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Contaminants: Reactive Media: Construction: Point of Contact: |
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A pilot-scale permeable reactive barrier (PRB) was constructed in April 1996 at the former NAS Moffett Field in Mountain View, CA, by the U.S. Navy Engineering Field Activity-West. Previous investigations identified extensive ground-water contamination on the site from dissolved chlorinated hydrocarbons—trichloroethylene (TCE), dichloroethylene (1,2-DCE), and perchloroethylene (PCE)—much of which originated offsite. Initial concentrations were 2,990 µg/L (TCE), 280 µg/L (1,2-DCE), and 26 µg/L (PCE) upgradient of the iron gate. The overall Moffett Field solvent plume is more than 10,000 ft long and about 5,000 ft wide. Subsurface sediments at the Moffett Field PRB site are a mixture of alluvial-fluvial clay, silt, sand, and gravel. Sands and gravels are present as lens-shaped, interbraided channel deposits that are presumed to have incised into the clay and silt layers. Contamination is present in two aquifer zones that extend from 5-60 ft below ground surface (bgs). These aquifer zones are separated by a discontinuous, semiconfining clay layer (aquitard) at approximately 25 ft bgs, ranging from 1-15 ft in thickness. Average linear flow velocities from onsite pumping tests were calculated to be about 1-4 ft/day. Hydraulic conductivity values for the separating aquitard layer range from 10-5 to 10-3 ft/min. Soil porosity values in the silts and sands ranged from 30-45%. A funnel-and-gate system was installed in the upper aquifer zone to just above the aquitard using a trenching method. The system includes a reactive iron gate that is 10 ft wide by 6 ft long and contains about 75 tons of granular zero-valent iron. The iron cell is bounded by 2-ft sections of pea gravel at upgradient and downgradient locations. Two 20-ft-long steel sheet pile funnels or wing walls are positioned on either side of the reactive iron gate. The U.S. Department of Defense Environmental Security Technology Certification Program (ESTCP) has sponsored the demonstration project at Moffett Field for the past three years. The Naval Facilities Engineering Service Center has collected performance monitoring and cost data to validate the PRB technology for potential use at DOD sites worldwide. Water quality sampling from 1996 (June and September) and 1997 (January, April, July, and October) from about 70 monitoring wells in or near the reactive barrier consistently have indicated significant degradation of chlorinated compounds. All principal contaminant concentrations had been reduced to below Maximum Contaminant Levels (MCLs) or non-detectable levels within the first 2-3 ft of the gate (iron cell). Bromide tracer testing at the PRB site revealed that flow velocities through the cell are about 0.5-2 ft/day. The final PRB technology evaluation report for the Moffett Field pilot demonstration project was published in November 1998. A summary version was published in December 1998.
Coring results have suggested that conditions exist for potential long-term formation of chemical precipitates in the iron cell. This may lead to an eventual reduction in the longevity and efficiency (permeability and reactivity) of the reactive barrier. The DOD ESTCP, U.S. Environmental Protection Agency (EPA), and U.S. Department of Energy (DOE), in partnership with the RTDF Permeable Reactive Barriers Action Team, are sponsoring additional performance and longevity evaluations to support widespread regulatory acceptance and encourage use of PRB technology. As part of these efforts to further investigate the potential concerns for biological fouling and chemical precipitation, annual water quality sampling and iron cell coring are planned at several PRB sites across the country, including Moffett Field, over the next three years. |
Sponsored by the Technology Innovation Program
Date Last Modified: January 14, 2000
Chlorinated Solvents