AN IN-SITU PERMEABLE REACTIVE BARRIER FOR THE TREATMENT OF ARSENIC IN GROUND WATER

Contamination of ground-water resources by arsenic is a widespread environmental problem; consequently, there is an escalating need for developments and improvements of remedial technologies to effectively manage arsenic contamination in ground water and soils. In June 2005, a 7 m long, 14 m deep, and 1.8 to 2.4 m wide (in the direction of ground-water flow) pilot-scale permeable reactive barrier (PRB) was installed at a former metal smelting facility, located near Helena, Montana. The reactive barrier was designed to treat ground water contaminated with moderately high concentrations of both arsenite and arsenate. The reactive barrier was installed over a 3-day period using bio-polymer slurry methods and modified excavating equipment for deep trenching. The reactive medium was composed entirely of granular iron which was selected based on long-term laboratory column experiments. In laboratory experiments, arsenic removal by zero-valent iron is controlled by adsorption and co-precipitation with iron corrosion products and results indicate removal capacities on the order of 10 mg arsenic per gram of granular iron. A monitoring network of approximately 50 ground-water sampling points was installed in July 2005. Early monitoring results indicate arsenic concentrations >25,000 mg/L in wells located hydraulically upgradient of the PRB. Within the PRB, arsenic concentrations are reduced to 1,500 to <10 mg/L. After 1 year of operation, monitoring points located within 1 m of the downgradient edge of the PRB do not show significant decreases in arsenic concentrations, although concentration reductions are expected in downgradient locations during the second year of operation based on estimated ground-water seepage velocities and arsenic uptake rates in the PRB. The presentation will cover aspects of site characterization, remedial design and implementation, and monitoring results. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.