Homogenization theory applied to soil vapor extraction in aggregated soils

Water Resources Research, 1999

... Mark N. Goltz. Department of Engineering and Environmental Management, Graduate School of Engineering, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio. Soil vapor extraction (SVE) is a technique that is frequently used to remediate unsaturated soils ...

Water Resources Research, 1994

Vapor extraction is a commonly used method for removing nonaqueous phase liquid volatile organic compounds (VOC) from the vadose zone. Experience indicates that in the absence of liquid VOC, the efficiency of vapor extraction systems decreases dramatically with time as effluent concentrations approach zero asymptotically. When such systems are restarted after a temporary shutdown, effluent concentrations are often found to recover for a short period before dropping back to preshutdown levels. This behavior is generally attributed to kinetic processes which limit the transfer of contaminant into the moving air. A numerical model is developed to simulate the rate-limited extraction of volatile compounds governed by first-order kinetic mass transfer processes. A sensitivity analysis is performed to identify model responses to various kinetic and equilibrium partitioning processes. The model is calibrated using experimental data collected from a pilot-scale experiment involving vapor extraction of trichloroethylene from fine sand. An analysis of the relationships between airflow rates and the kinetic mass transfer coefficients under various pumping schemes shows that for a given condition, increasing the flow rate has little effect beyond a certain point. It is also shown that pulsed pumping is generally less efficient than continuous pumping at a low rate. Introduction Volatile organic compounds (VOCs) are ubiquitous contaminants that have been found in the subsurface as pure liquid phase, dissolved in the soil moisture, sorbed to aquifer materials, and volatilized in soil gas. In the cleanup of contaminated sites, because of the high toxicity of VOCs essentially all of the contaminant in its various phases must be removed in order to meet drinking water standards. Experience shows that this goal can be difficult to meet. Soil vapor extraction (SVE), also known as soil venting, vapor stripping, or in situ volatilization, is an innovative technique for the removal of VOCs. SVE procedures are reviewed by Pedersen and Curtis [1991], while screening tools for the evaluation of SVE suitability are described by Johnson et al. [1988]. The technique is based on simple physical/chemical principles: By applying a vacuum, contaminated air in connected soil pores is extracted and replaced by clean atmospheric air; this perturbs the local chemical equilibrium, causing the transfer of contaminant from the residual phase, dissolved phase, and sorbed phase into the soft gas, where it can be removed. In theory, this should eventually lead to a complete cleanup. Experimental results for natural field-scale systems often show effluent concentrations below the theoretical maximum based on contaminant partial pressures [Baehr et al., 1989; Mackay et al., 1990; Rainwater et al., 1989]. The main reason for this is the nonuniform distribution of contaminants [Kueper and Frind, !988] and the lowered local permeability of zones containing pure phase residual, which will cause some of the air to bypass the residual zone. The

Water Resources Research, 1999

We present a model for the multicomponent vapor transport due to air venting in an unsaturated zone in the presence of free and trapped phases of residual nonaqueous phase liquid (NAPL). On the microscale the soil particles are assumed to form spherical aggregates with micropores filled with immobile water, trapped phases of NAPL and air. The interaggregate space is occupied with mobile air, and a thin film of free NAPL adheres on the aggregate surface. While the free NAPL can readily be in equilibrium with macropore vapor, the mass transfer from immobile phases in aggregates is rate-limited by aqueous diffusion. This model enables us to predict the vapor concentrations of various chemical species and the free NAPL saturation over the macroscale, based on the detailed understanding of the aqueous concentrations of the species and the trapped NAPL saturation within the aggregates. The model is compared favorably with some experimental data of sparging multicomponent vapor out of an intact core taken from a contaminated site. The distinctive features of multicomponent transport, clearly exhibited by the data, are further examined in the simulations of a hypothetical case of three-aromatic vapor transport under a radial flow field. It is found that while the vapor concentration of the most volatile component drops monotonically with time, those of the less volatile may rise as their mole fractions in the NAPL increase. The vapor concentration of a heavy component may have a local maximum at the evaporation front of the free NAPL. In the case of radial flow the free NAPL has two receding evaporation fronts. Condensation of the heavy component downstream of the far front causes a temporary increase of its total concentration there. With trapped NAPL and soil aggregation the macroscale transport is retarded, and the effluent concentrations end up in noticeable tailing. NG ET AL.: A MODEL FOR STRIPPING MULTICOMPONENT VAPOR [1991] and Ho et al. [1994], both using a local equilibrium model, have been able to predict the successive rise in the effluent vapor concentration of the heavier components upon depletion of the lighter ones. The temporal and spatial variations of the liquid compositions along the column were also investigated by Ho et al. [1994]. They found that the individual components propagate with separate evaporation fronts which move at a speed proportional to their vapor pressure or volatility. In addition, condensation of less volatile components was also observed to occur downstream of their evaporation fronts. These experiments suggest that in the absence of water content, the NAPL can readily equilibrate with vapor in the local pores. However, Hayden et al. [1994] and Wilkins et al. [1995] found that in columns emplaced with residual NAPL and residual water the deviation of effluent concentrations from the local equilibrium values is remarkable over extended periods of venting. The NAPL-vapor phase mass transfer appears to be rate-limited in the presence of soil moisture. Lingineni and Dhir [1997] also studied the kinetics in NAPL evaporation rate-limited by diffusion in the liquid phase. More recently, Ostendorf et al. [1997] carried out a venting experiment on an intact core taken from a site contaminated with weathered fuel. Their data clearly manifests the multicomponent effects. In particular, the vapor concentration of a heavy component may increase for some time and exhibits a maximum in the course of venting, which is in sharp contrast to the behavior of a lighter component whose concentration decreases monotonically with time, and has a smoother spatial distribution. They also obtained elution curves with appreciable tailing, a sign of departure from local equilibrium. Their findings have motivated us to develop a model which is capable of describing the vapor transport in an unsaturated medium with microscopic phase change from multicomponent residual NAPL which is not necessarily in local equilibrium with the vapor phase. In natural unsaturated soils the NAPL is often distributed into two phases: free and trapped [Parker and Lenhard, 1987].

Water Resources Research, 1997

Modeling the diffusive transport of volatile organic contaminants (VOCs) has been previously described by lumping together vastly different soil regions (e.g., interaggregate and intra-aggregate regions) and assuming local equilibrium with linear contaminant phase distributions. This approach has sometimes failed to adequately describe diffusive transport. In this work, a diffusive transport model was developed that separately considered diffusion in the intra-aggregate and interaggregate regions and utilized nonlinear contaminant distributions among the phases. Input parameters were determined from independent sources, calculations, and measurements. No adjustments were made to input parameters. The model was compared to breakthrough and desorption data for two VOCs (toluene and trichloroethylene) on a silt loam soil at moistures from 1.6 to 14%. Breakthrough predictions were significantly better than those from a commonly used model. Desorption predictions were excellent over the first 3 orders of magnitude in contaminant flux after which they deviated by a factor of-3. Background Chlorinated aliphatic hydrocarbons (CAHs) and benzene, toluene, and the xylene isomers (BTX) are frequently encountered groundwater contaminants, originating from leakage of petroleum products and degreasing solvents from underground storage tanks and transfer pipes, accidental spills, and improper waste disposal. Both CAHs and BTX have significant vapor pressures and aqueous solubilities, allowing them to be transported through any fluid phase of the soil system. Diffusive volatile organic contaminant (VOC) transport in the vadose zone can be significantly greater than advective transport under certain conditions and is often rate limiting in contaminant removal efforts. Advection of contaminants in the vadose zone occurs in response to external influences, such as vapor extraction systems, displacement due to water infiltration, significant changes in barometric pressure, and gaseous density differences. These can be intermittent, whereas diffusion occurs continuously according to localized or extensive concentration gradients. Diffusive transport in the vadose zone can be dominant when no active treatment or infiltration is occurring, for example, in an open, unconfined landfill area [Karimi et al., 1987; Lin et al., 1996]. The gas-filled volume fraction of a soil has a significant effect on contaminant diffusion. The effective diffusion coefficient Def t for benzene diffusing through moist soils has been observed to vary with the gas-filled volume fraction of the soil [Karimi et al., 1987].Johnson and Perrott [1991] noted that Def t for methane diffusion through a nearly water-saturated soil (-90% of voids filled with water) was 30 times greater than if liquid phase diffusion were the only transport mechanism. Sorption to hydrated soil surfaces from the fluid phases strongly affects contaminant diffusion. Sorption to these hy-•Now at Merck and Co., Rahway, New Jersey.

Journal of Hydrology, 2017

To greatly simplify their solution, the equations describing radial advective/dispersive transport to an extraction well in a porous medium typically neglect molecular diffusion. While this simplification is appropriate to simulate transport in the saturated zone, it can result in significant errors when modeling gas phase transport in the vadose zone, as might be applied when simulating a soil vapor extraction (SVE) system to remediate vadose zone contamination. A new analytical solution for the equations describing radial gas phase transport of a sorbing contaminant to an extraction well is presented. The equations model advection, dispersion (including both mechanical dispersion and molecular diffusion), and rate-limited mass transfer of dissolved, separate phase, and sorbed contaminants into the gas phase. The model equations are analytically solved by using the Laplace transform with respect to time. The solutions are represented by confluent hypergeometric functions in the Laplace domain. The Laplace domain solutions are then evaluated using a numerical Laplace inversion algorithm. The solutions can be used to simulate the spatial distribution and the temporal evolution of contaminant concentrations during operation of a soil vapor extraction well. Results of model simulations show that the effect of gas phase molecular diffusion upon concentrations at the extraction well is relatively small, although the effect upon the distribution of concentrations in space is significant. This study provides a tool that can be useful in designing SVE remediation strategies, as well as verifying numerical models used to simulate SVE system performance.

Transport in Porous Media, 2002

Groundwater contamination usually originates from surface contamination. Contaminants then move downward through the vadose zone and finally reach the groundwater table. To date, however, analytical solutions of multi-species reactive transport are limited to transport only in the saturated zone. The motivation of this work is to utilize analytical solutions, which were previously derived for single-phase transport, to describe the reactive transport of multiple volatile contaminants in the unsaturated zone. A mathematical model is derived for describing transport with phase partitioning of sequentially reactive species in the vadose zone with constant flow velocity. Linear reaction kinetics and linear equilibrium partitioning between vapor, liquid, and solid phases are assumed in this model.

Soil vapour extraction (SVE), is the primary method used in the world to remove volatile organic compounds (VOCs) from unsaturated subsurface porous media. The objective of this work is to study the change of the soil air relative permeability during continuous venting and removal of contaminant from a polluted soil. Once the flow starts, the contaminants will volatilize and the volume of liquid phases (Non aqueous phase liquid NAPL and water) will decreases with time accordingly as long as the venting process is continuous. A correlation that describes the evolution of the relative air permeability as a function of the gas saturation degree has been established by fitting pneumatic tests data, conducted a small-scale laboratory pilot, to an analytical gas flow model. The experimental correlation were compared to models developed previously [1,2]. A significant difference between simulated breakthrough curves, which illustrate evolution of contaminant concentration in the extracted ...

Journal of Contaminant Hydrology, 2006

The unsteady process of steam stripping of the unsaturated zone of soils contaminated with volatile organic compounds (VOCs) is addressed. A model is presented. It accounts for the effects of water and contaminants remaining in vapour phase, as well as diffusion and dispersion of contaminants in this phase. The model has two components. The first is a one-dimensional description of the propagation of a steam front in the start-up phase. This is based on Darcy's law and conservation laws of mass and energy. The second component describes the transport of volatile contaminants. Taking the view that non-equilibrium between liquid and vapour phases exists, it accounts for evaporation, transport, and condensation at the front. This leads to a moving-boundary problem. The moving-boundary problem is brought into a fixed domain by a suitable transformation of the governing partial differential equations, and solved numerically. For a broad range of the governing dimensionless numbers, such as the Henry, Merkel and Péclet numbers, computational results are discussed. A mathematical asymptotic analysis supports this discussion. The range of parameter values for which the model is valid is investigated. Diffusion and dispersion are shown to be of qualitative importance, but to have little quantitative effect in the start-up phase.

Vadose Zone Journal, 2008

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Journal of Contaminant Hydrology, 2000

The efficiency and effectiveness of soil vapor extraction SVE and bioventing BV systems for remediation of unsaturated zone soils is controlled by a complex combination of physical, Ž. chemical and biological factors. The Michigan soil vapor extraction remediation MISER model, a two-dimensional numerical simulator, is developed to advance our ability to investigate the performance of field scale SVE and BV systems by integrating processes of multiphase flow, multicomponent compositional transport with nonequilibrium interphase mass transfer, and aerobic biodegradation. Subsequent to the model presentation, example simulations of single well SVE and BV systems are used to illustrate the interplay between physical, chemical and biological processes and their potential influence on remediation efficiency and the pathways of contaminant removal. Simulations of SVE reveal that removal efficiency is controlled primarily by the ability to engineer gas flow through regions of organic liquid contaminated soil and by interphase mass transfer limitations. Biodegradation is found to play a minor role in mass removal for the examined SVE scenarios. Simulations of BV systems suggest that the effective supply of oxygen may not be the sole criterion for efficient BV performance. The efficiency and contaminant removal pathways in these systems can be significantly influenced by interdependent dynamics involving biological growth factors, interphase mass transfer rates, and air injection rates. Simulation results emphasize the need for the continued refinement and validation of predictive interphase mass transfer models applicable under a variety of conditions and for the continued