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

Abstract

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