PREDICTING THE DURATION AND EXTENT OF ACID DRAINAGE RELEASE FROM SULFIDE BEARING MINE WASTES

Mill tailings and waste rock are the dominant wastes derived from mining operations. Acidic mine drainage and the accompanying release of high concentrations of dissolved metals is associated with mine wastes throughout the world. Acidic drainage results from the microbially mediated oxidation of sulfide minerals and the subsequent transport of oxidation products through the mine wastes and underlying geological materials. Gas transport within mill tailings, the fine-grained residuals from the ore benefication process, is commonly limited to within the upper few meters of the waste by the high moisture content of these materials. Field investigations and laboratory studies, conducted on mill tailings impoundments of differing mineralogical composition and within varying hydrogeological regimes illustrate the complex interactions between hydrogeology, microbiology, geochemistry and mineralogy that control the severity and duration of acidic drainage. Geochemical models and reactive solute transport models provide a promising framework for understanding these highly coupled processes. Unlike mill tailings, waste rock piles are largely unsaturated, resulting in potential for more extensive and rapid movement of pore gases and heat, and development of more complex hydrological regimes. Because waste rock piles are unsaturated, extensive areas of these piles may undergo oxidation simultaneously. Furthermore, because of the large variations in grain size that are characteristic of waste rock, drainage may follow complex pathways and encounter large spatial variations in permeability and reactive surface area. As a consequence, water quality may be more challenging to understand and predict. Field and laboratory studies focused on waste rock systems illustrate the impact of these transport mechanisms on the geochemical evolution in waste rock piles. Improved understanding of the interaction of mechanisms controlling the generation and release of acidic drainage has the potential to lead to the development of new techniques for mine-waste disposal and new mitigation approaches.