Temperature and oxygen control on pyrite oxidation in frozen mine tailings

Sulphide oxidation rates in mine tailing systems are generally assumed to be negligible during winter in Arctic regions due to the low temperatures. Thermal covers have therefore been suggested as a solution to keep the mine tailings continuously frozen in permafrost areas, thereby hindering both the oxidation of sulphidic waste and the associated acid mine drainage (AMD) year round. This study evaluates oxygen gas (O2(g)) concentrations and consumption rates observed in pyritic mine tailings deposited at Nanisivik Mine on Baffin Island (Arctic Canada) and compares these observations with O2(g) consumption rates observed in the laboratory at temperatures from −12 to 12 °C at various oxygen concentrations. Laboratory results revealed that biotic and abiotic oxygen consumption due to sulphide oxidation proceeded at temperatures as low as −11 °C and could be described by exponential fits indicating a Q10 value of about 2 for temperatures above −2 °C and a value of 10 for temperatures below −2 °C. Oxygen consumption rates were found to be 0.7 to 0.8 order with respect to O2(g). Based on field observed depth-specific temperatures and O2(g) concentrations, the vertical distribution oxygen consumption was modelled using a diffusion-consumption model. Subsequently, the depth integrated oxygen consumption was calculated and found to be consistent with observed O2(g) fluxes across the surface. It is concluded that the relationships between observed subsurface O2(g) concentrations, vertical consumption rates and O2(g) fluxes can be described by diffusion as the main transport mechanism controlling subsurface O2(g) dynamics. Furthermore, it is documented that freezing temperatures will limit but not eliminate oxygen consumption by pyrite oxidation and that thermal covers should be designed to maintain mine tailing temperatures well below the freezing point.