This ESTCP project monitored and evaluated the performance of a funnel-and-gate permeable reactive wall and its ability to remediate groundwater contaminated with chlorinated solvents including trichloroethylene (TCE), perchloroethylene (PCE), and cis-1,2 dichloroethene (DCE). The reactive wall was installed at the former NAS Moffett Field, California, by the U.S. Navy Engineering Field Activity West in April 1996, and the Naval Facilities Engineering Services Center (NFESC) subsequently conducted the performance and cost evaluations. The permeable reactive wall is an innovative in-situ technology in which a reactive cell, typically composed of granulated iron, is emplaced across the contaminated plume. As the contaminants flow through the cell, an abiotic chemical reduction reaction occurs between zero-valent iron and the chlorinated hydrocarbons, which produces less toxic, readily biodegradable by-products, including non-chlorinated hydrocarbons and chloride, iron, and hydroxide ions.
The reactive cell has consistently reduced groundwater concentrations of PCE (2,900 ug/L), TCE (26 ug/L) cis-1,2-DCE (280 ug/L), and vinyl chloride (4 ug/L) to well below their respective drinking water maximum contaminant levels (MCLs) or to non-detect within the first few feet of the iron cell, without contributing significant levels of dissolved by-products. Although calcium and iron compounds were observed to be precipitating in the reactive cell, there was no apparent detrimental effect on hydraulic performance during the 20-month demonstration period. Water level measurements and bromide tracer tests showed that flow patterns through the reactive cell were heterogeneous, with more flow apparent through the deeper portions of the cell. Linear velocities were lower than expected from previous site characterization (pumping tests) and modeling, and equated to a 3-12 day residence time (0.5 to 1.5 ft/day flow velocities).
The estimated capital cost for a full-scale system at Moffett Field was $4.9 million, with an annual operating cost of $70K (monitoring) and an assumed iron replacement cost of $270K every 10 years. Although the initial capital investment is higher, the overall costs would be less than the conventional pump-and-treat option after about 6-8 years of operation, with cost savings ranging from 29% to 64% for a 20- to 30-year remediation period. Over 2,000 U.S. Department of Defense (DoD) sites are contaminated with chlorinated compounds, and implementation of reactive wall systems could potentially save the DoD millions of dollars annually.
Although the technology is ready for field use, permeable reactive walls must be carefully designed on a case-by-case basis for: (1) contaminant type and concentration; (2) size of plume; (3) groundwater velocity; (4) aquifer geochemistry; and (5) geotechnical considerations. Results from this demonstration indicated that the targeted groundwater was being captured and treated by the reactive barrier, although the iron cell was over-designed to treat greater than expected linear flows with a shorter residence time. Estimation of long-term maintenance costs is uncertain because the longevity of field systems and their hydraulic performance may be compromised by chemical precipitation build-up. General maintenance techniques for the replacement of the reactive cell have not yet been established. (Project Completed - 1999)