Mathematical Modeling of In Situ Vapor Stripping of Contaminated Soils
TL;DR: In this article, the feasibility of in situ vapor stripping for the removal of volatile organics from contaminated soils is evaluated using a simulation of both laboratory soil stripping columns and field-scale vacuum extraction wells.
Abstract: Mathematical models are developed for evaluating the feasibility of in situ vapor stripping for the removal of volatile organics from contaminated soils. The models simulate both laboratory soil stripping columns and field-scale vacuum extraction wells. Local equilibrium between the condensed and vapor phases is assumed for the volatile compounds; Henry's law or more complex isotherms may be used to describe this equilibrium. Methods for determining Henry's constants from lab data and for determining soil permeability from field data are presented. The models are used to predict the effects of well depth, Henry's constant, screened radius of well, and zone of influence radius. The use of impervious caps and passive vent wells to direct gas flow is discussed, and the effects of buried obstacles and evaporative cooling are examined. The possibility of recontamination from non-aqueous phase liquid (NAPL) underlying the vadose zone is investigated, as is the ability of the vapor stripping technique to remove pools of NAPL floating on the water table below the vadose zone. An estimate of the maximum time required for cleanup at a site is made.
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Patent•
23 Jul 1996
TL;DR: In this paper, a method for removing a contaminant from a treatment region of a contaminated region of the site in the earth having water therein and being contaminated with the contaminant includes heating the earth by establishing an electric field through the treatment region.
Abstract: A method for removing a contaminant from a treatment region of a contaminated region of a site in the earth having water therein and being contaminated with the contaminant includes heating the earth by establishing an electric field through the treatment region. The electric field gives rise to an electric conduction or displacement current through the treatment region. The electric current electrically heats at least a portion of the treatment region to a temperature below the boiling point of water to evaporate the water. A vacuum is drawn in a nether region of the site to collect water vapor evolved from the water and contaminant vapor evolved from the contaminant by movement of air from the surface of the earth, while the water vapor strips the contaminant from the earth. The contaminant vapor is disposed of in an innocuous manner.
30 citations
TL;DR: In this paper, a mathematical model for soil vapor stripping from porous fractured bedrock is presented, where one cannot make the assumption of local equilibrium between the stationary condensed phase and the moving vapor phase with respect to contaminant transport.
Abstract: A mathematical model for soil vapor stripping is presented which permits one to model vapor stripping from porous fractured bedrock, where one cannot make the assumption of local equilibrium between the stationary condensed phase and the moving vapor phase with respect to contaminant transport. Models for lab column operation and field operation of a vapor stripping well are presented. A lumped parameter approach is used to handle the kinetics of diffusion of the volatile contaminant from the interiors of the pieces of bedrock out to the moving soil gas. A method for estimating the time constant for this diffusion transport is presented, and the effects of the time constant on the performance of vapor stripping operations are assessed. The effects of impermeable circular caps on vapor stripping wells with and without passive vent wells are examined.
14 citations
TL;DR: In this article, the diffusivity of volatile organic compounds underlying the vadose zone in a pool of nonaqueous phase liquid (NAPL) in the vicinity of a soil vapor stripping well is modeled mathematically.
Abstract: The removal of volatile organic compounds (VOCs) underlying the vadose zone in a pool of nonaqueous phase liquid (NAPL) in the vicinity of a soil vapor stripping well is modeled mathematically. The diffusivity of the VOC is an important parameter determining the rate of removal of the NAPL; diffusivities of hexane, toluene, trichloroethylene, tetrachloroethylene, and 1,1,1-trichloroethane were determined in fine sand and found to be approximately 2 {times} 10{sup {minus}6} m{sup {minus}2}/s at 25C. Vapor stripping appears to be a practical method for the removal of NAPL floating on the water table or confined there by capillary pressure, as indicated by mathematical modeling and by bench-scale soil vapor stripping experiments. 64 refs.
10 citations
TL;DR: In this paper, a method for determining the ratio of the vertical and horizontal permeabilities of soils to air is described, which requires measurement of the gas flow rate in a vacuum well and the soil gas pressure at a suitably chosen point in the vicinity of the well.
Abstract: A method for determining the ratio of the vertical and horizontal permeabilities of soils to air is described. The method requires measurement of the gas flow rate in a vacuum well and the soil gas pressure at a suitably chosen point in the vicinity of the well. Soil gas pressures in the vicinity of a vaccum well in an anisotropic medium are calculated theoretically by using the method of images to construct a solution to Laplace's equation which satisfies the appropriate boundary conditions. Effects of well depth, depth of water table, and anisotropy are examined. A relaxation method is presented for determining soil gas pressure distributions when the permeability is both anisotropic and a function of position. It is shown that piezometer measurements in the vicinity of a vacuum well permit the location of strata of differing permeabilities.
8 citations
TL;DR: In this article, a mathematical model for simulating the operation of soil vapor stripping wells for the removal of volatile contaminants is extended to examine the possible benefits of using passive vent wells to increase clean-up rates.
Abstract: A mathematical model for simulating the operation of soil vapor stripping wells for the removal of volatile contaminants is extended to examine the possible benefits of using passive vent wells to increase clean-up rates. Contrary to expectation, these do not generally seem to improve soil vapor stripping performance. The model is also used to examine recontamination of a vapor stripped vadose zone by vaporization and diffusion of underlying nonaqueous phase liquid and to investigate the possibility of removal of underlying light nonaqueous phase liquid pools by vapor stripping from the vadose zone.
8 citations
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TL;DR: In this paper, a simple model for predicting the solubilization behavior of surfactants is developed and tested experimentally, and the recovered surfactant solution satisfactorily solubils biphenyl.
Abstract: Data on the solubilization of p-dichlorobenzene (DCB), naphthalene, and biphenyl in aqueous solutions of sodium dodecylsulfate (SDS) (0-100 mM concentration) indicate increases in effective solubilities of these hydrophobic compounds by factors of roughly 20 to 100. DCB is effectively removed from spiked clay-sand mixtures by leaching with SDS solutions in laboratory columns. Surfactant solutions loaded with DCB are satisfactorily treated by gentle extraction with hexane, and the recovered surfactant solution satisfactorily solubilizes biphenyl. A simple model for predicting the solubilization behavior of surfactants is developed and tested experimentally.
65 citations
TL;DR: In this article, the effect of an anisotropic Darcy's constant is examined and the compressibility of the extracting gas is taken into account in evaluating the feasibility of in-situ vapor stripping.
Abstract: Mathematical models are developed suitable for use in evaluating the feasibility of in-situ vapor stripping approaches for selected chemicals and site-specific environments. These models simulate the operation of both laboratory soil stripping columns and field-scale vacuum extraction wells (vent pipes). The effect of an anisotropic Darcy's constant is examined and the compressibility of the extracting gas is taken into account. The models incorporate the assumption of local equilibrium for the volatile compounds between the condensed and vapor phases. These models may use Henry's law or more complex isotherms for this equilibrium. A method is developed for calculating Henry's constant from field analytical data, and it is noted that use of Henry's constants calculated from laboratory data on solutions of volatile solutes in pure water can lead to very serious errors. It is shown that evacuation wells should be screened only down near the impermeable layer beneath the zone of stripping (unsaturat...
58 citations
TL;DR: In this article, a mathematical model for describing the solubilization of various types of contaminants in micelles of ionic and nonionic surfactants is described, and it is shown that the concentration of contaminant solubiliized is a linear function of the total surfactant concentration provided that this is above the critical micelle concentration of the surfactent, in agreement with experimental results.
Abstract: Mathematical models for describing the solubilization of various types of contaminants in micelles of ionic and nonionic surfactants are described. Contaminants which are purely hydrophobic compounds and those which are amphipathic are handled, and electrical effects are described by means of a Debye-Huckel theory approach. It is found that the concentration of contaminant solubilized is a linear function of the total surfactant concentration provided that this is above the critical micelle concentration of the surfactant, in agreement with experimental results.
25 citations
TL;DR: In this article, the effects of a gas-impervious cap on the velocity field of the moving soil gas are modeled and compared to the effects that a gas cap can have on soil vapor stripping.
Abstract: The clean up of soils contaminated by volatile compounds by in-situ vapor stripping was recently modeled by Wilson, Clarke, and Clarke. Their approach is modified to include the effects of a gas-impervious cap on the velocity field of the moving soil gas. Calculations indicate that such caps reduce the excessive flow of gas in the vicinity of the axis of the cylindrical volume of influence of a vent pipe, and they increase gas velocities near the periphery of the volume of influence. One thus expects use of impervious caps to improve the efficiency of in-situ soil vapor stripping: modeling of contaminant removal with such modified gas flow fields shows that this is indeed the case. Modeling of gas flow around buried obstacles indicates that these are not likely to interfere seriously with soil vapor stripping; some strategies are suggested to reduce their effects. The soil vapor stripping model is used to show that low soil permeabilities can be compensated for by increasing the radius of the str...
15 citations
TL;DR: In this article, a mathematical model for simulating the operation of soil vapor stripping wells for the removal of volatile contaminants is extended to examine the possible benefits of using passive vent wells to increase clean-up rates.
Abstract: A mathematical model for simulating the operation of soil vapor stripping wells for the removal of volatile contaminants is extended to examine the possible benefits of using passive vent wells to increase clean-up rates. Contrary to expectation, these do not generally seem to improve soil vapor stripping performance. The model is also used to examine recontamination of a vapor stripped vadose zone by vaporization and diffusion of underlying nonaqueous phase liquid and to investigate the possibility of removal of underlying light nonaqueous phase liquid pools by vapor stripping from the vadose zone.
8 citations