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ANNEX
D
SOIL
VENTING/ AIR SPARGING
Soil Venting
Description
Soil venting
is an in-situ technique for the extraction of volatile contaminants
from unsaturated zone soil (see Figure D1). Vacuum is applied
through extraction wells to create a pressure gradient that
induces gas phase volatile chemicals to move through the soil
to the wells. Contaminated soil is gradually cleaned up when
the volatile chemicals partition from the soil particles to
the air pockets between the soil particles and are then extracted.
Water vapour may also be extracted with the gases; however
soil vapour extraction cannot be used below the groundwater
table.
This method
is especially suitable for remediation in built-up areas because
wells can be placed between or below buildings, especially
using inclined drilling techniques. A system of multiple wells
at several depths is commonly used. Extracted vapours should
be treated by charcoal filters or catalytic oxidation units
to remove the contaminants before discharge.
Target
contaminants
- volatile
compounds with vapour pressure exceeding 1.0 mmHg at 25oC.
Suitable
for application in soil types
- coarser
grained materials (gravels and sands)
- field
pilot tests are necessary if suitability of soil types is
in doubt.
Limitations
- does
not work for low permeability soil or soil saturated with
groundwater
- does
not work well in heterogeneous soil since the air will likely
move through preferential zones which are more permeable.
Cleanup in less permeable zones then becomes difficult.
Treatment
time
The treatment
time generally falls within the range of 6 - 48 months.
Air
Sparging
Description
Air sparging
can be regarded as an extension of soil vapour extraction
in which air is injected into the saturated zone, stripping
volatile contaminants from groundwater, and transferring the
gaseous volatile contaminants to the vadose zone to be extracted
by soil venting (See Figure D1). Air sparging may result in
spreading of the contaminants in the horizontal direction.
If floating oil is present beneath a site, it should be removed
by recovery trenches or wells before air sparging. The behaviour
of injected air in the subsurface is highly dependent on the
subsurface conditions. Groundwater movement resulting from
air sparging is also site - specific. Therefore, field pilot-tests
are desirable to predict the response of the groundwater zone
to the operation of an air sparging system.
Target
contaminants
- volatile
compounds with a dimensionless Henry's constant greater
than 4.15 x 10-4, e.g. benzene, toluene, ethylbenzene and
xylene found in petroleum products, and chlorinated solvents.
Suitable
for application in soil types
- uniform,
coarser grained materials (gravels and sands)
Limitations
- If
the vertical air movement is restricted due to the presence
of a confining soil layer or by a large horizontal to vertical
permeability ratio, sparging can result in spreading of
the contaminants in the horizontal direction.
- Finer
grained soils or heterogeneous soils may lead to preferential
air channelling and poor air distribution.
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