Permeable Reactive Barriers
Description
Permeable Reactive Barriers (PRBs) are installed downgradient from or in the
flow path of a contaminant plume. The contaminants
in the plume react with the media inside the barrier to either break the
compound down into harmless products or immobilize contaminants by
precipitation or sorption.
The distinguishing feature about this technology is that it is a passive system
that requires no pumping.
The most common of the permeable barrier walls is the Iron Treatment Wall.
It is made up of zero-valent iron or iron-bearing minerals that reduce
chlorinated contaminants such as trichloroethylene (TCE)
and perchloroethylene (PCE). As the iron is oxidized, a chlorine atom is
removed from the compound using electrons supplied by the oxidation of iron.
The chlorinated compounds are reduced to nontoxic by-products.
Reactive walls are also used to immobilize metals such as uranium, chromium,
and arsenic. A variety of materials have been used in pilot tests, including
iron, peat, and bone char. Essentially, these materials either absorb the
metals or precipitate them, similar to soil stabilization and precipitation
technologies.
Limitations and Concerns
There has been concern that a wall might not capture an entire plume. In
areas where there are preferential groundwater
flow paths, ensuring total capture may be difficult. In many designs, an impermeable
material such as a slurry
wall or sheet pile flanks the reactive zone. This is called a funnel and gate
system, and it enables greater capture.
Because this technology is passive (that is, it depends on the natural flow
of the contaminant plume to pass through the wall), complete breakdown will
only occur after the entire plume has passed through the wall. This may take
many years. A groundwater monitoring system should be put in place to monitor
whether the technology is still working over the long term.
If the plume is too close to site boundaries or receptors, it may not
applicable. Additional treatment technologies are necessary if contamination
has already passed the wall's location.
The cost to install a treatment wall increases significantly at depths
greater than 80 feet.
Wall permeability
may decrease due to the precipitation of metal or salts, or from biological
activity. Passive treatment walls may also lose their reactive capacity over
time, and the iron may have to be replaced periodically.
If a wall is used for precipitation of metals, it is not certain how long it
will continue to be effective, nor is there sufficient information about what
environmental conditions may influence remobilization.
Iron may leach out of the wall and become a contaminant if concentrations
are high enough.
If the wall is used for precipitation of metals, the media may have to be
removed and disposed of as a hazardous waste, or contained in some other
fashion.
Applicability
Target contaminant groups for passive treatment walls are volatile organic compounds
(VOCs), metals,
and radioactive contaminants. A recent Defense Department study showed some
promising results with the use of a PRB to reduce energetics (RDX and TNT) in
groundwater.
Technology Development Status
This technology is commercially available.
Web Links
http://www.frtr.gov/matrix2/section4/4-41.html
http://www.sandia.gov/Subsurface/factshts/ert/reacbarr.pdf
http://www.clu-in.org/download/rtdf/prb/reactbar.pdf
http://clu-in.org/download/techdrct/tdfieldapp_prb.pdf
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA410697&Location=U2&doc=GetTRDoc.pdf
See the http://www.itrcweb.org/Documents/PBW-1.pdf Regulatory Guidance for Permeable Barrier Walls Designed to Remediate Chlorinated Solvents, 1999, http://www.itrcweb.org/Documents/PRB-3.pdf Regulatory Guidance for Permeable Reactive Barriers Designed to Remediate Inorganic and Radionuclide Contamination, 1999, and http://www.itrcweb.org/Documents/PRB-4.pdf Permeable Reactive Barriers: Lessons Learned/New Directions , 2005, and http://www.itrcweb.org/Documents/PRB-2a.pdf for a summary of Design Guidance for Application of Permeable Barrier Walls to Remediate Dissolved Chlorinated Solvents, Battelle for US Air Force, 2000.
Other Resources and Demonstrations
See http://www.serdp.org/research/CU/CU-107.pdf
for a description of the pilot-scale PRB at Dover Air Force Base, Delaware.
See http://www.clu-in.org/products/newsltrs/gwc/gwc0401.htm#funnel
for a description of a full-scale funnel and gate system at the Marzone
Superfund site near Tifton, GA, to treat ground water contaminated with
pesticides and other organics.
See http://www.estcp.org/Technology/upload/CU-9604-FR-01.pdf
for a report on a demonstration at Moffett Field.
See http://www.serdp.org/reporting/CU-1232.pdf
and http://www.estcp.org/Technology/upload/ER-0223-C&P-2.pdf
for reports on PRBs for Royal Demolition Explosive (RDX) and trinitrotoluene
(TNT). removal.
See http://www.epa.gov/nrmrl/pubs/600r08093/600r08093.pdf
for the application of PRBs for arsenic.
See http://www.sandia.gov/Subsurface/factshts/ert/reacbarr.pdf
for reactive barriers to stabilize metals, including uranium.