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Removal of the trapped NAPL generally requires mass transfer of the NAPL com-
ponents into either the gas phase or the aqueous phase, where they can be pumped
out, or destroyed by aqueous or gas phase chemical or biological reactions. Arguably,
the most successful NAPL/VOC remediation schemes have used the gas phase to
transfer contaminants out of the subsurface, or to deliver beneficial chemicals or
nutrients to the subsurface. Generally speaking, remediation methods that rely on
gas phase extraction are most effective for chemicals that have high vapor pressures
and Henry’s constants, while methods that rely on gas phase delivery of chemicals or
nutrients can attack a wider range of contaminants.
Obviously, remediation schemes based on gas phase transport are applicable in
the vadose zone, above the water table, however there are also effective remediation
methods that involve injecting air or steam below the water table. The following
sections describe some of the more popular gas-based remediation methods forVOCs.

22.2 SOIL VAPOR EXTRACTION
22.2.1 Introduction to SVE Applications, Induced Gas Flow

Soil vapor extraction is probably the simplest, most common, and most successful of
all of the in-situ techniques for removing VOC’s from the vadose zone. SVE systems
use one or more wells screened above the water table. A blower connected to the well
normally provides the vacuum (typically from 0.05 to 0.2 atmospheres) that induces
3
flow to the well. Typical gas flow rates are in the range of 50 to 300 cfm (0.024 m /s
3
to 0.14 m /s) depending on the vacuum in the well and the gas phase permeability.
Figure 22.1 from Shan et al. (1992) shows the calculated gas streamlines for a single
SVE well in a homogeneous, isotropic system that is open to the atmosphere. Each of
the streamtubes in this figure originate at the ground surface, and they each contribute
5% of the total gas flow to the well. As clean soil gas flows through the contaminated
zone, NAPLs evaporate, and dissolved or adsorbed VOCs partition into the moving
gas phase. Once the gas is removed from the well, it is treated, often by granular
activated carbon adsorption. The gas travel time from a location inside the SVE
well capture zone to the well determines how effectively that location is flushed by
gas. Considering that several thousand pore volumes of gas are often needed for

0.0
c d =0
Depth, meters 5.0

c d =1
10.0
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Radial distance, meters
Figure 22.1. Calculated gas streamlines around an SVE well for homogeneous, isotropic conditions,
with an open ground surface (from Shan et al. (1992))