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Characterization of the geochemistry of discharge waters, pore waters, primary and secondary minerals of an experimental waste rock pile, Cluff Lake Mine, Saskatchewan, Canada Wagner, Karin

Abstract

The objective of this study was to characterize the geochemistry of an experimental waste rock pile, focusing on the outflow waters, pore waters, and primary and secondary mineral phases. The experimental pile was located at the Guff Lake mine operated by Cogema Resources Incorporated in Saskatchewan. Constructed in 1998, instantaneous water samples were collected and flow was monitored continuously for a period of approximately six years before the pile was deconstructed and sampled in 2004. The water chemistry data and flow data were used to investigate the relationship between outflow water chemistry and flow rates. The data also allowed the total sulfate loading from the pile and the rate of sulfate and metal release to be estimated and compared to laboratory derived estimates. The primary minerals in the waste rock are quartz, k-feldspar, albite, chlorite, muscovite, kaolinite, smectite and amphibole. Acid base accounting results indicate that the waste rock is acid generating, with neutralization potentials less than 0.5. Due to the low pH of 3.6, almost all the primary minerals are weathering. Secondary minerals include gypsum, jarosite, ferryhydrite, goethite, annabergite and hydrated aluminum and magnesium sulfates. The total dissolved solids content of the pore water in the waste rock pile increases with depth indicating the ongoing acid mine drainage generation in the pile. The investigations of the outflow water revealed strong temporal and spatial variability in its quality. Sulfate is the major anion in the outflow water with concentrations mainly between 10,000 and 20,000 mg/1. The dominant cations are Mg, Al, Ca, Na, Ni, TJ and Mn with concentrations between 100 mg/1 and 3,500 mg/1. K, Co, Li, Sr, Zn, Ce, Fe, Cu, La, Be and As are present in the outflow water in minor concentrations. The chemical composition of the outflow water remains roughly the same in terms of the proportion of the existent cations and sulfate. Most of the cations except Ca and Fe show a strong correlation with the sulfate. The electrical conductivity of the outflow water correlates with the sulfate concentration. Thus, the simple measurement of the electrical conductivity yields a good estimate of the total dissolved solids content of the outflow water. A general inverse relation between the flow rates and the outflow chemistry is observed. In general, high flow rates correlate with fresher outflow water and slow flow correlates with more concentrated outflow water. The investigation of the outflow chemistry during and after infiltration events revealed that the outflow water becomes relatively fresh immediately after a rapid increase in the flow rates. A low-permeability cover that was put On the pile surface induced a decrease of the concentrations in the outflow water, suggesting that the oxygen concentrations decreased and that the pile is aging. During the four years between 2000 and 2003, approximately 5 % or 150 kg of the original primary, sulfur was released at the base of the pile.

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