Résumé - Abstract
Numerical models for simulation of reactive transport have seen significant advances over the past decades. These models are designed to simulate transport processes such as advection and diffusion of solute and gases, while undergoing geochemical reactions, such as sorption, ion exchange and mineral dissolution-precipitation. By nature, such models are well suited to simulate weathering reactions in the subsurface, including the long-term evolution of mine waste, specifically tailings and waste rock. Numerical analysis of mine tailings has been used in the past due to the nature of tailings material. In comparison to waste rock, tailings are relatively homogeneous in terms of grain size, permeability and chemical reactivity, the water table is commonly shallow. As a result, a one-dimensional approach for simulating reactive transport will often suffice to help quantify the long-term release of acidity, metals and sulfate from the vadose zone of the tailings. Waste rock on the other hand is much more complex, due to the highly heterogeneous nature of the material, structural features related to construction methods, and predominately unsaturated conditions throughout a pile. This facilitates rapid oxygen ingress and tends to accelerate sulfide mineral oxidation substantially, potentially causing pile-internal heating. Additional complexities arise in subarctic and arctic climates subject to permafrost conditions. In these environments, the development of permafrost can substantially inhibit the release of mine drainage and reduce mass loadings; however, this benefit might be lost, if sulfide oxidation causes internal heating, in turn inhibiting the formation of permafrost within a waste rock pile.
In this presentation, we will review the concepts of reactive transport modeling in mine waste, starting with an example in mine tailings, to demonstrate how these models can be used to simulate the evolution of weathering processes, the geochemical evolution of tailings, and mass loadings in drainage. We will then present recent results on the simulation of weathering of waste rock, as a function of heterogeneity, structure, scale, and climate regime. These simulations compare mass loadings from waste rock piles constructed by push dumping methods and end dumping methods, taking into consideration the characteristic segregation of materials due to these construction techniques. We will also show results comparing mass loadings from single-bench to multi-bench waste rock piles and discuss effects of structure and scale on maximum mass loading rates per unit mass of waste rock. In addition, we will discuss reactive transport simulations of waste rock piles in subarctic environments under current and warming climate conditions and subject to different sulfide weathering rates. Lastly, we will evaluate the effect of a thermal cover on mine waste weathering in a permafrost environment. Taken together, these simulations demonstrate how reactive transport modeling can be used as a supportive tool in developing process understanding in the context of mine waste management and help to anticipate short-term and long-term water treatment requirements.