The use of a Permeable Reactive Barrier (PRB) involves placement in the subsurface of a barrier consisting of a permanent, semi permanent or replaceable reactive media across the flow path of a contaminated groundwater plume. As the groundwater passes through the barrier under its natural gra­dient, contaminants are either degraded by, or retained in, the reactive media in a passive treatment system. A typical PRB involves the excavation and back­filling of a continuous trench with a reactive material designed to target par­ticular contaminants. Examples of reactive media used include iron, limestone, calcium phosphate-based minerals, compost and activated carbon, with iron being the most common.86 A review of the uses of these various reactive media is provided by Thiruvenkatachari et al.87 Zero-valent iron (ZVI) acts as a reactive medium through corrosion/oxidation of the metal in situ and donation of electrons from this process to organic and inorganic contaminants, such as halogenated hydrocarbons, U(VI) and Cr(VI), which are reduced thereby leading to degradation of the organic contaminant or metal immobilization.87 Conse­quently, the long-term efficiency of ZVI barriers is heavily dependent on the corrosion of Fe0, as continued use results in authigenic mineral formation which restricts the availability of reactive Fe0.88 However, the precipitation of ferrihydrite clusters found away from the immediate surface of the Fe0 barrier provides an increase in potential sites for metal adsorption thus prolonging the life of the PRB.88,89 A PRB containing ZVI was used in a remediation effort at Oak Ridge and is discussed in more detail later in this chapter.

Advantages of using a PRB include the in situ capture of contaminants, alleviating the need to manage the waste generated by pump and treat methods. Additionally, multiple contaminants, such as metals, radionuclides and organics, can be treated simultaneously;90 and both operating and maintenance costs are typically low.91 A review of the long-term performance of PRBs is presented by Henderson and Demond.92