In-Well
Air Stripping
Description
In-well air
stripping technology injects air
into a vertical well that has been screened at two depths. The lower
screen is
set in the saturated zone (i.e., groundwater), and the upper
screen is in the unsaturated zone (i.e.,
called the vadose zone, otherwise
referred to as soil).
Pressurized air is injected into the well below the water table, aerating the water. The
aerated water rises in
the well and flows out of the system at the upper screen. Contaminated
water is
drawn into the system at the lower screen. The volatile organic compounds (VOCs)
vaporize within the well at the top of the water table, as the air
bubbles out
of the water, very similar to an above-ground air stripper. The vapors are drawn off by a
soil vapor extraction (SVE) system and
treated.
The partially
treated groundwater is never
brought to the surface. After it is released to the unsaturated
zone, the water
percolates back down to the groundwater. Contaminant concentrations
are gradually
reduced as the process is repeated.
Limitations
and Concerns
These systems
only treat the water that passes
through the stripping well. Thus the radius of influence is limited by
the
pumping capacity of each well and the hydrogeologic
characteristics of the site.
Much remains to be learned about specific applications of in-well
stripping,
particularly regarding the questions of radius of influence and
groundwater
flow regime around the well.
Effective
installations require a well-defined
contaminant plume and well-placed
screens to prevent the
spreading of the contamination.
Some
contaminants are transferred from the
groundwater to the soil. Depending on site geology, these contaminants
may be sorbed to soil particles
and slowly desorbed
after treatment is complete.
In general,
in-well air strippers are most
effective at sites containing high concentrations of dissolved
contaminants.
Effectiveness
may be limited in shallow aquifers.
To prevent
smearing the contaminants in the area
immediately above the groundwater level, the process should not be used
at
sites containing non-aqueous phase liquids (NAPLs).
In-well
stripping generally takes a very long
time.
Applicability
This
technology remediates VOCs, some of the
lighter semi-volatile organic compounds (SVOCs),
and
fuels in groundwater. When combined with a vacuum extraction system, it
also
remediates contaminated soil. Typically, in-well air stripping systems
are more
cost-effective for remediating VOC-contaminated groundwater at sites
with deep
water tables because the water does not need to be brought to the
surface.
Technology
Development Status
In-well air
stripping is a commercial
technology. There are several variations of this technology. The most
common is
called Unterdruck-Verdampfer Brunner (UVB) and has been used at
numerous sites
in Germany.
Stanford
University has developed another
variation of this process called NoVOCs that has been evaluated as part
of the
U.S. Department of EnergyÍs Integrated Technology Demonstration
Program and by
the U.S. Navy. Wasatch Environmental, Inc. has developed an in-well
vapor
stripping system that enhances bioremediation and
discharges extracted vapors
into the unsaturated zone for degradation by naturally-occurring
microorganisms.
Web Links
http://www.frtr.gov/matrix2/section4/4-40.html
http://www.osti.gov/bridge/servlets/purl/7093-MSCtC9/webviewable/7093.PDF
Other
Resources and Demonstrations
See
descriptions of Air Sparging, Groundwater Circulation Wells,
and Soil Vapor Extraction.