Since the installation of a zero valent iron (ZVI) permeable reactive barrier (PRB) in November 2004 at Operable Unit (OU) 12, Hill Air Force Base, Utah, TCE concentra- tions downgradient of the barrier have not achieved the design target concentration of less than 5 micrograms per liter (μg/L), although the PRB has successfully reduced over- all plume mass downgradient of the PRB by one half. Initially downgradient TCE con- centrations decreased as a result of treatment by the PRB. However, after the first year, downgradient concentrations have generally increased while upgradient concentrations have remained relatively constant, indicating a reduction in treatment efficiency. The OU 12 TCE plume is 8,500 feet long extending off base beneath a residential area. The PRB fully intersects the plume and was constructed perpendicular to ground- water flow, approximately 1,500 feet upgradient of the leading edge of the plume, using a single-pass mechanical trencher. The 660-ft long, 30.5-ft deep, 1.5-ft wide PRB consists of approximately 37% iron and 63% sand, or an equivalent iron thickness of approxi- mately 0.55 ft. No safety factor was incorporated into the design specifications. The maximum influent TCE concentration is 150 μg/L.
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