Uncertainties in cleanup times for soil vapor extraction

Abstract

Abstract. Uncertainties in cleanup times for soil vapor extraction systems are estimated using mass removal curves generated through Monte Carlo analyses. The primary source of uncertainty is assumed to be soil permeability. Four general model components are coupled to quantify uncertainties: (1) a geological uncertainty model, (2) a spill distribution model, (3) a vapor flow model, and (4) a mass removal model. Spatial variability in permeability has a more significant effect on cleanup time than does overall average permeability because of the effects of channeling. For the examples considered in this study, field measurements of air conductivity have a relatively small effect on uncertainties in cleanup time. The examples also suggest that surface covers can significantly reduce cleanup time in cases where the extraction wells are not immediately within the zone of contamination and for relatively large spills. If wells are completed within the contaminated zone, however, these covers can actually increase average cleanup times. IntroductionSoil vapor extraction has become the most common innovative technology for treating subsurface soils contaminated with volatile and semivolatile organic compounds [U.S. Environmental Protection Agency, 1992]. This popularity is due, in part, to the low cost of vapor extraction relative to other available technologies, especially when contamination occurs relatively deep below the ground surface. Vapor extraction systems are also attractive because mitigation is completed in situ. This reduces the exposure of on-site workers and the off-site public to chemical contaminants. Vapor extraction also offers considerable flexibility in terms of installation and operation. This flexibility allows systems to be installed at crowded locations with existing structures, roadways, and other facilities and allows systems to be readily adjusted during the course of remediation to improve mass removal efficiency.Vapor extraction involves two major physical processes: mass transfer and mass transport. Mass transfer is the movement from one phase to another at a particular location. Volatile and semivolatile compounds will enter the vapor phase by desorption from the soil particles, through volatilization from the soil water, and by evaporation from nonaqueous phase liquids such as gasoline or liquid solvents. The rate at which this mass transfer occurs depends upon subsurface temperature, humidity, and pressure, the properties of the contaminant including vapor pressure and solubility, and the sorptive properties of the soil.The second major process involved in vapor extraction is mass transport, which is the movement of vapor from one location to another. This transport, which is primarily due to Copyright 2000 by the American Geophysical Union. Paper number 1999WR900305.0043-1397/00/1999WR900305509.00 advection, is caused by pressure gradients that are developed using extraction wells or trenches. In some instances, mass transport is enhanced through passive or active injection wells and...

Organic contaminant transport and fate in the subsurface: Evolution of knowledge and understanding

“…Optimization methods have been used to help design multiwell vapor extraction systems [Sun et al, 1996] and combined LNAPL skimming and vapor extraction systems [Qin et al, 2008]. Travel times to vapor extraction wells are influenced by permeability anisotropy [Shan et al, 1992], and cleanup times increase with increased spatial variability in permeability because channeled air flow reduces air-water interaction [Massmann et al, 2000]. Soil vacuum extraction has limited effectiveness for saturated zone NAPL remediation [Sleep and Sykes, 1993b].…”

Characterization and Remediation of Chlorinated Volatile Organic Contaminants in the Vadose Zone

“…The results of multiple investigations have demonstrated the impact of material‐property heterogeneity (e.g., spatially variable permeability), nonuniform contaminant distributions, and associated diffusion‐limited mass transfer on vapor‐phase mass removal (e.g., Johnson et al, 1990; Brusseau, 1991; Kearl et al, 1991; Silka et al, 1991; DiGiulio, 1992; Benson et al, 1993; Johnson and Ettinger, 1994; Rodriguez‐Maroto et al, 1994; Poulsen et al, 1996; Popovicova and Brusseau, 1997; DiGiulio et al, 1998; Kaleris and Croise, 1999; Massmann et al, 2000; Switzer et al, 2004; Stauffer et al, 2007; Switzer and Kosson, 2007). Gas will flow preferentially through zones of higher air permeability, effectively bypassing lower permeability regions.…”

Soil vapor extraction of chlorinated solvents at an industrial site in Brazil

“…It is known that the time required to clean the site using vapor extraction technology depends not only on the amount, nature and distribution of contaminants in the subsurface, but also upon the rates at which mass transfer and mass transport occur [3]. The spatial distribution of the intrinsic soil permeability exerts great influence on the performance of vapor extraction operations as well, mainly in the vicinity of the infiltrated NAPL.…”