jet fuel. The NAPL phase mole fractions of the chlorinated solvent in the mixture (XTCEN) that were investigated are typical of in situ values found at industrial and military waste sites (0.0001 ≥ XTCEN ≤ 0.1). The measured aqueous concentrations of TCE were essentially equal to the concentrations predicted with ideal dissolution theory (Raoult's law) at XTCEN values near 0.1. However, the ratio of the measured concentration to the ideal concentration, or the NAPL phase activity coefficient (γNTCE), increased nonlinearly as the XTCEN decreased. The γTCEN approached 6 at XTCEN = 0.0001. The UNIFAC method greatly underpredicts the γTCEN in this surrogate fuel. A NAPL‐mixture equilibrium‐dissolution model was developed that incorporates the observed nonideal dissolution. This model indicates that nonideal NAPL dissolution is 4 times faster than ideal dissolution for a hypothetical NAPL mixture with an initial XTCEN = 0.001. The magnitude of this effect becomes more important as the initial value of the XTCEN is decreased.

A magnetic resonance imaging study of dense nonaqueous phase liquid dissolution from angular porous media.

Magnetic resonance imaging (MRI) was used to determine the effects of pore-scale heterogeneity on the dissolution of a nonaqueous phase liquid (NAPL) in water-saturated flow-through columns (1.2 cm in diameter) packed with either approximately 500 or approximately 1,000 micron diameter angular silica gel (referred to as SG500 and SG1000, respectively). Columns were contaminated with 1,3,5-trifluorobenzene at residual saturation and then purged with water at a constant Darcy velocity of 1.83 m/day. Three-dimensional 19F images were acquired every 2-5 h at an imaging resolution of 59 x 234 x 234 microm3. Imaging results show that the specific NAPL surface area (a(t)) is linearly related to the NAPL volumetric fraction (theta(n)) and that the constant of proportionality between these parameters is determined by the blob size and geometry distribution. Overall (expressed as the modified Sherwood number, Sh') and intrinsic (expressed as the apparent Sherwood number, Sh(apt)) mass transfer rate coefficients were calculated. Values of Sh' and Sh(apt) for SG500 were approximately three times less than those for SG1000. For both solids, Sh' first increased or stayed the same and then decreased with decreasing theta(n), while Shapt generally increased with decreasing theta(n). These results suggest that during dissolution new flow paths were created (i.e., bypass zones were eliminated) as NAPL dissolved, decreasing the fraction of NAPL-water interfaces adjacent to pores filled with stagnant water and the average diffusion length scale. Since at for SG500 was dominated by less spherical multipore blobs (as opposed to more spherical singlets for SG1000), these results also suggest that the extent of flow bypassing (and the average diffusion length scale) increases in systems with more irregular blobs. These results are important because Sh' correlations and a "sphere" dissolution model do not account for transient changes in the fraction of NAPL surface area that contributes to dissolution or for the effect of initial blob size and geometry distribution on this fraction.