Agriculture Reference
In-Depth Information
an in situ clean-up process allows contaminated soil to be remediated without disturbance
or excavation (Nadim et al. 2000).
Soil vapor extraction (SVE) is an in situ unsaturated (vadose) zone soil remediation
technology in which a vacuum is applied to the soil to induce the controlled flow of air and
remove volatile and some semi-volatile contaminants from the soil. The gas leaving the soil
may be treated to recover or destroy the contaminants. The drawback in the use of SVE for
remediation of contaminated site is that SVE can not remove heavy oils, metals, PCBs, or
dioxins from contaminated soil; it is only effective for remediation of soil contaminated with
VOCs and Semi-VOCs. Because the process involves the continuous flow of air through the
soil, however, it often promotes the in situ biodegradation of low volatility organic
compounds that may be present.
3.1.2 In situ steam injection vapour extraction
Cold soil vapour extraction is a common technique for remediating volatile organic
compounds from the unsaturated subsurface. Limitations in efficiency can be overcome by
using thermal enhancement, e. g. steam as a fluid heat transport medium to speed up the
process (Sleep and Ma, 1997).
In situ steam extraction is a new technology and has had limited use across the globe. Steam
extraction can be used in two different systems; mobile and stationary. The mobile system
has a unit that volatilizes contaminants in small areas in a sequential manner by injecting
steam and hot air through rotating cutter blades that pass through the contaminated
medium. The stationary system uses steam injection as a means to volatilize and displace
contaminants from the undisturbed subsurface soil. In both systems, steam (at 200
0
C) and
compressed air (at 135
0
C) are forced through the soil medium and the mixture of air; vapor
and chemicals are collected by extraction wells (Nadim et al. 2000).
3.1.3 Air sparging
Air sparging is an in situ technology in which air is injected through a contaminated aquifer.
Injected air traverses horizontally and vertically in channels through the soil column,
creating an underground stripper that removes contaminants by volatilization (EPA, 2001).
Air sparging can also be explained as a method of site remediation that introduces air (or
other gases) into the saturated zone contaminated with VOCs. In addition to volatilization
of VOCs, air sparging promotes the growth of aerobic bacteria in saturated zones and may
oxidize reduced chemical species (Nadim et al 2000). Air sparging has been shown to be
effective in removing several types of contaminants such as the lighter petroleum
compounds (C
3
-C
10
) and chlorinated solvents (Marley et al. 1992; Reddy et al. 1995).
3.1.4 Excavation
Excavation (removal) is a fundamental remediation method involving the removal of
contaminated soil/media, which can be shipped off-site for treatment and/or disposal, or
treated on-site when contaminants are amenable to reliable remediation techniques.
Excavation is generally utilized for localized contamination and point source and is also
used for the removal of underground structures that are out of compliance or have been
identified as a potential or actual point source of contamination. The limiting factor for the
use of excavation is often represented by the high unit cost for transportation and final off-
site disposal. EPA (1991) further stated some limiting factors that may limit the applicability
and effectiveness of the process to include:
