Abstract: A multi-layered geosynthetic system that includes geosynthetic-clay liners (GCL) and a geonet-composite drain (GNGC) is being used to cap and stabilize a 178000 m(exp 2) heap-leach pad at the Summitville Mine Superfund Site in Colorado. Selected were materials on the basis of design requirements for permeability, strength, extreme site conditions and cost. The Summitville cleanup called for a heap-leach pad cap to provide a barrier that would prevent precipitation from infiltrating the pad material. This long-term remediation prevents the pile from becoming saturated and allowing water to overflow the downslope dike, which could cause instability to dike and pond. Three geosynthetic alternatives were proposed. The contractor, that was awarded the heap leach pad, phase 2 contract, decided for a geosynthetic clay liner cap placed directly on the redesigned slopes. Bentofix NW-8 was used as GCL and TexNet TN was selected as the geocomposite drain. Conformance testing, subgrade preparation review, geosynthetic installation/repair inspection and review of cover material placement, performed by independent construction-quality assurance, showed that GCL is a well-suited cap material for heap leach pads, where high wind, cold temperatures rain and high altitude hinder construction. The robust geosynthetic allowed on-site coarse material to be used in the subgrade and cover layer, which saved the cost of importing more expensive bedding material.

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.