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[1][2][3][4][5][6][7] and has been identified by the U.S. Environmental Protection Agency (EPA) as presumptive remedy.[8][9][10]
SVE Effectiveness[edit]
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, soil permeability, soil 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.[14][15]
Enhancement of SVE[edit]
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).[16][17][18] 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[edit]
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[19][20][21] 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[22] 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)[23] 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[24][25][23] provide approaches to evaluate system performance and obtain information on which to base decisions.
Related Technologies[edit]
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.