Permeable Reactive Barriers

Description

Permeable Reactive Barriers (PRBs) are installed in or down gradient from 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 the wall captures the 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 provides greater capture of the plume.

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.

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 the wall is used for precipitation of metals, it is not certain how long the walls 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

Technology Development Status

This technology is commercially available.

Web Links

http://www.frtr.gov/matrix2/section4/4_46.html

http://www.sandia.gov/Subsurface/factshts/ert/reacbarr.pdf

http://enviro.nfesc.navy.mil/erb/restoration/technologies/remed/phys_chem/phc-37.asp

http://clu-in.org/download/techdrct/tdfieldapp_prb.pdf

http://enviro.nfesc.navy.mil/ps/projects/permeabl.htm

http://www.estcp.org/projects/cleanup/199604v.cfm

Other Resources and Demonstrations

See the http://www.itrcweb.org/PBW_1.pdf Regulatory Guidance for Permeable Barrier Walls Designed to Remediate Chlorinated Solvents, 1999, http://www.itrcweb.org/PRB_3.pdf, Regulatory Guidance for Permeable Reactive Barriers Designed to Remediate Inorganic and Radionuclide Contamination, 1999, and http://www.itrcweb.org/user/PBW-2ExecSum.pdf for summary of Design Guidance for Application of Permeable Barrier Walls to Remediate Dissolved Chlorinated Solvents, Battelle for US Air Force, 2000.

See http://www.epa.gov/tio/products/newsltrs/gwc/gwccurre.htm#sodium for a description of a field-scale pilot study at the U.S. Coast Guard Support Center in Elizabeth City, NC that injected sodium dithionite into the aquifer and vadose zone to create a PRB system using naturally occurring iron for hexavalent chromium (Cr VI) reduction.

See http://www.epa.gov/radiation/cleanup/documents.html#frycanyon for a description of Field Demonstration of Permeable Reactive Barriers to Remove Dissolved Uranium from Groundwater, Fry Canyon, Utah.

See http://id.inel.gov/astd/pdf/98-TDI-34.PDF for status of PRB demonstration for radionuclides and metals.

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.serdp.org/reporting/CU-1232.pdf for a description of experiments with PRB for Royal Demolition Explosive (RDX) and trinitrotoluene (TNT).

See http://enviro.nfesc.navy.mil/erb/erb_a/restoration/technologies/remed/phys_chem/PRBSuccess.pdf and http://enviro.nfesc.navy.mil/erb/erb_a/restoration/technologies/remed/phys_chem/Moffett-cost_perf.pdf for cost and performance data for a pilot-scale PRB at Moffett Field.