This section provides a synopsis of the underlying principles governing the volatilization of contaminants in the vadose zone, and it is intended for readers who have practically no knowledge of soil vapor extraction (SVE) technology. It is not intended to be a thorough description of the underlying principles but rather a brief characterization of the basic principles so that one can understand the subsequent sections in this overview of SVE. Thorough descriptions of the underlying principles are provided in Chapter 4 and reported in a paper by Johnson et al. (1990a), an overview chapter of a review report by Jordan et al. (1995), and in Holbrook et al. (1998).

The primary goal of soil remediation by vapor extraction is the volatilization of contaminants. Remediation using SVE is achieved by inducing the flow of surface air through the contaminated zone, such as depicted at a grain scale in Figure 1, for the purpose of extracting the contaminant-laden vapors and promoting vaporization/volatilization and subsequent removal of liquid, dissolved, and sorbed contaminants.

The situation depicted in Figure 1 can occur wherever air flow can be maintained in the subsurface. Subsurface air flow is induced in a manner analogous to pumping groundwater: vacuum blowers (analogous to pumps) are used to reduce pressures in extraction vents (analogous to wells but completed in the vadose zone). Surface air, being at a higher pressure, flows towards the lower pressure in the extraction vents. Subsurface flow could likewise be induced by injecting air under pressures greater than atmospheric, but applying negative pressures (vacuum) allows the contaminated vapors to be captured, treated, and discharged in a safer fashion.

The flow of gases in the subsurface can be described mathematically using a continuity equation approach, with Darcy’s law for relating volumetric fluxes to potential gradients, and the ideal gas law as the equation of state (see Chapter 4 and Jordan et al. (1995)). Because gas density is small, the gravitational component of the fluid potential is typically ignored and flow is induced primarily by pressure gradients. Analytical solutions exist for idealized flow conditions (i.e., homogeneous, steady-state, and axisymmetric) in either one- or two-dimensional configurations (Johnson et al., 1990a; Shan et al., 1992; Falta, 1996). Numerical models can account for nonideal flow geometries and heterogeneities. By ignoring compositional effects on gas density and viscosity and linearizing the gas flow equation, then groundwater flow models can be used to simulate air flow induced by SVE (e.g., Baehr et al. (1995)).

Contaminant removal can also be described using a continuity equation (see Chapter 4 and Baehr and Hoag (1988)) approach with phase-partitioning (Henry’s law for air-water, Raoult’s law for NAPL-air and NAPL-water, linear sorption) between the organic, aqueous, gaseous, and sorbed phases. Nonequilibrium mass transfer has been observed to be important for chemical removal at a range of scales (Hiller and Gudemann, 1989; Brusseau, 1991; Gierke et al., 1992; Armstrong et al., 1994). It is common to characterize different phases of the removal process according to the dominant mechanisms: advection-dominant initially, transitioning to diffusion (nonequilibrium) dominant removal (Jordan et al., 1995). The advection-dominant phase is shorter as the degree of heterogeneity, in either the contaminant distribution or soil permeability, increases.

The effectiveness of an SVE system to remove vadose zone contamination is basically a function of: (1) the volatility of the contaminants and (2) the gas permeability of the contaminated soil. SVE also enhances in situ biodegradation of many organic contaminants, especially petroleum hydrocarbons, but this aspect is not considered in this section (see Section ??).

The property of volatility is characterized by a contaminant’s pure vapor pressure if it is present as a nonaqueous phase liquid (NAPL) and by its Henry’s constant if present only in dissolved and sorbed phases. Vapor pressure can be translated in terms of the carrying capacity of the gas phase for the contaminant. For example, a compound with a vapor pressure of 0.1-mm Hg at 25 ° C can achieve a vapor concentration up to 5.4 micromoles per liter of air. This example corresponds to what has been unofficially considered to be the minimum vapor pressure for which SVE is practical (cf. Hutzler et al. (1989)). Rather than relying on a rule of thumb, one should consider the practicality of employing volatilization where the maximum concentration is only a few milligrams per liter, realizing that the maximum concentration is never reached for reasons described below.

When contamination is present as a NAPL mixture, then the capacity of the vapor phase for each contaminant is reduced to an amount in direct proportion to its mole fraction in the NAPL phase. This effect is known as Raoult’s law (cf. Johnson et al. (1990a) for applications of Raoult’s law to SVE performance) and the contaminant removal observed by monitoring the SVE offgas may appear similar to the hypothetical curve shown in Figure 2.

The removal of contamination from the aqueous phase (and, since the soil is usually covered with moisture, then also the sorbed phase) is primarily a function of the chemical’s Henry’s constant, which is a function of both its vapor pressure and aqueous solubility. In general, compounds that have what would be considered a sufficiently high vapor pressure usually also have a high enough Henry’s constant for SVE to be effective (>1 L° atm/mole, cf. Jordan et al. (1995)). The most notable exceptions are miscible organics, such as many alcohols and acetone, which has a high vapor pressure (>80 mm Hg) but low Henry’s constant (<0.04 L° atm/mole) due to its infinite solubility.

Mixtures of dissolved contaminants can tend to slightly increase the volatility of most of the individual constituents as often their solubilities tend to decrease in the presence of other compounds. This effect is minimal and exceptions exist when substances are present that increase solubility, such as surfactants.

Contamination is always present in a heterogeneous distribution. Moreover, air flow will follow the paths of least resistant (i.e., shortest distance, highest permeability). Therefore not all of the induced air flow will contact contamination, a situation called "bypassing." This leads to offgas concentrations that are lower than the ideal concentration based on equilibrium calculations as shown in Figure 2. Grain-scale mass transfer processes also cause concentrations to be lower than equilibrium values. Both causes will exhibit abrupt increases in offgas concentrations when SVE flow is interrupted. From a practical view, it is not necessary to differentiate between causes of nonequilibrium, but it remains an area of active research by those developing and testing mathematical models for predicting SVE performance.

Separate from the issue of volatility is the ability of the soil to allow sufficient vapor flow for practical achievement of cleanup goals. Furthermore, the contamination should be accessible by the flowing air for volatilization to be efficient and completed in a reasonable time period such as a few years rather than decades. Soil permeability, from the perspective of SVE operation, deals with the ability for air to flow through the vadose zone, described as the soil gas permeability. Gas density and viscosity also affect gas flow, but to a much lesser extent for typical SVE applications (Johnson et al., 1990a; Falta et al., 1989). Gas permeabilities are a complex function of gas-filled porosity and pore size distribution. In the vast majority of SVE projects, gas permeabilities are measured in situ during pilot-scale "air permeability" tests.

A practical minimum for the soil gas permeability would be a debatable criterion because feasibility depends also on the extent of contamination and on the degree of anisotropy and heterogeneity of the soils. Shallow contaminated zones of limited areal extent can be treated more efficiently than large zones of contamination. A highly heterogeneous soil may have a high permeability as measured in a pilot test, as most of the flow can be concentrated in rather narrow, high-permeability layers, but if the contamination is trapped primarily in low-permeable zones, then removal will be diffusion limited.

Soil vapor extraction is considered a presumptive remedy for volatile organic chemical (VOC) contamination in the vadose zone where the flow of air can be induced at a rate sufficient to flush the gas-filled porosity in the treatment zone on at most a daily basis. This qualitative criterion is consistent with the limited performance data available to date. For example, based on the projects listed in Table 1, several hundred to hundreds of thousands of gas pore volume flushes are required to reduce contamination levels to meet risk-reduction objectives. Quantitative guidance is not yet readily available because of a lack of predictive tools. Nevertheless, design and operation of SVE has been successful at many sites, such as those listed in Table 1, despite the lack of rigorously based approaches.

Soil vapor extraction was initially developed in the early 1980s. Identifying the "first" application is controversial and was the subject of at least one patent suit in the mid 1980s. The rapid acceptance of SVE as a soil treatment technology was due, in part, to the relative simplicity of the governing principles as outlined above, the early development of straightforward design guidance (Johnson et al., 1990b; USEPA, 1991; Michaelson, 1993), and the fact that standard equipment and materials are used (Hutzler et al., 1989). The primary design considerations are discussed briefly below.

SVE gained acceptance more rapidly than any other innovative treatment technology has to date (Gierke and Powers, 1997). The continued popularity of SVE has been the result of its performance at "successfully" remediating a number of sites where effective flows can be established (see more on the Status section below and in USEPA (1995 & 1998)). Success is used here as referring to the effectiveness at reducing health risks to an acceptable level so that treatment can be stopped. Demonstrations of complete removal to a pristine level are few, if any.

The basic design, installation, and operational practices have not changed substantially since those described in the first publications by Johnson et al. (1990b), USEPA (1991), and Michaelson (1993) and, more recently, a comprehensive text by Holbrook et al. (1998). Design refinements and new developments have been focused towards improvements in offgas treatment, blower performance, and durability and pneumatic and installation efficiency of screens. Predictive tools for forecasting SVE performance and optimizing the design of SVE systems are not yet proven (for more discussion of this, see Jordan et al. (1995) and the accompanying Case Study of Modeling SVE Performance). Permeability and volatility enhancements are described in the Augmenting Technologies section.

The basic design considerations for SVE are: (1) the number and placement of extraction vents, (2) selection of blower(s) to achieve desired flow rates, and (3) selection of "offgas" treatment system. These components are depicted in the context of a conventional SVE system in Figure 3. Subsurface vapors are captured in extraction vents, in which gas flow is induced using a vacuum blower. Surface air will flow into the treatment zone due to the vacuum induced in extraction vents as long as the site is not covered by an impermeable barrier. Where the treatment area is covered or where heterogeneities/anisotropic conditions exist that limit vertical movement of air, then subsurface flows can be affected by either allowing air to flow into inlet vents (i.e., vents open to the atmosphere) or by injecting air or treated offgas injection vents or sparge wells, which are installed below the groundwater table. Inlet vents are usually sufficient to prevent stagnant zones and encourage more flow deeper in heterogeneous/anisotropic soils. Injecting air potentially can cause contaminant vapors to move away from the treatment zone. It is common to configure extraction vents to be able to operate as either extraction or inlet vents.

Vents. Most SVE systems utilize water-well screens and casing that are installed vertically in the vadose zone, much like a water well is completed in an aquifer but only above the water table and preferably below the contaminated zone (USEPA, 1991; Shan et al., 1992). In shallow settings (<4 m deep), it is feasible, and sometimes more practical, to install vents horizontally to obtain more efficient vapor flow (USEPA, 1991; Aiken, 1992). The number of vents is usually selected by the size of the contaminated area and the so-called "radius of influence" (ROI) of the extraction vents. This is done simply by locating vents so that their ROI overlap in a fashion that will encompass the contaminated area (e.g., Johnson et al. (1990b) and USEPA (1991)). This oversimplified approach is increasingly recognized as inappropriate because it ignores gas residence times (flushing rates) and hence the contaminant removal rates. A more appropriate approach is to define the treatment zone around an extraction vent based on a desired flushing rate, which can be determined for homogeneous conditions using analytical approaches such as reported in Shan et al. (1992) or with numerical models for more general conditions (cf. Jordan et al. (1995)). In any case, it is important to realize that induced subsurface air flow is affected by heterogeneities, and rarely will actual flow patterns follow idealized predictions. Capping a site, proper vent installations, and inlet/injection vents are useful methods to encourage flows to be more "ideal" like.

Vertical vent installations are predominantly completed in unconsolidated deposits using hollow-stem augers and either pea-gravel or coarse-sand filterpacks as depicted in Figure 4a. Proper grouting near the ground surface is necessary to minimize short-circuiting of air through the filterpack. Direct-push technologies may be used to install vents in high-permeability, coarse-grained soils, but precautions should be taken to insure that screens do not become plugged with fine sediments as, unlike water-well development, there are no general screen-development methods for the unsaturated zone. Also, short-circuiting is likely when the top of the screen is near the surface. Horizontal vents can be installed in a back-filled trench as shown in Figure 4b or with directional-drilling techniques. Directional-drilling installations are susceptible to screen plugging unless precautions are taken to minimize contact of the screen with fines or procedures are performed to remove clogging. Stainless-steel, wire-wrap screens are least susceptible to chemical attack and are more pneumatically efficient in comparison to slotted screens. High-density polyethylene (HDPE) and polyvinyl-chlorine (PVC), slotted-screens are the most economical screens and are chemically resistant to petroleum hydrocarbons and chlorinated organics when concentrations are low. Steel and PVC are the two most common materials for vent casing and above-ground plumbing. Nominal diameters for screens, casing, and piping are usually between ¾ and 4 inches.

The above-ground plumbing should include valves and ports to allow flexibility in flow configurations, flow metering (rates and pressures), and ports for concentration monitoring for optimizing system performance. There is no readily available design guidance for the above-ground plumbing specific for SVE, so one must refer to a fluid mechanics handbook that includes gas flows. Pressure losses in the piping and fittings is significant and should not be neglected (Peramaki, 1993).

Blower Selection. Blower selection is very important to minimize power requirements. In permeable, sandy to sandy silt soils, dynamic-displacement blowers are typically used to induce gas flow. Positive-displacement, usually rotary-lobe, blowers are used where the soil permeability is low. Dynamic-displacement blowers can provide high flows at low vacuums, but blower performance tends to diminish rapidly as vacuums increase. Positive-displacement blowers operate at a constant flow rate over a wider range of pressures, but their capacities are lower in general.

Because reliable predictions of required vacuums are not available, in situ gas permeabilities must be measured in a pilot test. Therefore it is necessary to have a blower available to estimate the blower performance that will be needed for the full-scale operation. It is common to rent a blower for the pilot test and size the blower(s) that will be required for the full-scale remediation based on the pilot-scale performance (flows and vacuums) measurements, adjusted for the full-scale plumbing configurations. At sites where the soils are highly heterogeneous in terms of permeability, such as glacial deposits, it is prudent to perform several pilot tests in different locations to ensure that the desired flows can be achieved for the entire treatment area.

Thermally protected, intrinsically safe, explosion-proof equipment should be used. Blowers should not be throttled to control flow rates but rather plumbed to bleed in air from above ground; however, this condition should be avoided altogether by properly selecting a blower to minimize power usage. Blowers must be protected from dust using filters and liquid droplets using "knockout" drums as shown in Figure 3. Systems can be configured to shutdown, when the moisture separator is filled, with a float switch. The blower, moisture separator, and associated electrical controls are often purchased as a complete system and configured to the site requirements. If available, three-phase 230/460 voltage blower motors should be used to operate most efficiently.

Offgas Treatment. The offgas treatment system, if required, can be the costliest portion of the remediation system. Granular activated carbon (GAC) is the lowest capital cost but costs more to operate if the contamination exists as NAPL, especially as a mixture of different compounds, and is being removed at high concentrations. Combustion and thermal/catalytic oxidation units are more expensive to purchase than GAC but will be cheaper to operate when offgas concentrations are high and if the contaminants are combustible and/or can be oxidized. Offgas treatment units/systems can be rented and some vendors provide pilot-scale units to be tested during permeability tests. Pilot tests tend to overpredict contaminant removal rates from the soil. Therefore offgas treatment should be considered over the longterm by either providing for flexibility to adjust operating conditions when concentrations diminish or switch to other treatment options.

Installation of the extraction vents and the purchase of an offgas treatment system and blower(s) comprise the majority of capital costs. The operating-and-maintenance (O&M) costs are mostly needed to supply power for the blower(s) and to operate the offgas treatment system (e.g., replenishing fuel, replacement/regeneration of carbon, etc.). Initial site characterization and performance assessment and monitoring costs are often close to the costs of what can be separated out as the "remediation system" costs.

Conventional SVE configurations, as described above, are used for sites where the contaminants are volatile at ambient temperatures and soils are permeable to air. Technology augmentations can be implemented to enhance volatility (e.g., by heating the soil, see Section ??) and/or permeability (e.g., by fracturing (USEPA, 1997) or augering (Siegrist et al., 1995; see Section ??)) to employ SVE at sites where it is not normally feasible from a permeability consideration.

Heating soils will tend to favor the partitioning behavior of volatile and semi-volatile organics towards the gaseous phase, which will of course improve SVE performance. Heating using hot-air injection has been shown to improve removal rates (see Section ??)

Soil vapor extraction is not usually effective in low-permeability soils, especially those containing clays, because effective air flushing rates can not be achieved with conventional equipment. Rock and soil formations can be fractured to enhance the permeability. Pneumatic fracturing has been shown to increase SVE performance in glacial drift as well as fractured shale (Frank and Barkley, 1995). Hydraulic fracturing has also been used to enhance SVE (Murdoch et al., 1991). The long-term complete cleanup of fractured systems is not known because contaminants in the unfractured matrix may still require a long time to diffuse to the fractures (Grathwohl, 1998).

Deep soil mixing is a process that disrupts the soil fabric within the radius of motion of auger blades over a depth interval limited to the vertical movement of the auger shaft (see Siegrist et al. (1995) and Section ??).

Large-scale, small-pressure disturbances associated with weather systems can cause gas flow into and out of the subsurface: a process called, "barometric pumping." Barometric pumping is used as a long-term, low-operating cost form of SVE for slow removal of diffusion-limited contamination through a combination of volatilization and enhanced bioremediation.

Critical factors affecting performance

The variables typically monitored during SVE operation are listed in Table 2. The value of some measurements are questionable given that they are not necessarily representative of subsurface conditions. For example, subsurface gas pressures are needed during pilot tests for determining gas permeabilities, however, during full-scale operation they are not necessarily indicative of subsurface gas velocities nor even useful for identifying areas where flow is occurring as even stagnant zones will exhibit nonzero vacuums. A more effective measure of vent influence is changes in soil-gas concentrations of the contaminants or other tracer such as oxygen.

Sites where contaminants are present as mixtures are difficult to monitor in terms of concentrations. Typically, several constituents are selected as contaminants of concern (COC), e.g., BTEX. Equivalent and comprehensive measures are also used, such as: total hydrocarbons/VOCs or total petroleum hydrocarbons (gasoline range organics, diesel range organics), respectively. Reductions in COC concentrations do not necessarily correlate to overall contaminant removal or vice versa.

At a base level, flow and concentration measurements are needed to ascertain system performance and potentially will allow operational improvements. When removals are advection dominant and transitioning towards diffusion-limited, increases in extraction rates will increase mass removal rates even though offgas concentrations may decrease as a result of a higher proportion of bypassing (gas flow missing contamination) or reductions in gas residence times (less time for equilibration). When the removal rate is diffusion-limited (Figure 2), then increased extraction rates provide little or no increase in mass removal rate. Combustion and catalytic oxidation methods for offgas treatment benefit from high vapor concentrations, so monitoring concentrations, in terms of fuel value, from individual extraction vents can be used to optimize the performance of offgas treatment. Flow and concentrations are used to determine contaminant mass removed but this rarely corresponds to independent measures of contaminant mass present nor removed based on soil sample analyses, due to limitations of the latter.

Comprehensive site characterization in terms of both permeability and contaminant distributions are useful for locating extraction vents in the most-permeable, highest-concentration areas and operating the system to maximize extracted vapor concentrations, which lead to maximum offgas treatment efficiency.

Two summary reports (USEPA 1995 & 1998) of case studies provide performance details of SVE. The information summarized in these reports include site and contaminant characteristics, system configuration and key design criteria, operational performance, costs (capital and O&M), regulatory issues, lessons learned, technical contacts, and additional references. Table 1 lists some of the important site and contaminant conditions that illustrate the range of applications of SVE. The sites included range in size from 650 cubic yards of treated soil to over 200,000 cubic yards. Chlorinated solvents and/or fuel contaminants were the most common, amounts of which varied from low levels (probably only dissolved and sorbed phases) to substantial NAPL contamination (upwards of 40 pounds of contaminants per cubic yard of soil). Reported costs varied from a few dollars per cubic yard for large sites with only low levels of contamination to over a thousand dollars per cubic yard for sites with severe geological limitations and lots of contamination. Moreover, some of the reported projects were completed in a little over a year while others are provided as work in progress. The information in these reports are provided in concise format that is useful for compiling evidence of the feasibility of SVE for many sites.

Aiken, B. (1992) Horizontal wells are cost-effective for in situ air stripping, Ground Water Monitoring Review; 12(3), 55.

Armstrong, J.E., Frind, E.O., McClellan, R.D. (1994) Nonequilibrium mass transfer between the vapor, aqueous, and solid phases in unsaturated soils during vapor extraction, Water Resources Research, 30(2), 355-368.

Baehr, A.L., Hoag, G.E., (1988) A modeling and experimental investigation of induced venting of gasoline-contaminated soils, in Soil Contaminated by Petroleum, John Wiley and Sons, New York, 113-123.

Baehr, A.L., Joss, C.J. (1995) An updated model of induced airflow in the unsaturated zone, Water Resources Research, 31(2), 417-421.

Brusseau, M.L. (1991) Transport of organic chemicals by gas advection in structured or heterogeneous porous media: Development of a model and application to column experiments, Water Resources Research, 27(12), 3189-3199.

Falta, R.W. (1996) A program for analyzing transient and steady-state soil gas pump tests, Ground Water, 34(4), 750-755.

Falta, R.W., Javandel, I., Pruess, K., Witherspoon, P.A. (1989) Density-driven flow of gas in the unsaturated zone due to the evaporation of volatile organic compounds, Water Resources Research, 28(2), 323-335.

Frank, U., Barkley, N. (1995) Remediation of low permeability subsurface formations by fracturing enhancement of soil vapor extraction, Journal of Hazardous Materials, 40, 191-201.

Gierke, J.S., Powers, S.E. (1997) Increasing implementation of in situ treatment technologies through field-scale performance assessments, Water Environment Research, 69(2), 196-205.

Gierke, J.S., Hutzler, N.J., McKenzie, D.B. (1992) Vapor transport in unsaturated soil columns: Implications for vapor extraction, Water Resources Research, 28(2), 323-335.

Grathwohl, P. (1998) Diffusion in Natural Porous Media: Contaminant Transport, Sorption/Desorption and Dissolution Kinetics, Kluwer Academic Publishers, Boston, Massachusetts, 207 pp.

Hiller, D., Gudemann, H. (1989) Analysis of vapor extraction data from applications in Europe, Proceedings of Third International Conference on New Frontiers for Hazardous Waste Management, EPA/600/8-89/072, Pittsburgh, PA, September 10-13, 1989.

Holbrook, T.M., Bass, D.H., Boersma, P.M., DiGiulio, D.C., Eisenbeis, J.J., Hutzler, N.J., Roberts, E.P. (1998) Vapor Extraction and Air Sparging, American Academy of Environmental Engineers, Annapolis, Maryland.

Hutzler, N.J., Murphy, B.E., Gierke, J.S. (1989) State of Technology Review: Vapor Extraction Systems, EPA 600/2-89-024.

Johnson, P.C., Kemblowski, M.W., Colthart, J.D. (1990a) Quantitative analysis for the cleanup of hydrocarbon-contaminated soils by in-situ soil venting, Ground Water, 28(3), 413-429.

Johnson, P.C., Stanley, C.C., Kemblowski, M.W., Byers, D.L., Colthart, J.D. (1990b) A practical approach to the design, operation, and monitoring of in-situ soil-venting systems, Ground Water Monitoring Review, 10(2), 159-178.

Jordan, D.L., Mercer, J.W., Cohen, R.M. (1995) Review of Mathematical Modeling for Evaluating Soil Vapor Extraction Systems, EPA/540/R-95/513.

Michaelson, G. (1993) Guidance for Design, Installation and Operation of Soil Venting Systems, PUBL-SW185-93, Emergency and Remedial Response Section, Wisconsin Department of Natural Resources, Madison, Wisconsin.

Murdoch, L.C. Losonsky, G., Cluxton, P., Patterson B., Klich, I., Braswell, B. (1991) The Feasibility of Hydraulic Fracturing of Soil to Improve Remedial Actions. Final Report, EPA600/2-91-012, NTIS Report PB91-181818, 298 pp.

Peramaki, M.P. (1993) Soil-vapor extraction system piping design and blower selection: A spreadsheet program, Proceedings of the Seventh National Outdoor Action Conference and Exposition on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods, Ground Water Management Book 15, May 25-27, 1993, Las Vegas, Nevada, 15-29.

Shan, C., Falta, R.W., Javendal, I. (1992) Analytical solutions for steady state gas flow to a soil vapor extraction well, Water Resources Research, 28(4), 1105-1120.

Siegrist, R.L., West, O.R., Morris, M.I., Pickering, D.A., Greene, D.W., Muhr, C.A., Davenport, D.D., Gierke, J.S. (1995) In situ mixed region vapor stripping in low-permeability media. 2. Full-scale field experiments, Environmental Science & Technology, 29(9), 2198-2207.

USEPA (1991) Soil Vapor Extraction Technology Reference Handbook, EPA/540/2/91/003.

USEPA (1995) Remediation Case Studies: Soil Vapor Extraction, EPA-542-R-95-004.

USEPA (1998) Remediation Case Studies: In Situ Soil Treatment Technologies (Soil Vapor Extraction, Thermal Processes), Volume 8, EPA-542-R-98-012.

USEPA (1997) Analysis of Selected Enhancements for Soil Vapor Extraction, EPA542R97007, National Service Center for Environmental Publications, Cincinnati, Ohio.

Figure 1. Grain-scale view of soil vapor extraction process where fresh air (white arrow) is drawn into a contaminated zone (liquid contaminant depicted as red) under an induced vacuum. The fresh air displaces soil gas previously equilibrated with the contaminant and thus causes vaporization/volatilization of liquid, dissolved, and sorbed contaminants, potentially until chemical equilibrium is achieved. The colored arrows depict increasingly higher concentrations of contaminants in the soil gas, which eventually is extracted from the ground and treated.

Figure 2. Characteristic offgas concentrations observed during SVE in conventional configurations in permeable soils with NAPL contamination (adapted from Hiller and Gudemann (1989) and Johnson et al. (1990a)).

Figure 3. Conventional SVE configurations for removal of volatile contaminants from the vadose zone depicted here for a leaky underground storage tank (LUST) scenario.

Figure 4. Vent configurations: (a) vertical and (b) horizontal trench.

Table 1. Summary of published SVE performance at field sites in the U.S.

In field_summary.xls

Table 2. Variables monitored during SVE design activities and operation.