|Field Application of a Permeable Reactive Barrier for Treatment of Arsenic in Ground Water (EPA/600/R-08/093) September 2008
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 9.1 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 zerovalent iron is controlled by adsorption and co-precipitation with iron corrosion products. Previous studies indicate removal capacities on the order of 1 to 10 mg arsenic per gram of granular iron. A monitoring network of approximately 40 ground-water sampling points was installed in July 2005. Monitoring results indicate arsenic concentrations >25 mg L-1 in wells located hydraulically upgradient of the PRB. Within the PRB, arsenic concentrations are reduced to 2 to <0.01 mg L-1. After 2 years of operation, monitoring points located within 1 m of the downgradient edge of the PRB showed significant decreases in arsenic concentrations at depths intervals impacted by the emplaced zerovalent iron. Arsenic removal in the PRB results from several pathways involving adsorption to iron oxide and iron sulfide surfaces. These different uptake processes lead to multiple oxidation states and bonding environments for arsenic in the reactive medium as indicated using spectroscopic methods. This report covers aspects of site characterization, remedial design and implementation, and monitoring results for this pilot-scale PRB, including a flux-based analysis for arsenic.
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