Performance Evaluations At The Moffett Field And Department Of Defense Permeable Barrier Sites

A pilot scale permeable reactive barrier (PRB) demonstration project was initiated by the<br>US Navy Engineering Field Activity (EFA) West at the former Naval Air Station (NAS) Moffett<br>Field site in Mountain View, California in late 1995. Performance evaluations and cost-benefit<br>analyses were performed by US Naval Facilities Engineering Service Center (NFESC) at the<br>Moffett Field site, which were sponsored by the Department of Defense (DOD) Environmental<br>Security Technology Certification Program (ESTCP). The Moffett Field PRB uses a funnel-andgate<br>system design. The funnel is made of interlocking steel sheet piles and the gate consists of a<br>reactive cell filled with zero-valent granular iron. Performance monitoring was conducted at the<br>site to demonstrate the effectiveness of the PRB technology in capturing and remediating ground<br>water that contained dissolved chlorinated hydrocarbon compounds. The primary contaminants<br>of concern at Moffett Field in the vicinity of the PRB were trichloroethene (TCE), cis-1,2<br>dichloroethene (cDCE), and perchloroethene (PCE) at upgradient concentrations of about 2,900<br>micrograms per liter tug/L), 280 ug/L, and 26 ug/L, respectively. Monitoring events included<br>measuring water levels, testing field parameters, and ground-water quality sampling at about 75<br>monitoring points. Tracer tests using bromide solutions and flow-velocity meter testing were<br>also completed in April and August 1997. Iron cell coring samples were collected and analyzed<br>in December 1997 for early indications of chemical precipitation. The iron cell coring analyses<br>and geochemical modeling from Moffett Field indicated that changes in inorganic chemistry may<br>be caused by precipitation of calcium carbonates, iron-sulfide, and hydroxide compounds.<br>Chemical precipitates are of significant concern because of the potential loss of reactivity and<br>permeability in the iron cell. Long-term performance and life-expectancies of PRBs are generally<br>unknown. The DOD ESTCP, Environmental Protection Agency (EPA), and Department of<br>Energy (DOE) are sponsoring additional performance evaluations at several PRB sites to help<br>find answers to the longevity concerns. In the meantime, these agencies are also attempting to<br>help gain widespread regulatory acceptance and user confidence in implementing the PRB<br>technology.