The Naval Facilities Engineering Service Center and Battelle Memorial Institute conducted a performance evaluation of permeable reactive barrier (PRB) installations at several Department of Defense (DoD) sites containing chlorinated solvents in groundwater. This project was jointly funded by SERDP and ESTCP and was coordinated with similar studies conducted by the U.S. Environmental Protection Agency and Department of Energy. Its objective was to evaluate the hydraulic performance and longevity of PRBs, primarily those containing zero-valent iron.
PRBs are passive remediation systems consisting of a trench designed to intercept contaminated groundwater, into which, reactive material (e.g., zero-valent iron) is placed and used for chlorinated solvent reduction. A lingering question is long-term PRB efficacy as well as potentially limiting factors (e.g., precipitation on reactive iron surfaces). Since PRBs are initially more expensive to install than traditional pump-and-treat systems, it is imperative to further qualify these systems.
The hydraulic performance evaluation was conducted at the former Lowry Air Force Base (AFB), Colorado; Seneca Army Depot, New York; the former Naval Air Station (NAS) Moffett Field, California; and Dover AFB, Delaware. These sites represented different hydrogeologic conditions and different PRB designs (e.g., continuous reactive barriers and funnel-and-gate systems). The longevity evaluation initially focused on field measurements, such as groundwater sampling and iron core sampling. However, due to inherent limitations in making long-term projections based on these measurements, a laboratory simulation was conducted to evaluate the longevity of the PRBs at Lowry and Moffett Field. In both the Lowry and Moffett Field PRBs, it was estimated that it would take approximately 30 years for the reactivity of the iron to decline by a factor of 2. The mass flux of native dissolved solids in the groundwater determined the longevity of the iron. Although total dissolved solids levels were higher at Lowry, the groundwater flow was much slower than at Moffett Field. The resulting mass flux of dissolved solids and, therefore, the rate of loss of reactivity were projected to be similar at the two sites.
Potentially, up to 1,000 DoD sites are contaminated with chlorinated solvents and other contaminants that could benefit from the use of PRBs. The benefits from this project are improved PRB designs by site owners and greater acceptance of this technology by regulators. Unlike pump-and-treat systems, PRBs, once installed, can be difficult and expensive to modify. This project enhances the understanding of the short-term (i.e., hydraulics) and long-term (i.e., geochemistry and cost) performance of existing PRBs and provides guidance for designing future PRB installations. The benefit to DoD is that current PRB installations can be better managed and future PRBs can be designed to optimize performance and cost. (Project Completed - 2003)