Permeable Reactive Barrier Successfully Treats Plume at U.S. Coast Guard Facility in Region 4
Permeable Reactive Barriers (PRBs) are a new technique for treating and containing contaminated plumes in ground water. PRBs, also known as treatment walls, are installed across the flow path of a contaminated plume and often keyed into lower permeability material. Water flows through the wall while contaminants are degraded or retained in place. Zero-valent iron (ZVI) is the most common treatment agent used in PRBs.
In June 1996 a PRB was installed at the U.S Coast Guard (USCG) Support Center in Elizabeth City, North Carolina, to treat ground water contaminated with chromium and trichloroethylene (TCE) discharging from an old chrome plating shop on the base. The chromium originated from an electroplating shop that operated for more than 30 years before closing in 1984. TCE came from several sources, including the shop. Initial maximum concentrations entering the wall were more than 4,320 micrograms per liter (ug/L) for TCE and over 3,430 ug/L for Cr6+.
At the time it was constructed, the USCG PRB was the longest PRB on record. The wall is about 150 long and 2 feet thick, begins about 3 feet below ground surface, and extends to a depth of 24 feet. It was installed more rapidly and inexpensively by using an innovative trenching technique. A 60-ton trencher used to install drainage pipes in landfills was used to install the treatment wall. The trencher is a track-mounted vehicle that uses a large cutting chain excavator system to create the trench. As native material is excavated, granular zero-valent iron is simultaneously placed in the trench. This was the first time that direct trenching was used to install ZVI. The wall was installed in only one day and contains about 450 tons of granular iron.
The PRB was constructed down gradient of the source area to use the natural groundwater hydraulic gradient to transport the contaminants to the treatment zone. Contaminants are chemically reduced while passing through the iron filings. The wall cost $500,000 to install, including design, construction, materials, and iron, which cost about $175,000. According to the USCG, the use of the PRB will save nearly $4,000,000 over a traditional pump-and-treat system. In six years of operation, the PRB has required no maintenance. The average annual cost for monitoring over 4 years is $50,000.
The goal of the treatment wall is to contain and treat the plume emanating from the old plating shop at the base. This has successfully been accomplished, with the complete removal of the chromium and reduction of TCE to below treatment goals (0.05 mg/L for Cr6+ and 5 ug/L for TCE) for the plating shop plume. However, monitoring data indicate that some TCE may be located lower than the depth of the wall and migrating under the wall. TCE contamination also may be present west of the PRB and outside its influence. Additional characterization of other sources of TCE are now required to extend the system to fully treat all sources of TCE at the base.
To address the source of the chromium contamination. U.S. EPA's Robert S. Kerr Environmental Research Center conducted a pilot-scale test injecting sodium dithionate to reduce the Cr+6 to Cr+3. This test was successful and the USCG is now considering a full-scale remedy for the source area.
For further information contact Robert Puls at (580) 436-8543.