Control of BTEX migration using a biologically enhanced permeable barrier

A permeable barrier system, consisting of a line of closely spaced wells, was installed perpendicular to ground water flow to control the migration 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. Laboratory batch reactor experiments were conducted to identify concrete mixtures that slowly released oxygen over an extended time period. Concretes prepared with urea hydrogen peroxide were unsatisfactory, while concretes prepared with calcium peroxide and a proprietary formulation of magnesium peroxide (ORC (R)) gradually released oxygen at a steadily declining rate. The 21 percent MgO2 concrete cylinders and briquets released oxygen at measurable rates for up to 300 days, while the 14 percent CaO2 briquets were exhausted by 100 days. A full-scale permeable barrier system using ORC was constructed at a gasoline-spill site. During 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 transport through the barrier. Over time, BTEX treatment efficiencies declined, indicating the barrier system had become less effective in releasing oxygen and nutrients to the highly contaminated portion of the aquifer. Point dilution tests and sediment analyses performed at the conclusion of the project indicated that the aquifer in the vicinity of the remediation wells had been clogged by precipitation with iron minerals. This clogging is believed to result from high pH from the concrete and oxygen released by the ORC. Oxygen-releasing permeable barriers and other aerobic bioremediation processes should be used with caution in aquifers with high levels of dissolved iron.