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 EnergyÕs
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
Other
Resources and Demonstrations
See
descriptions of Air Sparging, Groundwater Circulation Wells,
and Soil Vapor Extraction.