The evaluation focuses on developing optimal strategies for delivering and distributing soluble ground-water amendments serving as organic substrates (the electron donor), thereby creating an in-situ biobarrier. The active approach relies on continuous recirculation of ground water into which amendments are injected, while the semipassive approach uses intermittent recirculation of amendments (Figure 2). Both approaches employ an electron donor in soluble form that allows easy distribution in the subsurface target zone.This strategy relies upon addition of electron donor in quantities high enough to achieve target perchlorate reductions, but low enough to avoid negative impacts on secondary water-quality characteristics. In contrast, a fully passive system involves injection of large quantities of electron donor during a single injection event, which maintains reducing conditions for several years but can more adversely affect secondary water-quality characteristics.
The Longhorn AAP semi-passive biobarrier was constructed downgradient of a former landfill, where earlier investigations identified a 250-foot-wide perchlorate plume with concentrations exceeding 3,000 ìg/L. A shallow aquifer extends to a depth of 35 feet bgs within interbedded sand, silt, and clay, and ground-water velocity in the treatment zone is approximately 37 ft/yr.