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The main advantage of in situ treatment is that it allows soil
to be treated without being excavated and transported, resulting
in potentially significant cost savings. However, in situ
treatment generally requires longer time periods, and there is
less certainty about the uniformity of treatment because of the
variability in soil and aquifer characteristics and because the
efficacy of the process is more difficult to verify.
Physical/chemical treatment uses the physical properties of
the contaminants or the contaminated medium to destroy (i.e.,
chemically convert), separate, or contain the contamination. Soil
vapor extraction uses the contaminant's volatility to separate it
from the soil. Soil flushing uses the contaminant's solubility in
liquid to physically separate it from the soil. Surfactants may
be added to the flushing solution to chemically increase the
solubility of a contaminant. Solidification/stabilization also
uses both physical and chemical means. Solidification
encapsulates the contaminant, while stabilization physically
alters or binds with the contaminant. Pneumatic fracturing is an
enhanced technique that physically alters the contaminated
media's permeability by injecting pressurized air to develop
cracks in consolidated materials.
Physical/chemical treatment is typically cost effective and
can be completed in short time periods (in comparison with
biological treatment). Equipment is readily available and is not
engineering or energy-intensive. Treatment residuals from
separation techniques will require treatment or disposal, which
will add to the total project costs and may require permits.
Extraction fluids from soil flushing will increase the mobility
of the contaminants, so provisions must be made for subsurface
recovery.
Available in situ physical/chemical treatment technologies
include electrokinetic separation, fracturing
(blast-enhanced, pneumatic, and lasagna process), soil
flushing, soil vapor extraction, and solidification/stabilization. These treatment
technologies are discussed in Section 4. Completed in situ
physical/chemical treatment projects for soil, sediment, bedrock and
sludge are shown in Table 3-5 and
additional information on completed demonstration projects are
shown on the FRTR Web Site.
Certain in situ physical/chemical treatment technologies are
sensitive to certain soil parameters. For example, the presence
of clay or humic materials in soil causes variations in
horizontal and vertical hydraulic parameters, which, in turn,
cause variations in physical/chemical process performance.
Stabilization/solidification technologies are less sensitive to
soil parameters than other physical/chemical treatment
technologies.
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