CONTROL OF BTEX MIGRATION USING A BIOLOGICALLY ENHANCED PERMEABLE BARRIER

A permeable barrier system, consisting of a line of closely spaced wel ls, was installed perpendicular to ground water flow to control the mi gration of a dissolved hydrocarbon plume. The wells were charged with concrete briquets that release oxygen and nitrate at a controlled rate , enhancing aerobic biodegradation in the downgradient aquifer. Labora tory batch reactor experiments were conducted to identify concrete mix tures that slowly released oxygen over an extended time period. Concre tes prepared with urea hydrogen peroxide were unsatisfactory, while co ncretes prepared with calcium peroxide and a proprietary formulation o f magnesium peroxide (ORC (R)) gradually released oxygen at a steadily declining rate. The 21 percent MgO2 concrete cylinders and briquets r eleased oxygen at measurable rates for up to 300 days, while the 14 pe rcent CaO2 briquets were exhausted by 100 days. A full-scale permeable barrier system using ORC was constructed at a gasoline-spill site. Du ring the first 242 days of operation, total BTEX decreased from 17 to 3.4 mg/L and dissolved oxygen increased from 0.4 to 1.8 mg/L during tr ansport through the barrier. Over time, BTEX treatment efficiencies de clined, indicating the barrier system had become less effective in rel easing oxygen and nutrients to the highly contaminated portion of the aquifer. Point dilution tests and sediment analyses performed at the c onclusion of the project indicated that the aquifer in the vicinity of the remediation wells had been clogged by precipitation with iron min erals. This clogging is believed to result from high pH from the concr ete and oxygen released by the ORC. Oxygen-releasing permeable barrier s and other aerobic bioremediation processes should be used with cauti on in aquifers with high levels of dissolved iron.