Nonaqueous Phase Liquid Dissolution Research Articles

Evolving interface between clean and nonaqueous phase liquid (NAPL)‐contaminated regions in two‐dimensional porous media

We studied the dissolution of nonaqueous phase liquids (NAPLs) at residual saturation in two‐dimensional systems using physical and numerical experiments. In all cases, preferential dissolution pathways developed in the NAPL‐contaminated regions. A permeability feedback mechanism led to the formation of centimeter‐scale dissolution fingers in homogeneous porous media, while fingers up to 1 order of magnitude wider formed in media with heterogeneous distributions of intrinsic permeability k, when σ2(lnk) was large. The structure of the interface between the NAPL‐contaminated and NAPL‐free regions was examined as dissolution progressed. In all cases the interface was a self‐affine fractal. The scaling relationship of Family and Vicsek [1985] fitted the interface data from the numerical simulations reasonably well.

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≈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 (≈0.02); however, little or no bioenhancement was observed with the higher mole fraction (≈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.

Pore-Scale Study of Nonaqueous Phase Liquid Dissolution in Porous Media Using Laser-Induced Fluorescence

Little quantitative, experimental pore-scale information exists regarding nonaqueous phase liquid (NAPL) ganglia undergoing dissolution in porous media. By using refractive index matched fluids and porous media, we have been able to nonintrusively visualize NAPL dissolution (at constant capillary numbers) in three dimensions using laser-induced fluorescence. The results provide dynamic, quantitative information regarding ganglia surface area, volume, position, and shape. It appears that ganglia smaller than a few pore volumes are spheroid, whereas larger ganglia exhibit a fractal area to volume scaling. Evidence of ganglia breakup is found for all capillary numbers studied. Mobilization, however, is only important at higher capillary numbers.

Dissolution of residual non-aqueous phase liquids in porous media: pore-scale mechanisms and mass transfer rates

Experiments designed to elucidate the pore-scale mechanisms of the dissolution of a residual non-aqueous phase liquid (NAPL), trapped in the form of ganglia within a porous medium, are discussed. These experiments were conducted using transparent glass micromodels with controlled pore geometry, so that the evolution of the size and shape of individual NAPL ganglia and, hence, the pore-scale mass transfer rates and mass transfer coefficients could be determined by image analysis. The micromodel design permitted reasonably accurate control of the pore water velocity, so that the mass transfer coefficients could be correlated in terms of a local (pore-scale) Peclet number. A simple mathematical model, incorporating convection and diffusion in a slit geometry was developed and used successfully to predict the observed mass transfer rates. For the case of non-wetting NAPL ganglia, water flow through the corners in the pore walls was seen to control the rate of NAPL dissolution, as recently postulated by Dillard and Blunt [Water Resour. Res. 36 (2000) 439–454]. Break-up of doublet non-wetting phase ganglia into singlet ganglia by snap-off in pore throats was also observed, confirming the interplay between capillarity and mass transfer. Additionally, the effect of wettability on dissolution mass transfer was demonstrated. Under conditions of preferential NAPL wettability, mass transfer from NAPL films covering the solid surfaces was seen to control the dissolution process. Supply of NAPL from the trapped ganglia to these films by capillary flow along pore corners was observed to result in a sequence of pore drainage events that increase the interfacial area for mass transfer. These observations provide new experimental evidence for the role of capillarity, wettability and corner flow on NAPL ganglia dissolution.

Nonaqueous‐phase‐liquid dissolution in variable‐aperture fractures: Development of a depth‐averaged computational model with comparison to a physical experiment

Dissolution of nonaqueous‐phase liquids (NAPLs) from variable‐aperture fractures couples fluid flow, transport of the dissolved NAPL, interphase mass transfer, and the corresponding NAPL‐water‐interface movement. Each of these fundamental processes is controlled by fracture‐aperture variability and entrapped‐NAPL geometry. We develop a depth‐averaged computational model of dissolution that incorporates the fundamental processes that control dissolution at spatial resolutions that include all scales of variability within the flow field. Thus this model does not require empirical descriptions of local mass transfer rates. Furthermore, the depth‐averaged approach allows us to simulate dissolution at scales that are larger than the scale of the largest entrapped NAPL blobs. We compare simulation results with an experiment in which we dissolved residual entrapped trichloroethylene (TCE) from a 15.4×30.3 cm, analog, variable‐aperture fracture. We measured both fracture aperture and the TCE distribution within the fracture at high spatial resolution using light transmission techniques. Digital images acquired over the duration of the experiment recorded the evolution of the TCE distribution within the fracture and are directly compared with the results of a computational simulation. The evolution with time of the distribution of the entrapped TCE and the total TCE saturation are both predicted well by the dissolution model. These results suggest that detailed parametric studies, employing the depth‐averaged dissolution model, can be used to develop a comprehensive understanding of NAPL dissolution in terms of parameters characterizing aperture variability, phase structure, and hydrodynamic conditions.

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.

Dissolution of residual tetrachloroethylene in fractional wettability porous media: incorporation of interfacial area estimates

Available data from a recent study of entrapped tetrachloroethylene (PCE) dissolution [Bradford et al., 1999] reveal that soil wettability and grain size distribution characteristics can dramatically influence dissolution behavior. This paper explores the modeling of the long‐term dissolution of residual nonaqueous phase liquids (NAPLs) entrapped in fractional wettability porous media. Here dissolution is modeled with a linear driving force expression, and the lumped mass transfer coefficient is represented using independent estimates of the film mass transfer coefficient and the NAPL‐water interfacial area. In fractional wettability porous media the NAPL saturation is assumed to occur as both NAPL films and ganglia, and NAPL‐water interfacial area is thus modeled as the sum of contributions from films and ganglia. The interfacial area model is calibrated by fitting several parameters to the available experimental data. Trends in fitted parameters suggest that (1) residual NAPL ganglia are less accessible to a flowing aqueous phase in finer textured soils; (2) the dissolution rate depends on the interfacial area in a nonlinear manner; and (3) NAPL ganglia entrapment depends on system wettability and grain size distribution. Mass transfer coefficient correlations are established from these fitted parameters. The calibrated model predicts that for a given NAPL saturation, an increasing PCE‐wet sand fraction results in longer periods of high effluent concentrations, followed by increased rates of concentration reduction and more persistent subsequent low‐ concentration tailing. The dependence of dissolution behavior on mean grain size is also captured by this model. Simulations demonstrate a significant improvement in dissolution predictions using the proposed model in comparison with those obtained using a previously developed mass transfer correlation based upon a power law function of saturation.

Mechanisms affecting the dissolution of nonaqueous phase liquids into the aqueous phase in slow‐stirring batch systems

Understanding the kinetics of the exchange processes between nonaqueous phase liquids (NAPLs) and water is important in predicting the fate of anthropogenic compounds such as petroleum hydrocarbons, i.e., benzene, toluene, ethylbenzene, and xylene (BTEX) as well as polynuclear aromatic hydrocarbons (PAHs). Exchange processes occurring in the environment resemble the experimental setup of the slow-stirring method (SSM) designed to determine solubilities and octanol-water partition coefficients. Data obtained from SSM experiments for diesel fuel compounds are interpreted by a linear transfer model that is characterized by an aqueous molecular boundary layer and the water/NAPL equilibrium partition coefficient. For the chosen experimental setup, the boundary layer thickness is 2.42 x 10(-2) cm. Typical equilibration times lie between 1 and 2 d. Due to the temperature dependence of the aqueous diffusivity, this time increases with decreasing temperature. Transport within the NAPL phase can slow down the exchange process for the more water-soluble compounds (e.g., benzene) provided that the stirring rate exceeds a critical value.

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, K diss a i, and NAPL/water total specific interfacial area, a i. K diss a i was computed as a function of modified Peclet number ( Pe′) for various NAPL saturations ( S N) and a i during drainage and imbibition and during dissolution without displacement. The largest contributor to a i was the interfacial area in the water-filled corners of chambers and tubes containing NAPL. When K diss a i was divided by a i, the resulting curves of dissolution coefficient, K diss versus Pe′ suggested that an approximate value of K diss could be obtained as a weak function of hysteresis or S N. Spatially and temporally variable maps of K diss a i 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 K diss a i 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.

Simulating the in situ oxidative treatment of chlorinated ethylenes by potassium permanganate

Several laboratory and field studies have demonstrated the potential viability of oxidation schemes using MnO4− for the in situ treatment of source areas, which are contaminated by chlorinated ethylenes (PCE, TCE, and DCE). Chemically, the chlorinated ethylenes are oxidized to CO2, Cl−, and MnO2. The goal of this study was to develop a theoretical framework for the chemical and physical processes involved. To this end, a computer model was created to simulate the coupled processes of nonaqueous phase liquid (NAPL) dissolution, chemical reactions, and solute mass transport in the in situ chemical oxidization scheme. The model incorporates a kinetic description of reactions between the MnO4− and the chlorinated ethylenes and the rate of dissolution of the NAPL. A Strang operator‐splitting method, which coupled the different physical and chemical processes and an exponentially expressed solution of the kinetic equations, led to a significant speed up in the solution process. The products were calculated based on the stoichiometry of the reaction. We demonstrated the capabilities of this code using already published results of column, test cell, and field experiments. Generally, the simulated results matched well with experimental measurements. The computer model provides a useful tool for assisting in the design and the prediction of the oxidization processes under field conditions.

Influence of Heterogeneity and Sampling Method on Aqueous Concentrations Associated with NAPL Dissolution

The purpose of this work is to examine the effects of nonuniform distributions of nonaqueous-phase liquid (NAPL) saturation, porous-media heterogeneity, and sampling method on the magnitude of aque...

A distributed-site model for non-equilibrium dissolution of multi-component residually trapped NAPL

An approach for modeling the dissolution of residually-trapped multi-component non-aqueous phase liquids (NAPLs) from a physically heterogeneous porous medium is presented. The Distributed-Site (DS) model presented uses the beta probability density function to characterize the initial distribution of residual NAPL blob volumes. Mass transfer from the trapped idealized NAPL blobs is simulated using as a basis a correlation for mass transfer coefficients, a linear concentration driving force, and multi-component partitioning. The main advantage of the approach outlined in this work is that by using reasonable assumptions the Distributed-Site model is able to model the mass transfer behavior of a complex dissolution phenomena using fewer fitted parameters than current multi-site models. Application of this model to column experimental data for the dissolution of benzene, toluene, and m-xylene from a tridecane NAPL source is presented. The incorporation of several NAPL blob sizes (and corresponding mass transfer rate coefficients) was able to account for the behavior of the effluent concentrations as they passed through the exponential decay and into the asymptotic region, an important feature lacking in the two-site modeling approach. When fitted to data for benzene, the DS model was found to predict experimental data satisfactorily for the dissolution of toluene and m-xylene from tridecane without adjustment of the parameters. This indicates that the dissolution process can be generalized for various compounds even when mass transfer limitations are present and that dissolution data obtained for one compound can be useful for predicting the long term dissolution history.

A note on statistical analysis of the rate coefficient of nonaqueous phase liquid dissolution in porous media

A statistical analysis of the rate coefficient of nonaqueous phase liquid dissolution in porous media is performed based on general functional correlation relationships found in literature. The input variables are treated as random and assumed to be lognormally distributed. The dissolution rate coefficient statistics are derived theoretically, and the influence of various parameters and their uncertainties are examined and discussed. The impact of uncertainties in the mean grain diameter is found to be the greatest on the dissolution rate coefficient uncertainties.

Effect of groundwater flow dimensionality on mass transfer from entrapped nonaqueous phase liquid contaminants

Mass transfer from entrapped nonaqueous phase liquids (NAPLs) at subsurface locations of environmental contamination typically takes place in three‐dimensional groundwater flow fields. Yet most laboratory studies of NAPLs dissolution have been one‐dimensional, eliminating more realistic conditions such as the heterogeneous distribution of entrapped NAPL ganglia and the potential for flow bypassing due to reduced water permeability in contaminated zones. In this study, experiments in two‐dimensional flow fields were used to evaluate the effects of flow dimensionality on NAPL dissolution. Modifications of the transport code MT3D provided the capability to simulate NAPL dissolution. Regression analysis, matching experimental observations to simulated predictions, provided parameter values for a proposed phenomenological model of dissolution in two‐dimensional flow fields. The proposed model predicted lower NAPL dissolution rates relative to models developed on the basis of published one‐dimensional experimental measurements. The results indicate potential for significant errors using the one‐dimensionally based models for NAPL dissolution in field applications.

Development of a pore network simulation model to study nonaqueous phase liquid dissolution

A pore network simulation model was developed to investigate the fundamental physics of nonequilibrium nonaqueous phase liquid (NAPL) dissolution. The network model is a lattice of cubic chambers and rectangular tubes that represent pore bodies and pore throats, respectively. Experimental data obtained by Powers [1992] were used to develop and validate the model. To ensure the network model was representative of a real porous medium, the pore size distribution of the network was calibrated by matching simulated and experimental drainage and imbibition capillary pressure‐saturation curves. The predicted network residual styrene blob‐size distribution was nearly identical to the observed distribution. The network model reproduced the observed hydraulic conductivity and produced relative permeability curves that were representative of a poorly consolidated sand. Aqueous‐phase transport was represented by applying the equation for solute flux to the network tubes and solving for solute concentrations in the network chambers. Complete mixing was found to be an appropriate approximation for calculation of chamber concentrations. Mass transfer from NAPL blobs was represented using a corner diffusion model. Predicted results of solute concentration versus Peclet number and of modified Sherwood number versus Peclet number for the network model compare favorably with experimental data for the case in which NAPL blob dissolution was negligible. Predicted results of normalized effluent concentration versus pore volume for the network were similar to the experimental data for the case in which NAPL blob dissolution occurred with time.

Nonaqueous Phase Liquid Dissolution in Porous Media: Current State of Knowledge and Research Needs

Our understanding of nonaqueous phase liquid (NAPL) dissolution in the subsurface environment has been increasing rapidly over the past decade. This knowledge has provided the basis for recent developments in the area of NAPL recovery, including cosolvent and surfactant flushing. Despite these advances toward feasible remediation technologies, there remain a number of unresolved issues to motivate environmental researchers in this area. For example, the lack of an effective NAPL‐location methodology precludes effective deployment of NAPL recovery technologies. The objectives of this paper are to critically review the state of knowledge in the area of stationary NAPL dissolution in porous media and to identify specific research needs. The review first compares NAPL dissolution‐based mass transfer correlations reported for environmental systems with more fundamental results from the literature involving model systems. This comparison suggests that our current understanding of NAPL dissolution in small‐scale (on the order of cm) systems is reasonably consistent with fundamental mass transfer theory. The discussion then expands to encompass several issues currently under investigation in NAPL dissolution research, including: characterizing NAPL morphology (i.e. effective size and surface area); multicomponent mixtures; scale-related issues (dispersion, flow by-passing); locating NAPL in the subsurface and enhanced NAPL recovery. Research needs and potential approaches are discussed throughout the paper. This review supports the following conclusions: (1) Our knowledge related to local dissolution and remediation issues is maturing, but should be brought to closure with respect to the link between NAPL emplacement theory (as it impacts NAPL morphology) and NAPL dissolution; (2) The role of nonideal NAPL mixtures, and intra-NAPL mass transfer processes must be clarified; (3) Valid models for quantifying and designing NAPL recovery schemes with chemical additives need to be refined with respect to chemical equilibria, mass transfer and chemical delivery issues; (4) Computational and large-scale experimental studies should begin to address parameter up-scaling issues in support of model application at the field scale; and (5) Inverse modeling efforts aimed at exploiting the previous developments should be expanded to support field-scale characterization of NAPL location and strength as a dissolving source.

A Physically Based Model of Dissolution of Nonaqueous Phase Liquids in the Saturated Zone

The design of remediation strategies for nonaqueous phase liquid (NAPL) contaminants involves predicting the rate of NAPL dissolution. A physically based model of an idealized pore geometry was developed to predict nonaqueous phase liquid dissolution rate coefficients. A bundle of parallel pores in series model is used to represent NAPL dissolution as a function of three processes: pore diffusion, corner diffusion, and mixing and multiple contact. The dissolution rate coefficient is expressed in terms of the modified Sherwood number (Sh′) and is a function of Peclet (Pe) number. The model captures the complex behavior of Sh′ versus Pe data for both water-wet (Powers, 1992) and NAPL-wet (Parker et al., 1991) media. For water-wet media, the observed behavior can be broken down into four distinct regions. Each region represents a different physical process controlling NAPL dissolution: the low-Pe region is controlled by pore diffusion; the low- to moderate-Pe region is a transition zone; the moderate-Pe region is controlled by mixing and multiple contact; and the high-Pe region is controlled by corner diffusion. For the high-Pe conditions typical of most column experiments, the model involves only one fitting parameter. For NAPL-wet media, NAPL dissolution is governed exclusively by corner diffusion, and the model again involves only one fitting parameter.

Influence of porous media and airflow rate on the fate of NAPLs under air sparging.

Nonequilibrium air-water mass transfer experiments using a laboratory-scale single-air channel setup were conducted to investigate the influence of porous media and air velocity on the fate of nonaqueous phase liquids (NAPLs) under air sparging conditions. Benzene was used as a NAPL while silica sand 30/50 (dp 50 = 0.305 mm, uniformity coefficient, UC = 1.41) and silica sand 70/100 (dp 50 = 0.168 mm, UC = 1.64) were used as porous media. Air velocities ranged from 0 to 1.4 cm/s. Mass transfer coefficients for the dissolution of NAPLs were estimated by numerical methods using a two-dimensional dissolution-diffusion-volatilization model. The study showed that the presence of advective airflow in air channels controlled the spreading of the dissolved phase but the overall removal efficiency was independent of airflow rate. Removal efficiencies and dissolution rates of the NAPL were found to be strongly affected by the mean particle size of the porous media during air sparging. More than 50% reduction in the removal rate of benzene was found when silica sand 70/100 was used instead of silica sand 30/50. Mass transfer coefficients for the dissolution of benzene NAPL were estimated to be 0.0041 cm/min for silica sand 70/100 and 0.227 cm/min for silica sand 30/50. Increasing the air velocity from 0.6 to 1.4 cm/s for silica sand 30/50 did not result in a higher removal rate. Quantitative estimation of the dissolution rates of benzene NAPL indicated that the dissolution rates (between 0.227 and 0.265 cm/min) were similar in magnitude for the same porous media but different air flow rates. Based on the visualization study, air sparging may be used to control the spreading of the dissolved phase even when the glob of NAPL is several centimeters away from the air-water interface of the air channels.

Analytical Modeling of Nonaqueous Phase Liquid Dissolution with Green's Functions

Equilibrium and bicontinuum nonequilibrium formulations of the advection–dispersion equation (ADE) have been widely used to describe subsurface solute transport. The Green's Function Method (GFM) is particularly attractive to solve the ADE because of its flexibility to deal with arbitrary initial and boundary conditions, and its relative simplicity to formulate solutions for multi‐dimensional problems. The Green's functions that are presented can be used for a wide range of problems involving equilibrium and nonequilibrium transport in semi‐infinite and infinite media. The GFM is applied to analytically model multi‐dimensional transport from persistent solute sources typical of nonaqueous phase liquids (NAPLs). Specific solutions are derived for transport from a rectangular source (parallel to the flow direction) of persistent contamination using first‐, second‐, or third‐type boundary or source input conditions. Away from the source, the first‐ and third‐type condition cannot be expected to represent the exact surface condition. The second‐type condition has the disadvantage that the diffusive flux from the source needs to be specified a priori. Near the source, the third‐type condition appears most suitable to model NAPL dissolution into the medium. The solute flux from the pool, and hence the concentration in the medium, depends strongly on the mass transfer coefficient. For all conditions, the concentration profiles indicate that nonequilibrium conditions tend to reduce the maximum solute concentration and the total amount of solute that enters the porous medium from the source. On the other hand, during nonequilibrium transport the solute may spread over a larger area of the medium compared to equilibrium transport.

The influence of NAPL dissolution characteristics on field-scale contaminant transport in subsurface

A numerical model based on the two-phase method of characteristics (2PMOC) is developed to account for rate-limited dissolution processes and to examine the groundwater and soil gas contamination resulting from an immobile non-aqueous phase liquid (NAPL) residual in water-saturated and variably unsaturated field-scale systems. Recently developed dissolution rate coefficient correlation formulations are used to simulate the rate-limited dissolution processes. One-dimensional aqueous simulations are performed to examine differences between these correlations by comparing effluent concentrations. Damkohler number analysis is used to determine degree of equilibrium for each correlation and extent of error introduced by assuming local equilibrium between the NAPL and aqueous phases. Mathematical models are also used to simulate two-dimensional two-phase flow and contaminant transport to evaluate the potential of groundwater contamination resulting from a NAPL residual. While the simulation results illustrate the significance of the system scale on the rate of mass transfer between phases and the degree of equilibrium, the exact extent of the system scale effects remains to be answered. The modeling results from this study suggest more efforts to address the differences between laboratory and field-scale dissolution characteristics before a mature understanding of NAPL dissolution processes in subsurface can be achieved.