A practical model for mobile, residual, and entrapped NAPL in water-wet porous media

Flow of nonvolatile nonaqueous phase liquid (NAPL) and aqueous phases that account for mobile, entrapped, and residual NAPL in variably saturated water-wet porous media is modeled and compared against results from detailed laboratory experiments. Residual saturation formation in the vadose zone is a process that is often ignored in multifluid flow simulators, which might cause an overestimation of the volume of NAPL that reaches the ground water. Mobile NAPL is defined as being continuous in the pore space and flows under a pressure gradient or gravitational body force. Entrapped NAPL is defined as being occluded by the aqueous phase, occurring as immobile ganglia surrounded by aqueous phase in the pore space and formed when NAPL is replaced by the aqueous phase. Residual NAPL is defined as immobile, nonwater entrapped NAPL that does not drain from the pore spaces and is conceptualized as being either continuous or discontinuous. Free NAPL comprises mobile and residual NAPL. The numerical model is formulated on mass conservation equations for oil and water, transported via NAPL and aqueous phases through variably saturated porous media. To account for phase transitions, a primary variable switching scheme is implemented for the oil-mass conservation equation over three phase conditions: (1) aqueous or aqueous-gas with dissolved oil, (2) aqueous or aqueous-gas with entrapped NAPL, and (3) aqueous or aqueous gas with free NAPL. Two laboratory-scale column experiments are modeled to verify the numerical model. Comparisons between the numerical simulations and experiments demonstrate the necessity to include the residual NAPL formation process in multifluid flow simulators.     

Non-equilibrium alcohol flooding model for immiscible phase remediation: 2. Model development and application

A non-equilibrium, two-phase, three-component compositional model for the simulation of alcohol flooding has been developed and tested. Inter-phase mass transfer algorithms allow for transfer of all three components at high concentrations and high mass flux rates using a two-film model. The model has been used to simulate alcohol floods where the alcohol has an affinity for either the water-rich phase, or the organic-rich phase. Calibration, using experimental effluent data from an alcohol flood which used a 2-propanol (IPA)-water-tetrachlorethene (PCE) ternary system, indicates that inter-phase mass transfer parameters can be non-unique. Sensitivity studies, completed using the non-equilibrium model for the IPA-water-PCE system, indicate that experimentally derived organic-rich phase composition data should lead to better estimates of the non-wetting phase film thickness. For alcohol flooding experiments where the primary mechanism of non-aqueous phase liquid (NAPL) removal is enhanced dissolution, near-equilibrium conditions may be achieved with NAPL recovery similar for conditions of near-equilibrium and equilibrium. However, for systems where remobilization is the primary mechanism of NAPL recovery, it is expected that although local conditions may approach equilibrium, the resulting NAPL recovery can be significantly lower than would be attained if equilibrium conditions persisted.     

Electrical Resistance Heating of Clay Layers in Water‐Saturated Sand

Electrical resistance heating (ERH) experiments were performed in a two‐dimensional water‐saturated porous medium comprising an electrically conductive, low‐permeability clay lens embedded within a less electrically conductive, higher permeability silica sand. These were compared to experiments performed in homogeneous silica sand. All experiments were performed in the absence of a non‐aqueous phase liquid (NAPL) or dissolved volatile organic compound (VOC). Temperature monitoring showed preferential heating in the clay lens and higher overall heating rates throughout the test cell compared to the homogeneous case. Gas production was localized around the sand–clay interface due to high temperature and low capillary displacement pressure. Above the clay lens, unexpected temperature plateaus were observed, similar to those observed in previous experiments during NAPL–water co‐boiling. A conceptual model based on the consumption of thermal energy as latent heat of vaporization in the highly localized heating and gas production region adjacent to the clay lens is proposed to explain the temperature plateaus. Supporting data is drawn from images of the gas phase and electric current measurements. These results show that the use of co‐boiling plateaus as an indicator of NAPL–water co‐boiling could be misleading during applications of ERH at sites containing electrically conductive, low‐permeability clay lenses embedded within less electrically conductive, higher‐permeability sands.     

Experimental Investigation of Surfactant-Enhanced Dissolution of Residual NAPL in Saturated Soil

Nonaqueous phase liquid (NAPL) is a long-term source of ground water contamination as the pollutant slowly partitions into the air and water phases. The objective of this work was to study the efficacy of aqueous surfactant solution to enhance the dissolution of a residual NAPL below the capillary fringe, hence reducing the time needed for aquifer restoration. An analytical technique was developed to measure the concentration of NAPL in a nonionic surfactant. Soil column experiments simulated conditions in the saturated soil where a NAPL may become trapped as a discontinuous immobile phase. Experimental results indicate that dissolution was a rate-limited process, approaching equilibrium concentrations after 24 hours. The relative permeability of the aqueous phase initially decreased as surfactant was injected, but increased over time as the saturation of residual NAPL was reduced through mass transfer into the surfactant-enhanced aqueous phase. These findings suggest that enhancing the aqueous phase with a nonionic surfactant may significantly enhance the in situ recovery or residual NAPL.     

Transport and fate of nonaqueous phase liquid (NAPL) in variably saturated porous media with evolving scales of heterogeneity

 A stochastic simulation is performed to study multiphase flow and contaminant transport in fractal porous media with evolving scales of heterogeneity. Numerical simulations of residual NAPL mass transfer and subsequent transport of dissolved and/or volatilized NAPL mass in variably saturated media are carried out in conjunction with Monte Carlo techniques. The impact of fractal dimension, plume scale and anisotropy (stratification) of fractal media on relative dispersivities is investigated and discussed. The results indicate the significance of evolving scale of porous media heterogeneity to the NAPL transport in the subsurface. In general, the fractal porous media enhance the dispersivities of NAPL mass plume transport in both the water phase and the gas phase while the influence on the water phase is more significant. The porous media with larger fractal dimension have larger relative dispersivities. The aqueous horizontal dispersivity exhibits a most significant increase against the plume scale.     

The influence of wettability on NAPL dissolution fingering

Laboratory experiments and numerical simulations in homogeneous porous media were used to investigate the influence of porous medium wettability on the formation and growth of preferential dissolution pathways, dissolution fingers, during nonaqueous phase liquid (NAPL) dissolution. As the porous medium became increasingly NAPL-wet, dissolution fingers grew wider and slower. This result was observed in physical experiments with 0% and 100% NAPL-wet conditions and confirmed with numerical simulations at these and intermediate wettabilities. A previously derived expression for an upscaled mass transfer rate coefficient that accounts for the growth of dissolution fingers was used to quantify the effect of fingering on overall NAPL removal rates. For the test cases evaluated, NAPL dissolution fingering controlled the overall rate of NAPL dissolution after the dissolution front moved 4 cm in 0% NAPL-wet conditions and 18 cm in 100% NAPL-wet conditions. Thus, even in completely NAPL-wet media dissolution fingering may control the overall rate of NAPL dissolution after relatively short travel distances. The importance of NAPL dissolution fingering in heterogeneous systems with spatially varying NAPL saturations, though, remains an important question for future work.     

Using Phase Diagrams to Predict the Performance of Cosolvent Floods for NAPL Remediation

Cosolvent flooding using water miscible solvents such as alcohols has been proposed as an in situ NAPL remediation technique. This process is conceptually similar to enhanced oil recovery (EOR) using alcohols and some surfactant formulations. As a result of interest in the EOR aspects of these systems, analytical and graphical methods based on fractional flow theory were developed in the petroleum engineering literature for modeling theses floods. The existing fractional flow solutions have not been used previously in environmental applications of cosolvent flooding, but they are applicable and provide many useful insights into the process. These applications are discussed, with an emphasis on emplaining the mechanisms which tend to mobilize trapped NAPL during a cosolvent flood. The theory provides a simple way to predict the general behavior of a cosolvent flood using the phase diagram. It is concluded that the one-dimensional performance of a cosolvent flood can be predicted largely by inspection of the ternary phase diagram. In particular, the nature of the cosolvent flood depends primarily on the position of the cosolvent injection concentration relative to a critical the line extension which passes through the plait point, tangent to the binodal curve.     

Effect of NAPL Source Morphology on Mass Transfer in the Vadose Zone

The generation of vapor‐phase contaminant plumes within the vadose zone is of interest for contaminated site management. Therefore, it is important to understand vapor sources such as non‐aqueous‐phase liquids (NAPLs) and processes that govern their volatilization. The distribution of NAPL, gas, and water phases within a source zone is expected to influence the rate of volatilization. However, the effect of this distribution morphology on volatilization has not been thoroughly quantified. Because field quantification of NAPL volatilization is often infeasible, a controlled laboratory experiment was conducted in a two‐dimensional tank (28 cm × 15.5 cm × 2.5 cm) with water‐wet sandy media and an emplaced trichloroethylene (TCE) source. The source was emplaced in two configurations to represent morphologies encountered in field settings: (1) NAPL pools directly exposed to the air phase and (2) NAPLs trapped in water‐saturated zones that were occluded from the air phase. Airflow was passed through the tank and effluent concentrations of TCE were quantified. Models were used to analyze results, which indicated that mass transfer from directly exposed NAPL was fast and controlled by advective‐dispersive‐diffusive transport in the gas phase. However, sources occluded by pore water showed strong rate limitations and slower effective mass transfer. This difference is explained by diffusional resistance within the aqueous phase. Results demonstrate that vapor generation rates from a NAPL source will be influenced by the soil water content distribution within the source. The implications of the NAPL morphology on volatilization in the context of a dynamic water table or climate are discussed.     

Effect of NAPL film stability on the dissolution of residual wetting NAPL in porous media: A pore-scale modeling study

Wettability profoundly affects not only the initial distribution of residual NAPL contaminants in natural soils, but also their subsequent dissolution in a flowing aqueous phase. Under conditions of preferential NAPL wettability, the residual NAPL phase is found within the smaller pores and in the form of continuous corner filaments and thick films on pore walls. Such films expose a much greater interfacial area for mass transfer than would be exposed by the same amount of non-wetting NAPL. Importantly, capillary and hydraulic continuity of NAPL filaments and thick films is essential for sustaining NAPL–water counterflow during the course of NAPL dissolution in flowing groundwater—a mechanism which maintains and even increases the interfacial area for mass transfer. Continued dissolution results in gradual thinning of the NAPL films, which may become unstable and rupture causing disconnection of the residual NAPL in the form of clusters. Using a pore network simulator, we demonstrate that NAPL film instability drastically modifies the microscopic configuration of residual NAPL, and hence the local hydrodynamic conditions and interfacial area for mass transfer, with concomitant effects on macroscopically observable quantities, such as the aqueous effluent concentration and the fractional NAPL recovery with time. These results strongly suggest that the disjoining pressure of NAPL films may exert an important, and hitherto unaccounted, control on the dissolution behaviour of a residual NAPL phase in oil wet systems.     

Cyclodextrin-Enhanced Solubilization of Organic Contaminants with Implications for Aquifer Remediation

Reagents that enhance the aqueous solubility of non-aqueous phase organic liquid (NAPL) contaminants are under investigation for use in enhanced subsurface remediation technologies. Cyclodextrin, a glucose-based molecule, is such a reagent. In this paper, laboratory experiments and numerical model simulations are used to evaluate and understand the potential remediation performance of cyclodextrin. Physical properties of cyclodextrin solutions such as density, viscosity, and NAPL-aqueous inter-facial tension are measured. Our analysis indicates that no serious obstacles exist related to fluid properties that would prevent the use of cyclodextrin solutions for subsurface NAPL remediation. Cyclodextrin-enhanced solubilization for a large suite of typical ground water contaminants is measured in the laboratory, and the results are related to the physicochemical properties of the organic compounds. The most-hydrophobic contaminants experience a larger relative solubility enhancement than the less-hydrophobic contaminants but have lower aqueous-phase apparent solubilities. Numerical model simulations of enhanced-solubilization flushing of NAPL-contaminated soil demonstrate that the more-hydrophilic compounds exhibit the greatest mass-removal rates due to their greater apparent solubilities, and thus are initially more effectively removed from soil by enhanced-solubilization-flushing reagents. However, the relatively more hydrophobic contaminants exhibit a greater improvement in contaminant mass-removal (compared with water flushing) than that exhibited for the relatively hydrophilic contaminants.     

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.     

Partition coefficients for alcohol tracers between nonaqueous-phase liquids and water from UNIFAC-solubility method

In this work, we have applied a group-contribution activity-coefficient model, UNIFAC, and the solubility of alcohols in water to estimate partition coefficients for alcohol tracers between water and nonaqueous-phase liquids (NAPLs). The effects of temperature and mutual solubility between NAPL and aqueous phases on the estimation of partition coefficients were also investigated. By comparing the estimated results with experimental partition coefficients for 30 alcohol tracers between 10 NAPLs and water, we found that: i) the UNIFAC-solubility method, in which the UNIFAC model in its infinite-dilution form is applied to the NAPL phase and the solubility of tracers in water is used for estimation of the activity coefficient in the aqueous phase, works better than the UNIFAC model; ii) a linear relation between the logarithm of partition coefficients and the logarithm of tracer solubility in water is observed for those tracers having a similar chemical structure (i.e. the same number of branched methyl groups). This can serve as a useful tool for quick selection of the tracers that exhibit the desired partition coefficients; iii) the effect of mutual solubility between NAPL and aqueous phases can be neglected because such miscibility is very small, usually of the order of 10⁻³ mole/mole unit; and iv) temperature variation between 15° and 25°C does not significantly affect partition coefficients.     

DNAPL to LNAPL Transitions During Horizontal Cosolvent Flooding

Cosolvent flooding is a technology with the potential to remove nonaqueous phase liquid (NAPL) sources from the subsurface. It can be used to initiate separate phase mobilization, which allows removal of NAPL within very few pore volumes. Mobilization may result in a sinking DNAPL bank during horizontal flooding of NAPLs denser than water. Reversal of phase density difference between aqueous and DNAPL phases could potentially avoid this downward migration of mobilized DNAPLs. We achieved phase density difference reversal and made DNAPLs float using two components in the cosolvent flooding solution. A low-density cosolvent partitions preferentially into the DNAPL and swells it, which causes a reduction in density of the DNAPL and reversal of the density difference between the NAPL and aqueous phases. A highdensity additive that remains in the aqueous phase allows the cosolvent flooding solution overall to have a density greater than that of water and permits control of the flooding instability. This study focused on tert-butanol as the swelling cosolvent and tetrachloroethylene as the contaminant. In batch tests with sucrose and glycerol as dense additives, phase density difference reversal occurred. To investigate the applicability of phase density difference reversal as a remediation technology, horizontal column and sandbox experiments were performed. These experiments demonstrated the occurrence of phase density difference reversal and effective remediation in horizontal cosolvent floods.     

Influence of mass transfer resistance on detection of nonaqueous phase liquids with partitioning tracer tests

Partitioning interwell tracer tests (PITTs) are a relatively new technique for measuring the amount of nonaqueous phase liquid (NAPL) within saturated porous media. In this work we examined the influence of mass transfer limitations on the accuracy of measured NAPL from PITTs. Two mathematical models were used along with laboratory column experiments to explore the influence of tracer partition coefficient, tracer detection limit, and injected tracer mass on NAPL measurements. When dimensionless mass transfer coefficients were small, NAPL measurement errors decreased with decreasing tracer partition coefficient, decreasing tracer detection limit, and increasing injected tracer mass. Extrapolating breakthrough curves exponentially reduced but did not eliminate systematic errors in NAPL measurement. Although transport in a single stream tube was used in the mathematical models and laboratory experiments, the results from this simplified domain were supported by data taken from a three-dimensional computational experiment, where the NAPL resided as large pool. Based on these results, we suggest guidelines for interpreting tracer breakthrough data to ascertain the importance of mass transfer limitations on NAPL measurements.     

Pore Water Characteristics Following a Release of Neat Ethanol onto Pre-existing NAPL

Neat ethanol (75.7 L) was released into the upper capillary zone in a continuous-flow, sand-packed aquifer tank (8.2 m³) with an average seepage velocity of 0.75 m/day. This model aquifer system contained a residual nonaqueous phase liquid (NAPL) that extended from the capillary zone to 10 cm below the water table. Maximum aqueous concentrations of ethanol were 20% v/v in the capillary zone and 0.08% in the saturated zone at 25 and 30 cm downgradient from the emplaced NAPL source, respectively. A bench-scale release experiment was also conducted for a similar size spill (scaled to the plan area). The concentrations of ethanol in ground water for both the bench- and pilot-scale experiments were consistent with advective–dispersive limited mass transfer from the capillary to the saturated zone. Concentrations of monoaromatic hydrocarbons and isooctane increased in the pore water of the capillary zone as a result of both redistribution of residual NAPL (confirmed by visualization) and enhanced hydrocarbon dissolution due to the cosolvent effect exerted by ethanol. In the tank experiment, higher hydrocarbon concentrations in ground water were also attributed to decreased hydrocarbon biodegradation activity caused by preferential microbial utilization of ethanol and the resulting depletion of oxygen. These results infer that spills of highly concentrated ethanol will be largely confined to the capillary zone due to its buoyancy, and ethanol concentrations in near-source zone ground water will be controlled by mass transfer limitations and hydrologic conditions. Furthermore, highly concentrated ethanol releases onto pre-existing NAPL will likely exacerbate impacts to ground water, due to NAPL mobilization and dissolution, and decreased bioattenuation of hydrocarbons.     

A novel experimental procedure to investigate the biodegradation of NAPL under unsaturated conditions

Soils need to be thoroughly investigated regarding their potential for the natural attenuation of non-aqueous phase liquids (NAPL). Laboratory investigations truly representative of degradation processes in field conditions are difficult to implement for porous media partially saturated with water, NAPL and air. We propose an innovative protocol to investigate degradation processes under steady-state vadose zone conditions. Experiments are carried out in glass columns filled with a sand and, as bacteria source, a soil from a diesel-fuel-polluted site. Water and NAPL (n-hexadecane diluted in heptamethylnonane (HMN)) are added to the porous medium in a two-step procedure using ceramic membranes placed at the bottom of the column. This procedure results, for appropriate experimental conditions, in a uniform distribution of the two fluids (water and NAPL) throughout the column. In a biodegradation experiment non-biodegradable HMN is used to provide NAPL mass, while keeping biodegradable n-hexadecane small enough to monitor its rapid degradation. Biodegradation is followed as a function of time by measuring oxygen consumption, using a respirometer. Degradative activity is controlled by diffusive transfers in the porous network, of oxygen from the gas phase to the water phase and of n-hexadecane from the NAPL phase to the water phase.     

Pore-scale simulation of entrapped non-aqueous phase liquid dissolution

We investigated the dissolution of non-aqueous phase liquids (NAPLs) in a three-dimensional random sphere-pack medium using a pore-scale modeling approach to advance fundamental understanding and connect rigorously to microscale processes. Residual NAPL distributions were generated using a morphological approach and the entrapped non-wetting phase was quantitatively characterized by calculating volume, orientation, interfacial area, and shape of isolated NAPL regions. With a detailed aqueous-phase flow field obtained by a multiple-relaxation time lattice Boltzmann approach, we solved the advective–diffusive equation in the pore space using a high-resolution, adaptive-stencil finite-volume scheme and an operator-splitting algorithm. We show good agreement between the mass transfer rates predicted in the computational approach and previously published experimental observations. The pore-scale simulations presented in this work provide the first three-dimensional comparison to the considerable experimental work that has been performed to derive constitutive relations to quantify mass transfer from a residual NAPL to a flowing aqueous phase.     

Effect of NAPL exposure on the wettability and two‐phase flow in a single rock fracture

Surface‐wetting properties are an important cause of changing the groundwater and two‐phase fluid flows. Various factors affecting the surface wettability were investigated in a parallel‐walled glass fracture with non‐aqueous phase liquid (NAPL) (gasoline, diesel, trichloroethylene, and creosote) wetted surfaces. First, the effect of the duration of NAPL exposure on wettability change was considered at pre‐wet fracture surfaces using the various NAPL species, and the result showed that the surface became hydrophobic after the exposure time of NAPL exceeded 2000 min. Second, the initial wetting state of the surface affected the timing when the wettability change begins as well as the extent of the wettability change in an NAPL‐wetted rock fractures. Under the dry condition, the wettability change was completed within a very short time of exposure to NAPL (~5 min), and then it finally reached the intermediate and weakly NAPL wetting (contact angle of 118°). Under the pre‐wet condition, a relatively long time of exposure (~5000 min) was needed to observe the obvious change of the surface wettability, which was changed up to strongly NAPL wetting (contact angle of 142°). Third, the wettability changed by NAPL exposure was stable and maintained for a long time, regardless of water flushing rate and temperature. Finally, the wettability change by the exposure of NAPL on parallel fracture surfaces was evaluated at various groundwater flow velocities. Result showed that groundwater flow velocity has an important impact upon measured contact angle. Although fracture surfaces were exposed to NAPL at the low groundwater flow velocity, the wettability was not changed from hydrophilic to hydrophobic when the contact time between NAPL and mineral surfaces was not sufficient owing to the pulse‐type movement of NAPL. This implies that the variation of exposure pattern due to groundwater flow on the wettability change can be an important factor affecting the wettability change of fracture surface and migration behaviour at natural fractured rock aquifers in case of NAPL spill. Copyright © 2015 John Wiley & Sons, Ltd.     

DNAPL accumulation in wells and DNAPL recovery from wells: Model development and application to a laboratory study

Dense nonaqueous phase liquid (DNAPL) accumulation and recovery from wells cannot be accurately modeled through typical pressure or flux boundary conditions due to gravity segregation of water and DNAPL in the wellbore, the effects of wellbore storage, and variations of wellbore inflow and outflow rates with depth, particularly in heterogeneous formations. A discrete wellbore formulation is presented for numerical modeling of DNAPL accumulation in observation wells and DNAPL removal from recovery wells. The formulation includes fluid segregation, changing water and DNAPL levels in the well and the corresponding changes in fluid storage in the wellbore. The method was added to a three-dimensional finite difference model (CompSim) for three phase (water, gas, DNAPL) flow. The model predictions are compared to three-dimensional pilot scale experiments of DNAPL (benzyl alcohol) infiltration, redistribution, recovery, and water flushing. Model predictions match experimental results well, indicating the appropriateness of the model formulation. Characterization of mixing in the extraction well is important for predicting removal of highly soluble organic compounds like benzyl alcohol. A sensitivity analysis shows that the incorporation of hysteresis is critical for accurate prediction. Among the multiphase flow and transport parameters required for modeling, results are most sensitive to soil intrinsic permeability.     

Dependence of lumped mass transfer coefficient on scale and reactions kinetics for biologically enhanced NAPL dissolution

Residual dense non-aqueous liquids (NAPLs) in aquifers constitute a great challenge for groundwater cleanup. Active engineered treatment of regions that contain residual NAPLs is often required to shorten the long-term impact of NAPLs on groundwater quality. Enhanced residual NAPL cleanup can be achieved by promoting biodegradation of NAPL components in the aqueous phase, thereby increasing contaminant fluxes from the NAPL phase. Reaction-enhanced NAPL dissolution is often mathematically simulated under the assumption that lumped mass transfer coefficients, used to describe the dissolution behavior of the NAPL phase, are independent of the reactions. However, this assumption is not warranted because reactions occurring near the water–NAPL interface can reduce characteristic mass transfer lengths, which tend to enhance mass transfer over the no-reaction case.In this study, we mathematically investigated the connections between lumped mass transfer coefficients and reaction kinetics over an idealized residual NAPL domain. Since mass transfer is frequently a scale-dependent process, we also examined the influence of system extent on mass transfer coefficients. For our idealized domain with an assumed first-order decay reaction, the results show that lumped mass transfer coefficients depend on reaction kinetics and system scale. The mass transfer coefficient derived from the non-reactive case cannot properly represent the mass transfer process under the reactive conditions. When the advection time scale is long in comparison to the transverse dispersion time scale in the system, a fast reaction can increase significantly the lumped mass transfer coefficient. The mass transfer coefficient used for simulation was also found to be affected by the nature of the numerical scheme used.