Characterization of Metals Distribution and Their Leaching Behavior in a Uranium Mine Waste Rock Pile, Key Lake, Saskatchewan, Canada
The potential for soil and groundwater contamination by uranium mine waste-rock piles is an environmental concern. The objective of this study was to characterize the vertical distribution of sulphide/sulphate and trace metals (nickel, arsenic, and uranium) at the study site DWR-39 located on the Deilmann waste-rock pile at the Key Lake uranium mine in northern Saskatchewan. Seven standpipe lysimeters were installed within the pile and below it to measure soil suction at depths between 5 m and 33 m. Soil suction was monitored between May 00 and Aug 2002. Based on these data and moisture-content profiles measured through the pile measured using a neutron probe, it was determined that the waste rock was at residual water contents, suggesting that the waste-rock pile was well drained. Detailed vertical core samples to a depth of 33 m were collected at DWR-39. These samples were used to define the lithology of the pile. The waste-rock pile was determined to be comprised of a basement-gneissic layer underlain by a sandstone layer. A thin layer with high organic content was encountered below the sandstone layer and was identified as the original lake sediments. Sulphide contents were lower in the sandstone (0.01 %, n = 12) compared to basement rock (0.11 %, n = 11). Sulphate concentration profiles suggested a downward migration of sulphate from the basement rock into the sandstone layer. However, the depth profiles of nickel, arsenic, and uranium total assays from these core samples proved inconclusive for verifying downward leaching to the water table. Sequential extraction analyses showed that 75% of Ni was bound to the residual phase, 40% of U was bound to carbonate phase, 25% As in carbonate, Fe-Mn, and residual. These data suggested that the vast majority of Ni, As, and U are not mobile in the pile. In the groundwater below DWR-39, elevated sulphate concentration was observed and this may originate from oxidation reactions in the basement materials of the waste rock pile. Elevated concentrations of metals (Ni and U) dissolved in groundwater may have originated from oxidation reaction in the basement materials. The lack of agreement between the groundwater chemistry immediately underlying the test site and the sequential extraction and the profiles of total metal assays may be attributed to the integrating effect (i.e. lateral mixing) of groundwater sample, and the heterogeneity of the basement rock in the waste-rock pile. Alternately, the groundwater chemistry immediately below the study site may not be the result of reactions in the waste-rock immediately above it.