Permeable Reactive Barriers
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.
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.
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-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.