Nonaqueous Phase Liquid Dissolution Research Articles

Enhanced mass transfer of residual NAPL by convection in stagnant zone

Cosolvent flushing is a promising method to improve NAPLs (non-aqueous phase liquids) removal. Nevertheless, the removal performance is undermined by the tailing phenomenon, i.e., the contaminant concentration tends to remain unchanged in later period as NAPL is residually trapped in the stagnant zone. High proportion of stagnant zone leads to more severe tailing phenomenon and how to accelerate dissolution of NAPL in the stagnant zone is important for NAPL remediation. However, few work has studied the remediation process at pore-scale due to limited observation techniques. As a result, the fundamental process of convection enhancement remains unclear. In this work, dynamics of NAPL removal in the stagnant zone is quantitatively investigated in transparent micromodels via the use of Raman spectroscopy. Impacts of injection condition and properties of the porous media are systematically investigated. Results indicate that convection dominants the mass transfer in aqueous phase and enhances the NAPL dissolution. Increasing the flushing velocity can enhance convection, but when the flushing velocity exceeds 3 cm/min, convection in the region away from the dominant channel is little affected by the flushing velocity. Differently, increasing the pore throat size will enhance convection in the entire stagnant zone and accelerate the whole dissolution process.

Modeling of Complex, Multi‐Component NAPL Remediation for Decision Support

AbstractIn situ remediation of nonaqueous phase liquid (NAPL)‐impacted sites is difficult and costly. Even with enhancements (e.g., chemical and thermal) and partial NAPL recovery, mass transfer constraints associated with partitioning from residual NAPL typically control the depletion rate of sources and attainment of cleanup goals. Practical methodologies are needed to support strategic evaluation, planning, and implementation of cost‐effective remedial approaches that are considered protective. A modeling system averaging over the NAPL‐impacted saturated soil volume was developed and demonstrated at the former Williams Air Force Base. The system combines upscaled, physically based mass transfer coefficients for multi‐component NAPL dissolution with theoretical enhancements specific to multiple remediation processes. Dissolution increases in the presence of aqueous phase reactions, according to first‐ or second‐order kinetics, by increasing concentration gradients. Slow biological processes are considered first order in modeling natural attenuation and enhanced biological degradation. Fast reactions associated with chemical oxidation are considered second order. These enhancement models are equally applicable to numerical simulations of NAPL remediation. Pump and treat enhances dissolution in proportion to increases in the characteristic velocity associated with dissolution. The demonstration yielded realistic predictions, with greater certainty, for outcomes from of multiple technologies intended to reduce remedial timeframes and life cycle costs. The enhanced dissolution modeling provides a site‐specific, quantitative assessment of changes in NAPL source discharge concentration and mass discharge over time for various remedial options that is equivalent to assessments from complex numerical transport models, given typical input data limitations.

Numerical Modeling and Data‐Worth Analysis for Characterizing the Architecture and Dissolution Rates of a Multicomponent DNAPL Source

AbstractA numerical solute transport model was calibrated to a high‐resolution monitoring data set to characterize a multicomponent source of nonaqueous phase liquids (NAPLs) and evaluate the uncertainty of estimated parameters. The dissolution of NAPL mass was simulated using SEAM3D with parameter zones including adjustable NAPL saturations and mass transfer coefficients, representing the heterogenous architecture of the source zone. Source zone parameters were simultaneously estimated using PEST from aqueous‐phase concentrations measured in a multilevel monitoring transect and from mass recovery rates measured at extraction wells during a controlled field experiment. Data‐worth analyses, facilitated by PEST ancillary software, linked maximum aqueous‐phase concentrations of all compounds to reductions in the pre‐calibration uncertainty of mass transfer coefficients. In turn, decreasing concentrations of the most soluble NAPL fraction constrained the source mass estimation. The accurate estimation of model parameters was possible by removing concentrations measured during early NAPL dissolution stages, identified as drivers of model bias using the iterative ensemble smoother PESTPP‐iES. Although uncertainty analyses highlighted model limitations for representing sub‐grid‐scale heterogeneity of NAPL distribution and mass transfer rates, final stages of NAPL dissolution measured at multilevel ports eliminated parameter bias and produced long‐term projections of multi‐stage source zone depletion. Including mass discharge rates for model calibration further improved the accuracy of estimated residual source mass, complementing multilevel monitoring constraints on the saturation distribution and mass transfer coefficients.

Modeling Dissolution of Soluble Compounds from Multi-Component NAPL Using a Desorption Approximation.

Groundwater professionals require methods to estimate the potential time required to achieve remedial goals, including locations within and downgradient of zones containing non-aqueous phase liquids (NAPLs). NAPLs have long been recognized as persistent contaminant sources to groundwater. Dissolution of multi-component NAPLs is particularly complex, and numerical models that explicitly simulate it are not widely available. This study introduces an equilibrium partitioning approximation to simulate the dissolution of the most soluble chemical components from multi-component NAPL containing a significant fraction of relatively insoluble mass. The effective distribution coefficient that describes depletion of a specific compound from NAPL is estimated based on the properties of the NAPL and the porous medium. This study also presents numerical modeling results that support the utility of the method, with verification using published empirical data collected during dissolution of residual coal tar in a controlled laboratory sand tank experiment. The numerical modeling method uses equilibrium partitioning as an approximation and matched the concentrations of the two most soluble NAPL components in and downgradient of the NAPL zone with reasonable accuracy. The results suggest that the method should be useful for screening level assessments and can be adapted to compare relative groundwater restoration timeframes of select NAPL components for various remedial alternatives.

A review on dissolution of multi-component non-aqueous phase liquids: recent studies, mechanisms and mass transfer limitations

A review on dissolution of multi-component non-aqueous phase liquids: recent studies, mechanisms and mass transfer limitations

Forward and back diffusion of reactive contaminants through multi-layer low permeability sediments

Contaminants stored in the low permeability sediments will continue to threaten the adjacent shallow groundwater system after the aquifer is remediated. Understanding the storage and discharge behavior of contaminants in the aquitards is essential for the efficient remediation of contaminated sites, but most of the previous analytical studies focused on nonreactive solutes in a single homogenous aquitard. This study presents novel analytical solutions for the forward and back diffusion of contaminants through multi-layer low permeability sediments considering abiotic and biotic environmental degradation. Three representative source depletion patterns (i.e., instantaneous, linear, and exponential patterns) were selected to describe the dissolution of dense non-aqueous phase liquids (DNAPL) in the aquifer more realistically. At the forward diffusion stage, the mass storage of contaminants in the aquitards with the instantaneous pattern is the largest, nearly twice that with the exponential pattern. A simple equivalent homogeneous model is generally adopted in the risk assessment. However, relative to the proposed multi-layer model, it will significantly underestimate the onset of the back-diffusion of heterogeneous aquitards and overestimate the persistence of aquifer plumes. The previously-reported semi-infinite boundary assumption is also not applicable, with a maximum error of over 200% in the long-term prediction of back diffusion behavior of a thin aquitard. Moreover, when the degradation half-life is less than 16 years, less than 10% of the contaminants stored in the aquitards will diffuse into the overlying aquifer, suggesting that biostimulation or bioaugmentation can effectively mitigate back-diffusion risk. Overall, the proposed diffusion-reaction coupled model with multi-layer media is of great value and high demand in predicting the back-diffusion behavior of heterogeneous aquitards and guiding the soil bioremediation.

Application of a Developed Numerical Model for Surfactant Flushing Combined with Intermittent Air Injection at Field Scale

Surfactant flushing with intermittent air injection, referred to as enhanced flushing, has been proposed at a site in Korea contaminated by military activity to overcome the difficulty of treatment caused by a layered geological structure. In this study, we developed a simple numerical model for exploring the effects of various physical and chemical processes associated with enhanced flushing on pollutant removal efficiency and applied it in a field-scale test. This simple numerical model considers only enhanced hydraulic conductivity rather than all of the interacting parameters associated with the complex chemical and physical processes related to air and surfactant behavior during enhanced flushing treatment. In the numerical experiment, the removal efficiency of residual non-aqueous phase liquid (NAPL) was approximately 12% greater with enhanced, rather than conventional, flushing because the hydraulic conductivity of the low-permeability layer was enhanced 5-fold, thus accelerating surfactant transport in the low-permeability layer and facilitating enhanced dissolution of residual NAPL. To test whether the enhanced flushing method is superior to conventional flushing, as observed in the field-scale test, successive soil flushing operations were simulated using the newly developed model, and the results were compared to field data. Overall, the simulation results aligned well with the field data.

Temperature influence on the NAPL-water interfacial area between 10 °C and 60 °C for trichloroethylene

Underground thermal energy storage (UTES) can contribute to renewable energy usability, especially in urban areas with the most demand and available infrastructure. But UTES may interact in those areas with non-aqueous phase liquids (NAPL) by increasing the temperature in storage formations. To determine temperature effects on NAPL dissolution rates into groundwater, the effective specific interfacial area (anw) between trichloroethylene (TCE) and water, as a function of temperature and TCE pore saturations, was calculated. The interfacial tension between the flushing solution and the NAPL, the adsorption coefficient and the retardation of a reactive tracer were determined by the drop weight method and interfacial tracer tests at 10 °C, 30 °C and 60 °C. From 10 to 60 °C anw increased by a factor of six to eight. Based on the results, a function to describe the anw between TCE and water was developed, which could improve numerical models on NAPL dissolution rates. The main mechanisms for the increase in anw are suggested to be NAPL blob migration on pore scale, and thermal-induced changes in wettability and in blob shape correlating with a temperature-induced decline in effective porosity of up to 32%. These results contribute to the understanding and predictability of UTES in contaminated aquifers, the general response of NAPL behavior on artificial increasing aquifer temperatures and the improvement of thermal and other groundwater remediation techniques.

Dissolution and remobilization of NAPL in surfactant-enhanced aquifer remediation from microscopic scale simulations

In this paper, the dissolution and mobilization of non-aqueous phase liquid (NAPL) blobs in the Surfactant-Enhanced Aquifer Remediation (SEAR) process were upscaled using dynamic pore network modeling (PNM) of three-dimensional and unstructured networks. We considered corner flow and micro-flow mechanisms including snap-off and piston-like movement for two-phase flow. Moreover, NAPL entrapment and remobilization were evaluated using force analysis to develop the capillary desaturation curve (CDC) and predict the onset of remobilization. The corner diffusion mechanism was also applied in the modeling of interphase mass transfer to represent NAPL dissolution as the dominant mass transfer process. In addition, the effect of pore-scale heterogeneity on mass transfer rate coefficient and recovered residual NAPL was considered in the simulations. Sodium dodecyl sulfate (SDS) and Triton X-100 were used as the surfactant for the SEAR process. The results indicate that although surfactants enhance NAPL recovery during two-phase flow, surfactant-enhanced remediation of residual NAPL through dissolution is highly dependent on surfactant type. When SDS ─as a surfactant with high critical micelle concentration (CMC) and low micelle partition coefficient (Km)─ was injected into a NAPL contaminated site, the mass transfer rate coefficient decreased (due to considerable changes in interface chemical potentials) which leads to a significant reduction in NAPL recovery after the end of two-phase flow. In contrast, Triton X-100 (with low CMC and high Km) improved NAPL recovery, by enhancing solubility at surfactant concentrations greater than CMC which overcompensates the interphase mass transfer reduction.

Pore-scale investigation of wettability impact on residual nonaqueous phase liquid dissolution in natural porous media

Understanding the dissolution behavior of residual nonaqueous phase liquids (NAPLs) in soil and aquifer is essential for remediation activities and related prediction models. In this study, the impact of wettability of pore surfaces on the residual NAPL dissolution was investigated in natural porous media using X-ray microtomography. Three-dimensional pore-scale characteristics of NAPL blobs were determined in water-wet and neutral-wet media. Time-resolved images enable to estimate the temporal changes in the residual saturation and interfacial area, then to calculate the mass transfer coefficient. The results showed that the wettability has a significant impact on the morphology of residual NAPL; thus, affecting the dissolution behavior in natural porous media. In the neutral-wet system, residual NAPL blobs are trapped in small and large pores, resulting in a uniform distribution and a higher specific total interfacial area. The NAPL dissolution rate in the neutral-wet media is faster due to the higher specific interfacial area and more uniform spatial distribution. Considering the total interfacial area as the active mass transfer place, the wettability has a slight effect on the mass transfer coefficient. The mass transfer coefficient is independent of the residual saturation, whereas the lumped mass transfer coefficient decreases significantly during the dissolution process. Under different flow rates, the experimental data of Sherwood number Sh and modified Sherwood number Sh' could be well fitted with Sh ~ Ren and Sh′ ~ Renθn. These results are essential because the effect of wettability on dissolution was first studied from the perspective of pore scale, and the prediction models will help optimize the remediation strategies.

Pore-scale investigation on nonaqueous phase liquid dissolution and mass transfer in 2D and 3D porous media

Fluid–fluid interphase mass transfer in porous media plays an important role in the remediation of soil and groundwater contaminated by nonaqueous phase liquid (NAPL). In this study, we experimentally elucidated the pore-scale dissolution process and the macroscopic interphase mass transfer coefficient inside the porous media. Further, the local characteristics of the residual phase, including saturation and interfacial area, were determined during the dissolution process using nondestructive visualization technologies. The dynamic dissolution process indicated that not all blobs are equally exposed to flowing water and the dead-end pores considerably decreased the dissolution rate. According to the linear driving force model, the NAPL concentration in mobile water was predicted from the residual saturation. Further, the local and overall mass transfer coefficients corrected with concentration and interfacial area were estimated. The results showed that the NAPL concentration in mobile water increased along the water injection direction because of the NAPL dissolved in water. The local mass transfer coefficient exhibited a uniform distribution along the sample, indicating that the mass transfer coefficient is independent of the concentration difference. The effect of pore structure on the dissolution process of entrapped NAPL was studied by comparing a two-dimensional (2D) micromodel and a three-dimensional (3D) packed bed. The overall mass transfer coefficient was higher in the 3D packed bed under Darcy flow conditions. The major differences between the 3D packed bed and 2D micromodel can be attributed to the heterogeneity of pore geometry and the differences in pore connectivity.

Bioenhanced back diffusion and population dynamics of Dehalococcoides mccartyi strains in heterogeneous porous media

Diffusion, sorption-desorption, and biodegradation influence chlorinated solvent storage in, and release (mass flux) from, low-permeability media. Although bioenhanced dissolution of non-aqueous phase liquids has been well-documented, less attention has been directed towards biologically-mediated enhanced diffusion from low-permeability media. This process was investigated using a heterogeneous aquifer cell, packed with 20–30 mesh Ottawa sand and lenses of varying permeability (1.0 × 10−12-1.2 × 10−11 m2) and organic carbon (OC) content (<0.1%–2%), underlain by trichloroethene (TCE)-saturated clay. Initial contaminant loading was attained by flushing with 0.5 mM TCE. Total chlorinated ethenes removal by hydraulic flushing was then compared for abiotic and bioaugmented systems (KB-1® SIREM; Guelph, ON). A numerical model incorporating coupled diffusion and (de)sorption facilitated quantification of bio-enhanced TCE release from low-permeability lenses, which ranged from 6% to 53%. Although Dehalococcoides mccartyi (Dhc) 16S rRNA genes were uniformly distributed throughout the porous media, strain-specific distribution, as indicated by the reductive dehalogenase (RDase) genes vcrA, bvcA, and tceA, was influenced by physical and chemical heterogeneity. Cells harboring the bvcA gene comprised 44% of the total RDase genes in the lower clay layer and media surrounding high OC lenses, but only 2% of RDase genes at other locations. Conversely, cells harboring the vcrA gene comprised 50% of RDase genes in low-permeability media compared with 85% at other locations. These results demonstrate the influence of microbial processes on back diffusion, which was most evident in regions with pronounced contrasts in permeability and OC content. Bioenhanced mass transfer and changes in the relative abundance of Dhc strains are likely to impact bioremediation performance in heterogeneous systems.

Experimentally based pore network modeling of NAPL dissolution process in heterogeneous porous media

Practical designs of non-aqueous phase liquids (NAPLs) remediation strategies require reliable modeling of interphase mass transfer to predict the retraction of NAPL during processes such as dissolution. In this work, the dissolution process of NAPL during two-phase flow in heterogeneous porous media is studied using pore-network modeling and micromodel experiments. A new physical-experimental approach is proposed to enhance the prediction of the dissolution process during modeling of interphase mass transfer. In this regard, the normalized average resident solute concentration is evaluated for describing the dissolution process at pore-level. To incorporate the effect of medium heterogeneities, a new experimental factor is considered for enhancing corner diffusion modeling. In addition, capillary desaturation curves (CDCs) are predicted during hydraulic flow modeling to estimate initial residual NAPL saturation. The developed network model can predict residual NAPL saturations and mass transfer rate coefficient for a NAPL-water system at different injection rates and fluid saturations. The evaluated mass transfer rate coefficients using the proposed physical-experimental approach show a significant improvement compared to either mechanistic or empirical methods. The proposed approach in this study can be attractive for possible applications in commercial simulators of contaminant transport in porous media.

Effects of Pore‐Scale Heterogeneity on Macroscopic NAPL Dissolution Efficiency: A Two‐Scale Numerical Simulation Study

AbstractInterphase mass transfer or dissolution coefficient of nonaqueous phase liquids (NAPL) is an important parameter in predicting the transport of contaminant species in porous media. While the literature offers valuable insights into the dependence of this coefficient on different parameters at the continuum scale (e.g., contaminant saturation and Darcy velocity), effects of pore‐scale heterogeneity on macroscopic dissolution coefficient have received little attention. In this work a three‐dimensional pore‐scale model is developed to simulate interphase mass transfer over different synthetic pore network structures with various pore radii correlation lengths. The pore network modeling simulates dissolution of immobile NAPL into water (single phase) through diffusive throats for the water‐NAPL interface. The impacts of pore network spatially correlated heterogeneities, NAPL saturation/distribution, and aqueous phase velocity on NAPL mass transfer coefficient and water‐NAPL interfacial surface area are studied. These macroscopic properties are then employed in two‐dimensional continuum‐scale domains formed by concatenating 20 by 20 pore networks in x and y directions. The results highlight the impact of pore‐scale heterogeneity on the distribution of NAPL and subsequently on the dissolution rate (i.e., dissolution coefficient). An uncorrelated distribution of pore radii consistently leads to higher NAPL dissolution coefficient than spatially correlated heterogeneity. The results of continuum modeling show that NAPL dissolution rates are only different between domains formed by correlated and uncorrelated pore networks at very high flow rates and Darcy velocities. However, for typical values of Darcy velocity in groundwater systems, variation in mass transfer coefficient due to pore‐scale heterogeneity is minimal for efficient mass removal.

In Situ Persulfate Oxidation of 1,2,3-Trichloropropane in Groundwater of North China Plain.

In situ injection of Fe(II)-activated persulfate was carried out to oxidize chlorinated hydrocarbons and benzene, toluene, ethylbenzene, and xylene (BTEX) in groundwater in a contaminated site in North China Plain. To confirm the degradation of contaminants, an oxidant mixture of persulfate, ferrous sulfate, and citric acid was mixed with the main contaminants including 1,2,3-trichloropropane (TCP) and benzene before field demonstration. Then the mixed oxidant solution of 6 m3 was injected into an aquifer with two different depths of 8 and 15 m to oxidize a high concentration of TCP, other kinds of chlorinated hydrocarbons, and BTEX. In laboratory tests, the removal efficiency of TCP reached 61.4% in 24 h without other contaminants but the removal rate was decreased by the presence of benzene. Organic matter also reduced the TCP degradation rate and the removal efficiency was only 8.3% in 24 h. In the field test, as the solution was injected, the oxidation reaction occurred immediately, accompanied by a sharp increase of oxidation–reduction potential (ORP) and a decrease in pH. Though the concentration of pollutants increased due to the dissolution of non-aqueous phase liquid (NAPL) at the initial stage, BTEX could still be effectively degraded in subsequent time by persulfate in both aquifers, and their removal efficiency approached 100%. However, chlorinated hydrocarbon was relatively difficult to degrade, especially TCP, which had a relatively higher initial concentration, only had a removal efficiency of 30%–45% at different aquifers and monitoring wells. These finding are important for the development of injection technology for chlorinated hydrocarbon and BTEX contaminated site remediation.

The complex spatial distribution of trichloroethene and the probability of NAPL occurrence in the rock matrix of a mudstone aquifer

Methanol extractions for chloroethene analyses are conducted on rock samples from seven closely spaced coreholes in a mudstone aquifer that was subject to releases of the nonaqueous phase liquid (NAPL) form of trichloroethene (TCE) between the 1950's and 1990's. Although TCE concentration in the rock matrix over the length of coreholes is dictated by proximity to subhorizontal bedding plane fractures, elevated TCE concentrations in the rock matrix are not continuous along the most permeable bedding plane fractures. A complex configuration of subvertical and subhorizontal fractures appears to be responsible for the TCE distribution from prior TCE releases at land surface. Phase partitioning calculations of TCE in the rock matrix show that most TCE is adsorbed to solid surfaces because of the large fraction of organic carbon (foc) in the mudstone. Large TCE content in some cores indicate the likely presence of the NAPL form of TCE in the rock matrix. Using average values of porosity (n) and foc in phase partitioning calculations identifies a number of locations of possible NAPL occurrence in the rock matrix. Samples of mudstone analyzed for n and foc show variability in these properties over several orders of magnitude. Accounting for this variability in phase partitioning calculations identifies a probability of NAPL occurrence, PNAPL. The spatial variability of PNAPL along coreholes identifies a configuration that may be attributed to a TCE source zone that has evolved after emplacement due to NAPL dissolution, adsorption, and matrix diffusion.

Interaction between biofilm growth and NAPL remediation: A pore-scale study

In this paper, we introduce a pore-scale model to study the interaction between biofilm growth and non-aqueous-phase-liquid (NAPL) dissolution. Liquid flow and dissolved NAPL transport are coupled with a biofilm growth model to correctly describe the complex dynamics of the processes including fluid flow, NAPL dissolution/biodegradation and biofilm growth. Fluid flow is simulated using an immersed boundary-lattice Boltzmann (IB-LB) model; while solute transport is solved by a cut-cell finite volume method (FVM). A uniform dissolution approach is also adopted to capture the temporal evolution of trapped blobs. Spatio-temporal distributions of the biomass are investigated using a cellular automaton algorithm combined with the immersed boundary method (IBM). Simulations focused on NAPL dissolution in both abiotic and biotic conditions are conducted to assess the capability of the model. In abiotic conditions, we analyze the effects of the hydrodynamic regimes and the spatial distribution of NAPL blobs on the dissolution rate under different assumptions (i.e., blob size and Péclet number). In biotic conditions, a series of impact factors are also investigated (i.e., spatial distribution, reaction kinetics and NAPL-induced toxicity). Finally, the current model is used to evaluate the pore scale relevance of a local equilibrium assumption between fluid phase and biofilm phase in the vicinity of the NAPL source.

Numerical modelling of the impact of surfactant partitioning on surfactant-enhanced aquifer remediation

The partitioning of surfactants into non-aqueous phase liquids (NAPLs) during Surfactant-Enhanced Aquifer Remediation (SEAR) is potentially an important and non-negligible phenomenon that can strongly impact remediation efficiency. This paper numerically investigates the impact of surfactant partitioning on the enhanced NAPL dissolution and mobilization mechanisms and the overall NAPL removal from the subsurface. For demonstration, a multiphase model is used to simulate a hypothetical SEAR consisting of Triton X100 surfactant solution for the removal of perchloroethylene (PCE) entrapped in contaminated porous medium at the core/column scale. The simulations are conducted for two-dimensional homogenous and three-dimensional heterogeneous systems. By simultaneously incorporating spatial heterogeneity of porous media, injection rate, and endpoint mobility ratio into the model, we delineate the interplay of surfactant partitioning with flow and transport dynamics. Our results show that surfactant partitioning from the aqueous phase across the interface to the NAPL phase can undermine both efficiency of the enhanced dissolution and mobilization of NAPL species. This undermining is more pronounced for when aqueous phase mobility is less than the mobility of the NAPL phase. For such conditions interfacial tension between the two phases is reduced less for partitioning than non-partitioning cases (due to loss of surfactant into NAPL phase) and a secondary water front is formed due to partitioning that makes aqueous phase breaks through earlier.

An empirical model for the evaluation of the dissolution rate from a DNAPL-contaminated area.

This paper investigates dynamic variation in the morphologic distribution of dense non-aqueous phase liquids (DNAPLs), which take into account the coupled mass transfer. Experiments were carried out in a 2D tank representing a reconstructed aquifer model. DNAPL dissolution rates were investigated over a wide range of DNAPL saturations, several source configurations, and different hydraulic conditions. Morphometric indexes are presented that take into consideration further factors affecting the dissolution process. Local information regarding transport parameters related to the characteristics of the medium was obtained through a neural network and an optimization algorithm applied to experimental tracer tests. The history of DNAPL source architecture, in terms of saturation, indentation grade, and orientation, was determined by image analysis. Dissolved concentrations were registered and mass transfer rate coefficients were obtained for a wide range of source-zone configurations. A statistical analysis was performed to develop a constitutive equation that is descriptive of the mass transfer rate as a function of source-zone metric characteristics. A new empirical dissolution model using the proposed morphometric parameters is presented and compared with other models. The mass transfer correlation reported incorporates morphometric parameters and considers the complex and variable architecture of non-miscible contaminants. The proposed correlation can be used for an initial assessment of non-aqueous phase liquid (NAPL) dissolution rates over a wide range of saturation (residual and non-residual) conditions and different aqueous phase velocities within the NAPL source zone.

Comparison of theory and experiment for NAPL dissolution in porous media

Contamination of groundwater resources by an immiscible organic phase commonly called NAPL (Non Aqueous Phase Liquid) represents a major scientific challenge considering the residence time of such a pollutant. This contamination leads to the formation of NAPL blobs trapped in the soil and impact of this residual saturation cannot be ignored for correct predictions of the contaminant fate. In this paper, we present results of micromodel experiments on the dissolution of pure hydrocarbon phase (toluene). They were conducted for two values of the Péclet number. These experiments provide data for comparison and validation of a two-phase non-equilibrium theoretical model developed by Quintard and Whitaker (1994) using the volume averaging method. The model was directly upscaled from the averaged pore-scale mass balance equations. The effective properties of the macroscopic model were calculated over periodic unit cells designed from images of the experimental flow cell. Comparison of experimental and numerical results shows that the transport model predicts correctly - with no fitting parameters - the main mechanisms of NAPL mass transfer. The study highlights the crucial need of having a fair recovery of pore-scale characteristic lengths to predict the mass transfer coefficient with accuracy.