Multiphase flow and transport caused by spontaneous gas phase growth in the presence of dense non-aqueous phase liquid

Disconnected bubbles or ganglia of trapped gas may occur below the top of the capillary fringe through a number of mechanisms. In the presence of dense non-aqueous phase liquid (DNAPL), the disconnected gas phase experiences mass transfer of dissolved gases, including volatile components from the DNAPL. The properties of the gas phase interface can also change. This work shows for the first time that when seed gas bubbles exist spontaneous gas phase growth can be expected to occur and can significantly affect water-gas-DNAPL distributions, fluid flow, and mass transfer. Source zone behaviour was observed in three different experiments performed in a 2-dimensional flow cell. In each case, a DNAPL pool was created in a zone of larger glass beads over smaller glass beads, which served as a capillary barrier. In one experiment effluent water samples were analyzed to determine the vertical concentration profile of the plume above the pool. The experiments effectively demonstrated a) a cycle of spontaneous gas phase expansion and vertical advective mobilization of gas bubbles and ganglia above the DNAPL source zone, b) DNAPL redistribution caused by gas phase growth and mobilization, and c) that these processes can significantly affect mass transport from a NAPL source zone.     

Impact of nonaqueous phase liquid (NAPL) source zone architecture on mass removal mechanisms in strongly layered heterogeneous porous media during soil vapor extraction

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 impacts 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 later 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.     

Reaction-enhanced mass transfer and transport from non-aqueous phase liquid source zones

The objective of the following research is to theoretically quantify the enhancement of interphase mass transfer of dissolved non-aqueous phase liquid (NAPL) compounds from the non-aqueous phase to the aqueous phase and the enhancement of dispersive mass transport from a NAPL zone due to destruction of dissolved NAPL compounds. For relatively slow reaction rates, such as for permanganate and perchloroethene (PCE), local-scale mass transfer enhancement is expected to be small. Dispersive mass transport with reaction from a horizontal NAPL zone can be quantified using equations derived for a mathematically equivalent falling film reactor system. In contrast to local-scale interphase mass transfer, dispersive mass transport from NAPL zones may be significantly increased by reaction. Enhancement factors due to destruction of the NAPL compound(s) are mainly dependent on NAPL solubility and oxidant concentration and to a lesser extent on reaction rate, stoichiometry, and transverse dispersion coefficients. Higher NAPL solubility and/or lower oxidant concentration reduces the maximum expected enhancement factor. Reaction enhancement factors for mass transport from NAPL zones are expected to be in the range of 5-50 for permanganate and chlorinated solvents. Theoretical results suggest that assuming instantaneous reaction rates may be appropriate for dispersive mass transport from NAPL zones.     

DNAPL source depletion: linking architecture and flux response

The relationship between dense non-aqueous phase liquid (DNAPL) mass reduction and contaminant mass flux was investigated experimentally in four model source zones. The flow cell design for the experiments featured a segmented extraction well that allowed for analysis of spatially resolved flux information. This flux information was coupled with image analysis of the NAPL spatial distribution to investigate the relationship between flux and the up-gradient NAPL architecture. Results indicate that in the systems studied, the relationship between DNAPL mass reduction and contaminant mass flux was primarily controlled by the NAPL architecture. A specific definition of NAPL architecture was employed where the source zone is resolved into a collection of streamtubes with spatial variability in NAPL saturation along each streamtube integrated and transformed into an effective NAPL content for each streamtube. The distribution of NAPL contents among the streamtubes (NAPL architecture) controlled dissolution dynamics. Two simplified models, a streamtube model and an effective Damkohler number model, were investigated for their ability to simulate dissolution dynamics.     

Investigation of surfactant-enhanced dissolution of entrapped nonaqueous phase liquid chemicals in a two-dimensional groundwater flow field.

Because of their low solubility, waste chemicals in the form of nonaqueous phase liquids (NAPLs) that are entrapped in subsurface formations act as long-term sources of groundwater contamination. In the design of remediation schemes that use surfactants, it is necessary to estimate the mass transfer rate coefficients under multi-dimensional flow fields that exit at field sites. In this study, we investigate mass transfer under a two-dimensional flow field to obtain an understanding of the basic mechanisms of surfactant-enhanced dissolution and to quantify the mass transfer rates. Enhanced dissolution experiments in a two-dimensional test cell were conducted to measure rates of mass depletion from entrapped NAPLs to a flowing aqueous phase containing a surfactant. In situ measurement of transient saturation changes using a gamma attenuation system revealed dissolution patterns that are affected by the dimensionality of the groundwater flow field. Numerical modeling of local flow fields that changed with time, due to depletion of NAPL sources, enabled the examination of the basic mechanisms of NAPL dissolution in complex groundwater systems. Through nonlinear regression analysis, mass transfer rates were correlated to porous media properties, NAPL saturation and aqueous phase velocity. Results from the experiments and numerical analyses were used to identify deficiencies in existing methods of analysis that uses assumptions of one-dimensional flow, homogeneity of aquifer properties, local equilibrium and idealized transient mass transfer.     

The dissolution of benzene, toluene, m-xylene and naphthalene from a residually trapped non-aqueous phase liquid under mass transfer limited conditions

The results of dissolution experiments for benzene, toluene, m-xylene and naphthalene (BTXN) from a relatively insoluble oil phase (tridecane), residually trapped in a non-sorbing porous medium, are described. This mixture was chosen to simulate dissolution of soluble aromatic compounds from a petroleum hydrocarbon mixture, e.g., crude oil, for which a large fraction of the mixture is relatively insoluble. The experiments were carried out at a small source length to interstitial velocity ratio, L/v, so that dissolution would be mass transfer limited (MTL). When fitted to data for toluene, a multiregion mass transfer model was found to predict the experimental data satisfactorily for the other components without adjustment of the mass transfer rate parameters. These results indicate that the dissolution process can be generalized for various hydrophobic organic compounds present in a multicomponent non-aqueous phase liquid (NAPL) when mass transfer limitations are present. This also suggests that dissolution data obtained for one compound can be useful for predicting the dissolution histories for other compounds from petroleum hydrocarbon mixtures.     

Evaluation of simplified mass transfer models to simulate the impacts of source zone architecture on nonaqueous phase liquid dissolution in heterogeneous porous media

Nonaqueous phase liquid (NAPL) dissolution was studied in three-dimensional (3D) heterogeneous experimental aquifers (25.5 cm x 9 cm x 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.     

Effect of soil moisture dynamics on dense nonaqueous phase liquid (DNAPL) spill zone architecture in heterogeneous porous media

The amount, location, and form of NAPL in contaminated vadose zones are controlled by the spatial distribution of water saturation and soil permeability, the NAPL spill scenario, water infiltration events, and vapor transport. To evaluate the effects of these processes, we used the three-phase flow simulator STOMP, which includes a new permeability-liquid saturation-capillary pressure (k-S-P) constitutive model. This new constitutive model considers three NAPL forms: free, residual, and trapped. A 2-D vertical cross-section with five stratigraphic layers was assumed, and simulations were performed for seven cases. The conceptual model of the soil heterogeneity was based upon the stratigraphy at the Hanford carbon tetrachloride (CT) spill site. Some cases considered co-disposal of NAPL with large volumes of wastewater, as also occurred at the Hanford CT site. In these cases, the form and location of NAPL were most strongly influenced by high water discharge rates and NAPL evaporation to the atmosphere. In order to investigate the impact of heterogeneity, the hydraulic conductivity within the lower permeability layer was modeled as a realization of a random field having three different classes. For six extreme cases of 100 realizations, the CT mass that reached the water table varied by a factor of two, and was primarily controlled by the degree of lateral connectivity of the low conductivity class within the lowest permeability layer. The grid size at the top boundary had a dramatic impact on NAPL diffusive flux just after the spill event when the NAPL was present near the ground surface. NAPL evaporation with a fine grid spacing at the top boundary decreased CT mass that reached the water table by 74%, compared to the case with a coarse grid spacing, while barometric pumping had a marginal effect for the case of a continuous NAPL spill scenario considered in this work. For low water infiltration rate scenarios, the distribution of water content prior to a NAPL spill event decreased CT mass that reached the water table by 98% and had a significant impact on the formation of trapped NAPL. For all cases simulated, use of the new constitutive model that allows the formation of residual NAPL increased the amount of NAPL retained in the vadose zone. Density-driven advective gas flow from the ground surface controlled vapor migration in strongly anisotropic layers, causing NAPL mass flux to the lower layer to be reduced. These simulations indicate that consideration of the formation of residual and trapped NAPLs and dynamic boundary conditions (e.g., areas, rates, and periods of different NAPL and water discharge and fluctuations of atmospheric pressure) in the context of full three-phase flow are needed, especially for NAPL spill events at the ground surface. In addition, NAPL evaporation, density-driven gas advection, and NAPL vertical movement enhanced by water flow must be considered in order to predict NAPL distribution and migration in the vadose zone.     

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.     

Investigation of surfactant-enhanced mass removal and flux reduction in 3D correlated permeability fields using magnetic resonance imaging

Magnetic resonance imaging (MRI) was used to visualize the NAPL source zone architecture before and after surfactant-enhanced NAPL dissolution in three-dimensional (3D) heterogeneously packed flowcells characterized by different longitudinal correlation lengths: 2.1 cm (aquifer 1) and 1.1 cm (aquifer 2). Surfactant flowpaths were determined by imaging the breakthrough of a paramagnetic tracer (MnCl(2)) analyzed by the method of moments. In both experimental aquifers, preferential flow occurred in high permeability materials with low NAPL saturations, and NAPL was preferentially removed from the top of the aquifers with low saturation. Alternate flushing with water and two surfactant pulses (5-6 pore volumes each) resulted in approximately 63% of NAPL mass removal from both aquifers. However, overall reduction in mass flux (Mass Flux 1) exiting the flowcell was lower in aquifer 2 (68%) than in aquifer 1 (81%), and local effluent concentrations were found to increase by as high as 120 times at local sampling ports from aquifer 2 after surfactant flushing. 3D MRI images of NAPL revealed that NAPL migrated downward and created additional NAPL source zones in previously uncontaminated areas at the bottom of the aquifers. The additional NAPL source zones were created in the direction transverse to flow in aquifer 2, which explains the higher mass flux relative to aquifer 1. Analysis using a total trapping number indicates that mobilization of NAPL trapped in the two coarsest sand fractions is possible when saturation is below 0.5 and 0.4, respectively. Results from this study highlight the potential impacts of porous media heterogeneity and NAPL source zone architecture on advanced in-situ flushing technologies.     

Non-aqueous phase liquid spreading during soil vapor extraction

Many non-aqueous phase liquids (NAPLs) are expected to spread at the air-water interface, particularly under non-equilibrium conditions. In the vadose zone, this spreading should increase the surface area for mass transfer and the efficiency of volatile NAPL recovery by soil vapor extraction (SVE). Observations of spreading on water wet surfaces led to a conceptual model of oil spreading vertically above a NAPL pool in the vadose zone. Analysis of this model predicts that spreading can enhance the SVE contaminant recovery compared to conditions where the liquid does not spread. Experiments were conducted with spreading volatile oils hexane and heptane in wet porous media and capillary tubes, where spreading was observed at the scale of centimeters. Within porous medium columns up to a meter in height containing stagnant gas, spreading was less than ten centimeters and did not contribute significantly to hexane volatilization. Water film thinning and oil film pinning may have prevented significant oil film spreading, and thus did not enhance SVE at the scale of a meter. The experiments performed indicate that volatile oil spreading at the field scale is unlikely to contribute significantly to the efficiency of SVE.     

Modelling of iron cycling and its impact on the electron balance at a petroleum hydrocarbon contaminated site in Hnevice, Czech Republic

Over a period of several decades multiple leaks of large volumes from storage facilities located near Hnevice (Czech Republic) have caused the underlying Quaternary aquifer to be severely contaminated with nonaqueous phase liquid (NAPL) petroleum hydrocarbons. Beginning in the late 1980's the NAPL plume started to shrink as a consequence of NAPL dissolution exceeding replenishment and due to active remediation. The subsurface was classified geochemically into four different zones, (i) a contaminant-free zone never occupied by NAPL or dissolved contaminants, (ii) a re-oxidation zone formerly occupied by NAPL, (iii) a zone currently occupied by NAPL, and (iv) a lower fringe zone between the overlying NAPL and the deeper underlying contaminant-free zone. The study investigated the spatial and temporal variability of the redox zonation at the Hnevice site and quantified the influence of iron-cycling on the overall electron balance. As a first step inverse geochemical modelling was carried out to identify possible reaction models and mass transfer processes. In a subsequent step, two-dimensional (forward) multi-component reactive transport modelling was performed to evaluate and quantify the major processes that control the geochemical evolution at the site. The study explains the observed enrichment of the lower fringe zone with ferrihydrite as a result of the re-oxidation of ferrous iron. It suggests that once the NAPL zone started to shrink the dissolution of previously formed siderite and FeS by oxygen and nitrate consumed a significant part of the oxidation capacity for a considerable time period and therefore limited the penetration of electron acceptors into the NAPL contaminated zone.     

The significance of heterogeneity on mass flux from DNAPL source zones: an experimental investigation

Understanding the process of mass transfer from source zones of aquifers contaminated with organic chemicals in the form of dense non-aqueous phase liquids (DNAPL) is of importance in site management and remediation. A series of intermediate-scale tank experiments was conducted to examine the influence of aquifer heterogeneity on DNAPL mass transfer contributing to dissolved mass emission from source zone into groundwater under natural flow before and after remediation. A Tetrachloroethylene (PCE) spill was performed into six source zone models of increasing heterogeneity, and both the spatial distribution of the dissolution behavior and the net effluent mass flux were examined. Experimentally created initial PCE entrapment architecture resulting from the PCE migration was largely influenced by the coarser sand lenses and the PCE occupied between 30 and 60% of the model aquifer depth. The presence of DNAPL had no apparent effect on the bulk hydraulic conductivity of the porous media. Up to 71% of PCE mass in each of the tested source zone was removed during a series of surfactant flushes, with associated induced PCE mobilization responsible for increasing vertical DNAPL distributions. Effluent mass flux due to water dissolution was also found to increase progressively due to the increase in NAPL-water contact area even though the PCE mass was reduced. Doubling of local groundwater flow velocities showed negligible rate-limited effects at the scale of these experiments. Thus, mass transfer behavior was directly controlled by the morphology of DNAPL within each source zone. Effluent mass flux values were normalized by the up-gradient DNAPL distributions. For the suite of aquifer heterogeneities and all remedial stages, normalized flux values fell within a narrow band with mean of 0.39 and showed insensitivity to average source zone saturations.     

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.     

Three-dimensional visualization and quantification of non-aqueous phase liquid volumes in natural porous media using a medical X-ray Computed Tomography scanner

This study demonstrates the capabilities of a typical medical X-ray Computed Tomography (CT) scanner to non-destructively quantify non-aqueous phase liquid (NAPL) volumes, saturation levels, and three-dimensional spatial distributions in packed soil columns. Columns packed with homogeneous sand, heterogeneous sand, or natural soil, were saturated with water and injected with known quantities of gasoline or tetrachloroethene and scanned. A methodology based on image subtraction was implemented for computing soil porosity and NAPL volumes in each 0.35 mm x 0.35 mm x 1 mm voxel of the columns. Elimination of sample positioning errors and instrument drift artifacts was essential for obtaining reliable estimates of above parameters. The CT data-derived total NAPL volume was in agreement with the measured NAPL volumes injected into the columns. CT data-derived NAPL volume is subject to a 2.6% error for PCE and a 15.5% error for gasoline, at average NAPL saturations as low as 5%, and is mainly due to instrument noise. Non-uniform distributions of NAPL due to preferential flow, and accumulation of NAPL above finer-grained layers could be observed from the data on 3-D distributions of NAPL volume fractions.     

Modelling dissolution of multi-component non-aqueous phase liquids using Pareto distributions

A distributed-site non-aqueous phase liquid (NAPL) dissolution model that can capture the physical process of dissolution is presented. The model uses a Pareto distribution of effective interfacial area and a correlation for the mass transfer. Model parameters are obtained using a formal constrained non-linear optimisation technique. The model is applied on effluent data from two soil column experiments. The results of the Pareto distributed site model, with only two parameters to fit, were compared to two-site and multi-site models as well as the beta distributed site model. The Pareto distributed-site model produces equal or smaller error estimates than the other models.     

Bioenhancement of NAPL pool dissolution: experimental evaluation

Experiments were conducted to quantify nonaqueous phase liquid (NAPL) pool dissolution and its enhancement by in situ biodegradation. The experiments were performed using square cross-section, glass-bead packed column reactors with a small pool of a toluene-in-dodecane mixture (toluene mole fraction, X(tol) approximately 0.02 or 0.09). Experimental quasi-steady-state toluene dissolution fluxes were determined using a 14C-carbon mass-balance approach during water flushing with and without biodegradation. The experiments demonstrated a statistically significant bioenhancement of the toluene dissolution flux of up to roughly twofold at average pore water velocities of approximately 0.1 and 1 m/day when the toluene mole fraction was low ( approximately 0.02); however, little or no bioenhancement was observed with the higher mole fraction ( approximately 0.09). Although it cannot be determined conclusively, the weight of evidence based on biomass measurements and model analyses suggests that the reduced bioenhancement for the high mole fraction was due to higher dissolved toluene concentrations, which may have caused toxicity effects. Importantly, even though NAPL dissolution was not bioenhanced in every case, the biodegradation reduced toluene concentrations to low levels in the reactor effluents.     

Magnetic resonance imaging of nonaqueous phase liquid during soil vapor extraction in heterogeneous porous media

Soil vapor extraction (SVE) is commonly used to remediate nonaqueous phase liquids (NAPLs) from the vadose zone. This paper aims to determine the effect of grain size heterogeneity on the removal of NAPL in porous media during SVE. Magnetic resonance imaging (MRI) was used to observe and quantify the amount and location of NAPL in flow-through columns filled with silica gel grains. MRI is unique because it is nondestructive, allowing three-dimensional images to be taken of the phases as a function of space and time. Columns were packed with silica gel in three ways: coarse grains (250-550 microm) only, fine grains (32-63 microm) only, and a core of fine grains surrounded by a shell of coarse grains. Columns saturated with water were drained under a constant suction head, contaminated with decane, and then drained to different decane saturations. Each column was then continuously purged with water-saturated nitrogen gas and images were taken intermittently. Results showed that at residual saturation, a sharp volatilization front moved through the columns filled with either coarse-grain or fine-grain silica gel. In the heterogeneous columns, the volatilization front in the core lagged just behind the shell because gas flow was greater through the shell and decane in the core diffused outward to the shell. When decane saturation in the core was above residual saturation, decane volatilization occurred near the inlet, the relative decane saturation throughout the core dropped uniformly, and decane in the core flowed in the liquid phase to the shell to replenish volatilized decane. These results indicate that NAPL trapped in low-permeability zones can flow to replenish areas where NAPL is lost due to SVE. However, when residual NAPL saturation is reached, NAPL flow no longer occurs and diffusion limits removal from low-permeability zones.     

A functional relation for field-scale nonaqueous phase liquid dissolution developed using a pore network model

A pore network model with cubic chambers and rectangular tubes was used to estimate the nonaqueous phase liquid (NAPL) dissolution rate coefficient, Kdissai, and NAPL/water total specific interfacial area, ai. Kdissai was computed as a function of modified Peclet number (Pe') for various NAPL saturations (SN) and ai during drainage and imbibition and during dissolution without displacement. The largest contributor to ai was the interfacial area in the water-filled corners of chambers and tubes containing NAPL. When Kdissai was divided by ai, the resulting curves of dissolution coefficient, Kdiss versus Pe' suggested that an approximate value of Kdiss could be obtained as a weak function of hysteresis or SN. Spatially and temporally variable maps of Kdissai calculated using the network model were used in field-scale simulations of NAPL dissolution. These simulations were compared to simulations using a constant value of Kdissai and the empirical correlation of Powers et al. [Water Resour. Res. 30(2) (1994b) 321]. Overall, a methodology was developed for incorporating pore-scale processes into field-scale prediction of NAPL dissolution.