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Investigation of hydrological and geochemical properties and spatial relationships of an unsaturated waste rock pile, Key Lake, Saskatchewan Stockwell, Justin Emrich


Controls on environmental loading from acid mine drainage (ARD) are not well understood in waste rock material. In particular, relationships between subsurface flow and the timing, duration, and intensity of leaching of metals and acidity from waste rock dumps remain ill defined. This study investigates field-scale relationships between subsurface flow processes and geochemical processes in unsaturated waste rock. A 12-meter tall, unsaturated waste rock pile, constructed in 1994, was deconstructed and sampled at Key Lake, Saskatchewan, Canada, during the summer of 2000. A detailed sampling methodology was developed to characterize physical and chemical properties of waste rock material within the waste rock pile. Physical properties were characterized by measuring soil-water suction, volumetric water content, and grain-size distribution at 60 random and 20 grid locations within the pile. Paste pH, pore-water geochemistry, mineralogy, and analyses of selective extractions were used as metrics characterizing geochemical processes and sulfide oxidation at each of the 20 grid locations. Attempts to develop a field-based soil water characteristic curve failed due to spatial variability and hysteresis of waste rock properties. Grain-size distribution was used as a relative measure of subsurface flow. Geochemical characterization results demonstrate the preferential weathering of marcasite relative to pyrite and chalcopyrite. Dolomite is the main buffering carbonate mineral. Gyspum, jarosite and iron oxides are the main secondary minerals. Pore-waters contain up to 800, 11.7, and 6 mMolar (78,000, 690, 1400 mg/1) of sulfate, nickel, and uranium, respectively, suggesting significant weathering is occuring. Weathering rates calculated from estimating secondary mineral accumulation range from 0.3 - 49 and 0.8 - 61 M/kg/week for sulfate and iron release rates, respectively. Measures of weathering indicators show no discernable correlation with grain-size distribution nor do they show any spatial relationship within the sampled grid. Reasons for this may include limited duration of waste rock exposure to weathering and development of fluid flow-paths, and limitations of characterization methods used. Indeed, six years may not be long enough for weathering patterns to develop due to fluid flow-paths in this unsaturated waste rock pile.

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