Soil vapor extraction
Soil vapor extraction (SVE) is a physical treatment process for in situ remediation of volatile contaminants in vadose zone (unsaturated) soils (EPA, 2012). SVE (also referred to as in situ soil venting or vacuum extraction) is based on mass transfer of contaminant from the solid (sorbed) and liquid (aqueous or non-aqueous) phases into the gas phase, with subsequent collection of the gas phase contamination at extraction wells. Extracted contaminant mass in the gas phase (and any condensed liquid phase) is treated in aboveground systems. In essence, SVE is the vadose zone equivalent of the pump-and-treat technology for groundwater remediation. SVE is particularly amenable to contaminants with higher Henry’s Law constants, including various chlorinated solvents and hydrocarbons. SVE is a well-demonstrated, mature remediation technology (Hutzler et al., 1990; Pedersen and Curtis, 1991; Noyes, 1994; Stamnes and Blanchard, 1997; Suthersan, 1999; Kahn et al., 2004; Damera and Bhandari, 2007) and has been identified by the U.S. Environmental Protection Agency (EPA) as presumptive remedy (EPA, 1993, 1996, 2011).
The soil vapor extraction remediation technology uses vacuum blowers and extraction wells to induce gas flow through the subsurface, collecting contaminated soil vapor, which is subsequently treated aboveground. SVE systems can rely on gas inflow through natural routes or specific wells may be installed for gas inflow (forced or natural). The vacuum extraction of soil gas induces gas flow across a site, increasing the mass transfer driving force from aqueous (soil moisture), non-aqueous (pure phase), and solid (soil) phase into the gas phase. Air flow across a site is thus a key aspect, but soil moisture and subsurface heterogeneity (i.e., a mixture of low and high permeability materials) can result in less gas flow across some zones. In some situations, such as enhancement of monitored natural attenuation, a passive SVE system that relies on barometric pumping may be employed (Early et al., 2006; Kamath et al., 2010).
SVE has several advantages as a vadose zone remediation technology. The system can be implemented with standard wells and off-the-shelf equipment (blowers, instrumentation, vapor treatment, etc.). SVE can also be implemented with a minimum of site disturbance, primarily involving well installation and minimal aboveground equipment. Depending on the nature of the contamination and the subsurface geology, SVE has the potential to treat large soil volumes at reasonable costs.
The soil gas (vapor) that is extracted by the SVE system generally requires treatment prior to discharge back into the environment. The aboveground treatment is primarily for a gas stream, although condensation of liquid must be managed (and in some cases may specifically be desired). A variety of treatment techniques are available for aboveground treatment (EPA, 2006) and include thermal destruction (e.g., direct flame thermal oxidation, catalytic oxidizers), adsorption (e.g., granular activated carbon, zeolites, polymers), biofiltration, non-thermal plasma destruction, photolytic/photocatalytic destruction, membrane separation, gas absorption, and vapor condensation. The most commonly applied aboveground treatment technologies are thermal oxidation and granular activated carbon adsorption. The selection of a particular aboveground treatment technology depends on the contaminant, concentrations in the offgas, throughput, and economic considerations.
The effectiveness of SVE, that is, the rate and degree of mass removal, depends on a number of factors that influence the transfer of contaminant mass into the gas phase. The effectiveness of SVE is a function of the contaminant properties (e.g., Henry’s Law constant, vapor pressure, boiling point, adsorption coefficient), temperature in the subsurface, vadose zone soil properties (e.g., soil grain size, soil moisture content, permeability, carbon content), subsurface heterogeneity, and the air flow driving force (applied pressure gradient). As an example, a residual quantity of a highly volatile contaminant (such as trichloroethene) in a homogeneous sand with high permeability and low carbon content (i.e., low/negligible adsorption) will be readily treated with SVE. In contrast, a heterogeneous vadose zone with one or more clay layers containing residual naphthalene would require a longer treatment time and/or SVE enhancements. SVE effectiveness issues include tailing and rebound, which result from contaminated zones with lower air flow (i.e., low permeability zones or zones of high moisture content) and/or lower volatility (or higher adsorption). Recent work at U.S. Department of Energy sites has investigated layering and low permeability zones in the subsurface and how they affect SVE operations [Switzer and Kosson, 2007; Oostrom et al., 2007].
Enhancement of SVE
Enhancements for improving the effectiveness of SVE can include directional drilling, pneumatic and hydraulic fracturing, and thermal enhancement (e.g., hot air or steam injection) (Frank and Barkley, 1995; EPA, 1997; Peng et al., 2013). Directional drilling and fracturing enhancements are generally intended to improve the gas flow through the subsurface, especially in lower permeability zones. Thermal enhancements such as hot air or steam injection increase the subsurface soil temperature, thereby improving the volatility of the contamination. In addition, injection of hot (dry) air can remove soil moisture and thus improve the gas permeability of the soil. Additional thermal technologies (such as electrical resistance heating, six-phase soil heating, radio-frequency heating, or thermal conduction heating) can be applied to the subsurface to heat the soil and volatilize/desorb contaminants, but these are generally viewed as separate technologies (versus a SVE enhancement) that may use vacuum extraction (or other methods) for collecting soil gas.
Design, Optimization, Performance Assessment, and Closure
On selection as a remedy, implementation of SVE involves the following elements: system design, operation, optimization, performance assessment, and closure. Several guidance documents provide information on these implementation aspects. EPA and U.S. Army Corps of Engineers (USACE) guidance documents (EPA, 2001, 2004; USACE, 2002) establish an overall framework for design, operation, optimization, and closure of a SVE system. The Air Force Center for Engineering and the Environment (AFCEE) guidance (AFCEE, 2001) presents actions and considerations for SVE system optimization, but has limited information related to approaches for SVE closure and meeting remediation goals. Guidance from the Pacific Northwest National Laboratory (PNNL) (Truex et al., 2013) supplements these documents by discussing specific actions and decisions related to SVE optimization, transition, and/or closure.
Design and operation of a SVE system is relatively straightforward, with the major uncertainties having to do with subsurface geology/formation characteristics and the location of contamination. As time goes on, it is typical for a SVE system to exhibit a diminishing rate of contaminant extraction due to mass transfer limitations or removal of contaminant mass. Performance assessment is a key aspect to provide input for decisions about whether the system should be optimized, terminated, or transitioned to another technology to replace or augment SVE. Assessment of rebound and mass flux (Switzer et al., 2004; Brusseau et al., 2010; Truex et al., 2013) provide approaches to evaluate system performance and obtain information on which to base decisions.
Several technologies are related to soil vapor extraction. As noted above, various soil-heating remediation technologies (e.g., electrical resistive heating, in situ vitrification) require a soil gas collection component, which may take the form of SVE and/or a surface barrier (i.e., hood). Bioventing is a related technology, the goal of which is to introduce additional oxygen (or possibly other reactive gases) into the subsurface to stimulate biological degradation of the contamination. In situ air sparging is a remediation technology for treating contamination in groundwater. Air is injected and "sparged" through the groundwater and then collected via soil vapor extraction wells.
- AFCEE. 2001. United States Air Force Environmental Restoration Program: Guidance on Soil Vapor Extraction Optimization. Air Force Center for Environmental Excellence, Brooks Air Force Base, Texas.
- Brusseau, M.L., V. Rohay, and M.J. Truex. 2010. "Analysis of Soil Vapor Extraction Data to Evaluate Mass-Transfer Constraints and Estimate Source-Zone Mass Flux." Ground Water Monit. Remed., 30(3):57-64.
- Damera, R., and A. Bhandari. 2007. "Physical Treatment Technologies." In: Remediation Technologies for Soils and Groundwater. A. Bhandari, R.Y. Surampalli, P. Champagne, S.K. Ong, R.D. Tyagi, and I.M.C. Lo, ed. American Society of Civil Engineers, Reston, Virginia.
- Early, T., B. Borden, M. Heitkamp, B.B. Looney, D. Major, W.J. Waugh, G. Wein, T. Wiedemeier, K.M. Vangelas, K.M. Adams, and C.H. Sink. 2006. Enhanced Attenuation: A Reference Guide on Approaches to Increase the Natural Treatment Capacity of a System. WSRC-STI-2006-00083, Rev.1, Washington Savannah River Company, Aiken, South Carolina.
- EPA. 1993. "Presumptive Remedies: Site Characterization and Technology Selection For CERCLA Sites With Volatile Organic Compounds In Soils." EPA/540/F-93/048, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C.
- EPA. 1996. "User’s Guide to the VOCs in Soils Presumptive Remedy." EPA/540/F-96/008, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C.
- EPA. 1997. Analysis of Selected Enhancements for Soil Vapor Extraction. EPA/542/R-97/007, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C.
- EPA. 2001. Development of Recommendations and Methods to Support Assessment of Soil Venting Performance and Closure. EPA/600/R-01/070, U.S. Environmental Protection Agency, Washington, D.C.
- EPA. 2004. How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites. EPA/510/R-04/002, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C.
- EPA. 2006. Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice. EPA/542/R-05/028, U.S. Environmental Protection Agency, Office of Superfund Remediation and Technology Innovation, Washington, D.C.
- EPA. 2011. "Presumptive Remedies: Policy and Procedures" (website). U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C. Available online at: http://www.epa.gov/superfund/policy/remedy/presump/pol.htm (last accessed on July 30, 2013).
- EPA. 2012. "A Citizen’s Guide to Soil Vapor Extraction and Air Sparging." EPA/542/F-12/018, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C.
- Frank, U., and N. Barkley. 1995. "Remediation of Low Permeability Subsurface Formations by Fracturing Enhancement of Soil Vapor Extraction." J. Haz. Materials, 40(2):191-201.
- Hutzler, N.J., B.E. Murphy, and J.S. Gierke. 1990. "State of Technology Review: Soil Vapor Extraction Systems." EPA/600/S2-89/024, U.S. Environmental Protection Agency, Risk Reduction Engineering Laboratory, Cincinnati, Ohio.
- Khan, F.I., T. Husain, R. Hejazi. 2004. "An Overview and Analysis of Site Remediation Technologies." J. Environ. Mgmt., 71(2):95-122.
- Kamath, R., D.T. Adamson, C.J. Newell, K.M. Vangelas, and B.B. Looney. 2010. Passive Soil Vapor Extraction. SRNL-STI-2009-00571, Rev. 1, Savannah River National Laboratory, Aiken, South Carolina.
- Noyes, R. 1994. Unit Operations in Environmental Engineering. Noyes Publications, Park Ridge, New Jersey.
- Oostrom, M., M.L. Rockhold, P.D. Thorne, M.J. Truex, G.V. Last, and V.J. Rohay. 2007. "Carbon Tetrachloride Flow and Transport in the Subsurface of the 216-Z-9 Trench at the Hanford Site." Vadose Zone J., 6(4):971-984.
- Pedersen, T.A., and J.T. Curtis. 1991. Soil Vapor Extraction Technology. Noyes Data Corporation, Park Ridge, New Jersey.
- Peng, S., N. Wang, and J. Chen. 2013. "Steam and Air Co-injection in Removing Residual TCE in Unsaturated Layered Sandy Porous Media." J. Contam. Hydrol., in press. http://dx.doi.org/10.1016/j.jconhyd.2013.07.002.
- Stamnes, R., and J. Blanchard. 1997. "Engineering Forum Issue Paper: Soil Vapor Extraction Implementation Experiences." EPA 540/F-95/030, U.S. Environmental Protection Agency, Washington, D.C.
- Suthersan, S.S. 1999. "Soil Vapor Extraction." In: Remediation Engineering: Design Concepts, S.S. Suthersan, ed. CRC Press, Boca Raton, Florida.
- Switzer, C., and D.S. Kosson. 2007. "Soil Vapor Extraction Performance in Layered Vadose Zone Materials." Vadose Zone J., 6(2):397-405.
- Switzer, C. T. Slagle, D. Hunter, and D.S. Kosson. 2004. "Use of Rebound Testing for Evaluation of Soil Vapor Extraction Performance at the Savannah River Site." Ground Water Monit. Remed., 24(4):106-117.
- Truex, M.J., D.J. Becker, M.A. Simon, M. Oostrom, A.K. Rice, and C.D. Johnson. 2013. Soil Vapor Extraction System Optimization, Transition, and Closure Guidance. PNNL-21843, Pacific Northwest National Laboratory, Richland, Washington.
- USACE. 2002. Engineering and Design: Soil Vapor Extraction and Bioventing. EM 1110-1-4001, U.S. Army Corps of Engineers, Washington, D.C.
- U.S. EPA CLU-IN Soil Vapor Extraction Overview
- How To Evaluate Alternative Cleanup Technologies For Underground Storage Tank Sites
- Hyperventilate Users Manual: A Software Guidance System Created for Vapor Extraction Applications Environmental Protection Agency
- USACE Soil Vapor Extraction and Bioventing (EM 1110-1-4001)
- Soil Vapor Extraction System Guidance / Soil Vapor Extraction Endstate Tool (SVEET)
- Federal Remediation Technologies Roundtable (FRTR) Screening Matrix Section 4.8, Soil Vapor Extraction
- Center for Public Environmental Oversight (CPEO) Tech Tree: Soil Vapor Extraction (SVE)
- Center for Public Environmental Oversight (CPEO) Tech Tree: Soil Vapor Extraction Enhancements
- New Approach to Assess Volatile Contamination in Vadose Zone Provides Path Forward for Site Closure