Air
Sparging
Description
Air
sparging
is the process of injecting air directly into groundwater.
Air sparging remediates groundwater by volatilizing
contaminants
and enhancing biodegradation.
It is akin to blowing bubbles from a straw into a bowl of water. As the
bubbles
rise, the contaminants are removed from the groundwater by physical
contact
with the air (i.e., stripping) and are carried up into the unsaturated zone
(i.e., soil). As the contaminants move into the soil, a soil vapor
extraction
system is usually used to remove vapors. The addition of oxygen to
contaminated
groundwater and soils also enhances biodegradation of contaminants in
and above
the water table, as it acts as a nutrient for bacteria. There are
several
enhancements to air sparging.
Air
sparging is sometimes referred to as in situ
air
stripping. When used in combination with soil vapor extraction (SVE),
air
bubbles carry vapor phase contaminants to a SVE system which removes
them. The
SVE system controls vapor plume migration by creating a negative
pressure in
the unsaturated zone through a series of extraction wells. Using air
sparging
as an SVE enhancement technology increases contaminant movement and
enhances
oxygenation in the subsurface which increases the rate of contaminant
extraction.
Enhancements
The
first enhancement is called cometabolic air
sparging. Propane, which is injected along with air, acts as another
nutrient
for microorganisms.
Another
method is a newly patented technology, Bio-SpargeTM. It
addresses a major
problem of traditional pump-and-treat
systems. When a pump-and-treat system is employed, it pumps
contaminated water
to the surface and pulls the groundwater down into a Òcone of
depression.Ó At
the former upper extent of the groundwater (called the capillary fringe),
most of the sorbed contaminant remains. In many instances, this area
contains
the most heavily contaminated soil. When groundwater pumping ceases,
the
groundwater returns to its former level and becomes re-contaminated by
the
sorbed material. This is called a rebound.
Bio-SpargeTM induces desorption
within the capillary fringe by injecting a heated mixture of air,
water,
nutrients and bacteria. A downgradient well recovers the air for
reinjection.
As the air mixture moves across the contaminated zone, bacteria, with
the aid
of the nutrients, destroy many of the contaminants. Since the system is
a closed
loop, no release
to the environment occurs. That is, there is no need for off-gas
treatment and no need for an air emission permit.
Another
method is called the C-Sparge. C-Sparging periodically injects an
ozone/air
mixture in conjunction with a pulsing pump. This is a two-phase
process. First,
fine bubbles are injected into the groundwater to extract dissolved volatile
organic
compounds (VOCs)
out of contaminated groundwater. Second, the ozone
contained within the bubbles reacts rapidly with the volatile
compounds to decompose
them into end products consisting of carbon dioxide, dilute
hydrochloric acid,
and water.
Limitations
and Concerns
Air
flow through the saturated zone
may not be uniform.
There
can be uncontrolled movement of potentially dangerous vapors.
Air
sparging depends on two processes. One helps to biodegrade the
contaminants by
adding nutrients or other amendments, so transformation products should
be
monitored. The other process removes remaining contaminants from the
groundwater, possibly emitting them into the air.
Air
injection wells must be designed for site-specific conditions. Depth of
contaminants and specific site geology must be considered. Soil
heterogeneity
may cause some zones to be relatively unaffected.
Applicability
The
target contaminant groups for air sparging are VOCs and fuels in
groundwater.
Air sparging is generally more applicable to the lighter gasoline
constituents
such as benzene, ethylbenzene, toluene, and xylene. It is less
applicable to
heavier constituents such as diesel fuel and kerosene.
Technology
Development Status
Air
sparging has been demonstrated at numerous sites. It is a commercial
technology.
Web
Links
http://www.frtr.gov/matrix2/section4/4-34.html
http://www.serdp-estcp.org/content/download/3774/59748/file/199810.pdf
http://www.serdp-estcp.org/content/download/3762/59652/file/CU-9810-FR-01.pdf
http://www.clu-in.org/techfocus/default.focus/sec/Air_Sparging/cat/Overview/
http://www.clu-in.org/download/toolkit/sparge_o.pdf
http://www.clu-in.org/download/contaminantfocus/dnapl/Treatment_Technologies/Air_Sparg_TR-2193.pdf
http://www.serdp-estcp.org/content/download/3733/59410/file/CU-9808-FR-01.pdf
Other
Resources and Demonstrations
None