Mine tailings in cold regions climates : reactive transport modeling at two scales

Mine tailings have the potential to generate acid rock drainage (ARD) when sulfide gangue minerals are present. In cold regions, permafrost can naturally diminish the potential for ARD generation by limiting sulfide oxidation to the active layer, while reducing infiltration and contaminant release. However, the efficacy of these attenuation processes may be impacted in the long-term under conditions of climate change. Reactive transport models (RTMs) allow one to simulate complex coupled physical and geochemical processes and have proven useful for making projections on the long-term performance of tailings storage facilities (TSFs). In cold regions, mine waste is frequently stored in filtered tailings due to a lack of water and logistical issues associated with freezing temperatures. Filtered tailings have a low moisture content compared to conventional TSFs and to date have received limited attention in the context of RTM analysis. This thesis uses the code MIN3P-HPC to simulate variably saturated flow, reactive transport, and freeze-thaw cycles in such tailings. Models were developed at two scales. The first set of simulations was constrained by previous laboratory-scale column experiments. The weathering and evolution of pyrrhotite-rich tailings from the Raglan Nickel Mine were analyzed under ambient temperatures and freeze-thaw cycles. The second set of simulations focused on a sensitivity analysis of sulfide-bearing tailings at field scale, investigating performance of non-reactive covers for ARD reduction under current and warmer climate conditions. The lab-scale model at ambient temperature provided insights on geochemical processes including oxidation of pyrrhotite causing thiosulfate and sulfate release, neutralization of acidity by carbonate mineral and serpentine dissolution. Considering freeze-thaw conditions illustrated the major reduction in reactivity relative to ambient conditions. The field-scale model used the capability of RTMs to assess long-term trends, employing a simplified geochemical system. The analysis indicates that non-reactive covers can effectively eliminate ARD generation under current climatic conditions but also illustrates how cover performance may be compromised due to temperature increases. The simulations provide a framework for assessing ARD generation potential based on cover thickness and climate regime. Taken together, this work demonstrates how RTMs can be used to assess tailings for mine waste management in permafrost conditions.