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, Tc, U, V, NO3, PO4, and SO4. Small scale field studies have indicated the potential for treatment of Cd, Cr, Cu, Fe, Ni, Pb, NO3, PO4, and SO4. Permeable reactive barriers have been used in full-scale installations for the treatment of hexavalent chromium, dissolved constituents associated with acid-mine drainage, including SO4, 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, SO,, 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 l(-1) to <0.01 mg l(-1).

Abstract: Despite encrustation by Fe and Al hydroxides, limestone can be effective for remediation of acidic mine drainage (AMD). Samples of water and limestone (CaCO3) were collected periodically for 1 a at 3 identical limestone-filled drains in Pennsylvania to evaluate the attenuation of dissolved metals and the effects of pH and Fe- and Al-hydrolysis products on the rate of CaCO3 dissolution. The influent was acidic and relatively dilute (pH < 4; acidity < 90 mg) but contained 1-4 mg . L-1 of O-2, Fe3+, Al3+ and Mn2+. The total retention time in the oxic limestone drains (OLDs) ranged from 1.0 to 3.1 hr. Effluent remained oxic (O-2 > 1 mg . L-1) but was near neutral (pH = 6.2-7.0); Fe and Al decreased to less than 5% of influent concentrations. As pH increased near the inflow, hydrous Fe and Al oxides precipitated in the OLDs, The hydrous oxides, nominally Fe(OH)(3) and Al(OH)(3), were visible as loosely bound, orange-yellow coatings on limestone near the inflow. As time elapsed, Fe(OH)(3) and Al(OH)(3) particles were transported downflow. The accumulation of hydrous oxides and elevated pH (> 5) in the downflow part of the OLDs promoted sorption and coprecipitation of dissolved Mn, Cu, Co, Ni and Zn as indicated by decreased concentrations of the metals in effluent and their enrichment relative to Fe in hydrous-oxide particles and coatings on limestone. Despite thick (similar to 1 mm) hydrous-oxide coatings on limestone near the inflow, CaCO3 dissolution was more rapid near the inflow than at downflow points within and the OLD where the limestone was not coated. The high rates of CaCO3 dissolution and Fe(OH3) precipitation were associated with the relatively low pH and high Fe3+ concentration near the inflow. The rate of CaCO3 dissolution decreased with increased pH and concentrations of Ca2+ and HCO3- and decreased Pco(2). Because overall efficiency is increased by combining neutralization and hydrolysis reactions, an OLD followed by a settling pond requires less land area than needed for a two-stagetreatment system consisting of an anoxic limestone drain and oxidation-settling pond or wetland. To facilitate removal of hydrous-oxide sludge, a perforated-pipe subdrain can be installed within an OLD. (C) 1999 Elsevier Science Ltd.