Soil vapor extraction (SVE) is an in situremediation
process which removes volatile organic compounds (VOCs), such as
gasoline, solvents, and other relatively volatile compounds from
the soil by inducing and air flow through the soil. The basic
system used to accomplish this consists of a vapor extraction
well, which extends from the surface down to a depth where the
soil is contaminated, coupled with blowers or vacuum pumps, which
draw air through the contaminated soil up to the surface via the
well (see Figures 1-3). Several other procedures can be done in
order to enhance the performance of the system, such as passive
air injection, surface seals, or heating; but these are
site-specific enhancements and will be discussed in further
detail below. First, let's look at what scenarios soil vapor
extraction is best used for.
Photographs appear courtesy of Dr. Mark Widdowson.
The considerations which govern when to use SVE are: soil characteristics, contaminant location, contaminant vapor concentrations, contaminant removal rates, how much residual will be left, and the risk of negative effects. An important feature about SVE is that it only removes contamination from the vadose (unsaturated) zone of the soil. Furthermore, SVE is difficult to apply to soils with low permeability or complex stratification. However, recent studies, such as one conducted at a slightly permeable saprolitic soil, have demonstrated that implementation of SVE in low permeability soils is possible (Aelion, et al., 1995). Also, if a low permeability soil is largely heterogeneous or fractured, SVE may be successful (Widdowson, et al.,1995). Primarily, SVE is most applicable to contaminant zones which are relatively far beneath the surface, where excavation would be a highly uneconomical solution (DePaoli, et al., 1996).
Most often, SVE involves spills which contain diesel fuel, gasoline, or solvent mixtures. The most acceptable removal rates are obtained when the contaminant is a volatile organic compound (VOC) with a vapor pressure of at least 66 Pa, or a Henry's constant of at least 0.01 (DePaoli et al., 1996). It is important to determine the vapor pressures because the removal rate is directly related to the concentration of the volatile contaminant; that is to say, the removal rate declines as the contaminant concentration declines. The removal rate is also affected by the attainable vapor flow rate within the soil. The more permeable a soil is, the greater the vapor flow rate can be, and thus, the greater the contaminant removal rate can be. There are some limitations to removal, however. Residual will always be left behind because the concentrations of the volatile compounds will eventually drop to a level too low for SVE to remove at remotely efficient rates (due to increasing diffusional resistances with decreasing concentrations), and there will be non-volatile contaminants which are not removable via SVE (Johnson, et al., 1990).
There may also be some negative effects which result from SVE which one must consider. For instance, SVE may draw in contaminant vapors which are off the site which is being remediated. This can be remedied by placing a well which acts as a passive air supply (air not being pumped to the subsurface, to be discussed below) at the perimeter of the contaminant plume which is at the site being remediated (see Figure 4). Another possible negative effect is that there may be an upwelling in the water table due to vacuum pumping. This could result in the transport of the contaminant into the soluble phase, thereby requiring remediation of the groundwater as well. This effect can be negated, however, by adding a groundwater pumping well in the vicinity of the plume in order to stabilize the water table level (see Figure 5). (Johnson, et al.). Thus, site characteristics need to be carefully considered up front in order to determine if SVE is a viable solution and if enhancements need to be added.
Once one has decided SVE is the best option, one has to consider various configurations to optimize performance. The number of wells and the well locations must be selected such that the entire zone of contamination is reached and vapor flow outside the contaminant zone is minimized. Thus, careful modeling of the subsurface is very important for a good design. Vapor extraction trenches with surface seals are more economical where contamination zones are relatively shallow (less than approximately 4m from the surface). Surface seals are constructed to control vapor flow paths, and are typically constructed of such materials as: polymer-based liners asphalt, concrete, or clay. (Johnson, et al.).
For most contaminant zones, a typical vertical well is constructed of very basic materials: PVC pipe with slots located in the contaminant zone, coarse sand or gravel filter packing, and bentonite pellets and cement grout surrounding the filter packing (Johnson, et al.).
(Johnson, et al.; Metcalf and Eddy, 1996)
To ensure SVE is working properly, several characteristics of the soil and system need to be monitored. They are:
(Johnson, et al.)
For various reasons, SVE alone can prove to be inadequate or,
quite frankly, just too gosh-darn slow. However, with the help of
one or more modifications, SVE may still be the best option for
the remediation of a given site. Such enhancements include:
(Johnson, et al.; Metcalf and Eddy)
Send comments or suggestions to:
Student Authors: Jennifer Benning and Andy Sabalowsky
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
