Performance of a zerovalent iron reactive barrier for the treatment of arsenic in groundwater: Part 1. Hydrogeochemical studies Richard T. Wilkin ⁎, Steven D. Acree, Randall R. Ross, Douglas G. Beak 1 , Tony R. Lee U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Ground Water and Ecosystems Restoration Division, 919 Kerr Research Drive, Ada, Oklahoma 74820, United States article info abstract Article history: Received 14 August 2008 Received in revised form 3 December 2008 Accepted 8 December 2008 Available online 24 December 2008 Developments and improvements of remedial technologies are needed to effectively manage arsenic contamination in groundwater at hazardous waste sites. In June 2005, a 9.1 m long,14 m deep, and 1.8 to 2.4 m wide (in the direction of groundwater flow) pilot-scale permeable reactive barrier (PRB) was installed at a former lead smelting facility, located near Helena, Montana (USA). The reactive barrier was designed to treat groundwater contaminated with moderately high concentrations of both As(III) and As(V). 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. A monitoring network of approximately 40 groundwater sampling points was installed in July 2005. Monitoring results indicate arsenic concentrations N 25 mg L -1 in wells located hydraulically upgradient of the PRB. Of 80 groundwater samples collected from the pilot-PRB, 11 samples exceeded 0.50 mg As L -1 ; 62 samples had concentrations of arsenic at or below 0.50 mg L -1 ; and, 24 samples were at or below the maximum contaminant level (MCL) for arsenic of 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 depth intervals impacted by the emplaced zerovalent iron. This study indicates that zerovalent iron can be effectively used to treat groundwater contaminated with arsenic given appropriate groundwater geochemistry and hydrology. The study also further demonstrates the shortcomings of hanging-wall designs. Detailed subsurface characterization data that capture geochemical and hydrogeologic variability, including a flux-based analysis, are needed for successful applications of PRB technology for arsenic remediation. Published by Elsevier B.V. Keywords: Permeable reactive barrier Zerovalent iron Arsenic Contaminant flux 1. Introduction Permeable reactive barrier (PRB) technology has gained acceptance as an effective passive remediation strategy for the treatment of a variety of chlorinated organic and inorganic contaminants in groundwater (e.g., O'Hannesin and Gillham, 1998; Blowes et al., 2000). The technology combines subsurface fluid-flow management with contami- nant treatment by combinations of chemical and biological processes. Application of PRBs for the treatment of contami- nated groundwater has advantages over traditional pump- and-treat systems in that PRBs are passive and require minimal operation and maintenance expenditures. Yet few case studies are available that document the field perfor- mance of these in-situ systems at hazardous waste sites, especially with respect to the treatment efficiency of a variety of contaminant types and including examples from complex hydrogeologic environments. In some cases, PRB applications for groundwater remedia- tion have failed to achieve cleanup levels as expected from bench-scale tests. For example, Morrison et al. (2006) reported on a zerovalent iron system that showed sooner than expected breakthrough of molybdenum and uranium. Journal of Contaminant Hydrology 106 (2009) 1–14 ⁎ Corresponding author. Tel.: +1 580 436 8874; fax: +1 580 436 8703. E-mail address: wilkin.rick@epa.gov (R.T. Wilkin). 1 Current address: CSIRO, Division of Land and Water, Gate 4, Waite Road, Urrbrae, SA 5064, Australia. 0169-7722/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jconhyd.2008.12.002 Contents lists available at ScienceDirect Journal of Contaminant Hydrology journal homepage: www.elsevier.com/locate/jconhyd