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.
http://www.clu-in.org/download/Citizens/a_citizens_guide_to_in_situ_chemical_reduction.pdf
http://www.clu-in.org/download/Citizens/a_citizens_guide_to_permeable_reactive_barriers.pdf
http://t2.serdp-estcp.org/t2template.html#tool=PermeableMulchBiowalls&page=Biowalls
http://t2.serdp-estcp.org/t2template.html#tool=PRB&page=Introduction
Other Resources and Demonstrations
See http://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-107/ER-107
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.serdp-estcp.org/content/download/3420/56413/file/CU-9604-FR-01.pdf
for a
report on a demonstration at Moffett Field.
See http://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminants-on-Ranges/Protecting-Groundwater-Resources/ER-1232/ER-1232
and http://www.serdp-estcp.org/content/download/4355/65279/file/ER-0223-C&P.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.
See https://ert2.navfac.navy.mil/printfriendly.aspx?tool=PermeableMulchBiowalls for alternative to PRBs
using mulch.
See
http://t2.serdp-estcp.org/t2template.html#tool=Perchlorate&page=Intro1
for PRB
application to treat perchlorate.