Groundwater Quality: Remediation and Protection (Proceedings of the GQ'98 Conference held at 4g3 Tubingen, Germany, September 1998). IAHS Publ. no. 250, 1998. Treatment of dissolved metals using permeable reactive barriers D. W. BLOWES, C. J. PTACEK, S. G. BENNER, C. W. T. McRAE Department of Earth Sciences, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 R. W. PULS National Risk Management Research Laboratory, US EPA, Subsurface Protection and Remediation Division, PO Box 1198, Ada, Oklahoma 74820, USA Abstract Permeable reactive barriers are a promising new approach to the treatment of dissolved contaminants in aquifers. This technology has progressed rapidly from laboratory studies to full-scale implementation over the past decade. Laboratory treatability studies indicate the potential for treatment of a large number of inorganic contaminants, including As, Cd, Cr, Cu, Hg, Fe, Mn, Mo, Ni, Pb, Se, Te, U, V, N0 3 , P0 4 , and S0 4 . Small- scale field studies have indicated the potential for treatment of Cd, Cr, Cu, Fe, Ni, Pb, N0 3> P0 4 , and S0 4 . Permeable reactive barriers have been used in full-scale installations for the treatment of hexavalent chromium, dissolved constituents associated with acid-mine drainage, including S0 4 , Fe, Ni, Co and Zn, and dissolved nutrients, including nitrate and phosphate. A full-scale barrier designed to prevent the release of contaminants associated with inactive mine tailings impoundment was installed at the Nickel Rim mine site in Canada in August 1995. This reactive barrier removes Fe, S0 4 , Ni and other metals. The effluent from the barrier is neutral in pH and contains no acid-generating potential, and dissolved metal concentrations are below regulatory guidelines. A full-scale reactive barrier was installed to treat Cr(VI) and halogenated hydrocarbons at the US Coast Guard site in Elizabeth City, North Carolina, USA in June 1996. This barrier removes Cr(VI) from >8 mg f to <0.01 mg l 1 . INTRODUCTION The development and application of permeable reactive barriers for the treatment of groundwater contaminated by inorganic constituents has progressed rapidly over the past decade. Permeable reactive barriers have been developed and demonstrated to be effective for the treatment of dissolved metals (Blowes & Ptacek, 1992; Powell et al, 1995; Blowes et al, 1997; McRae et al, 1997), acid-mine drainage (Waybrant et al, 1995; Benner et al, 1997), and dissolved nutrients (Robertson & Cherry, 1995; Baker et al, 1997). Permeable reactive barriers are created by intercepting a plume of contaminated groundwater with a permeable reactive material. The reactive material properties are selected to promote the attenuation of the contaminant through degradation or precipitation/adsorption into a sparingly soluble phase. Reactive mixtures for the attenuation of inorganic species must be designed to maintain their permeability as secondary precipitates accumulate. The barrier design must also ensure that the contaminant will remain immobilized within the aquifer, or must be