The natural weathering of mine/mill tailings and waste rock containing reactive sulphide minerals is known to generate acid rock drainage (ARD), under a wide range of climatic conditions. Where acidic drainage occurs, mine decommissioning costs can be significant. The Yukon and Northwest Territories combined, are estimated to account for 3.5% of mine tailings and 2.3% of waste rock that are reactive or potentially acid generating within Canada. This translates to an estimated current liability of over $77 million.
In the last two decades an increasing emphasis has been placed on the development and application of strategies to control and mitigate the reactivity of these sulphide wastes. Two of the most advanced technologies for ARD control in Canada are: the use of water in underwater disposal or wet barriers; and engineered dry barriers.
A water cover can significantly reduce the effective diffusion rate of oxygen into tailings and is considered both an economical and an effective technique for the control of sulphide oxidation in the longer term. The impact of cold temperature conditions on the water cover scenario are modelled. Despite significantly reduced relative reaction rate constants at lower temperatures, the flux of oxygen into the waste is only estimated to decrease by a factor of 2.5 from 25 to 0C. This is not significant enough to preclude oxidation and thus freezing of reactive waste alone, under unsaturated conditions, is not considered sufficient to control ARD. In addition the effectiveness of saturating tailings to limit the oxygen influx is calculated to decrease as the temperature is lowered. In applying the water cover option to the Canadian north, the impact of reduced temperatures, ice formation and breakup, and snow cover are also discussed. An ice layer will isolate the water column from wind and wave related turbulence and limit the exchange of oxygen at the surface. It is suggested that ice scouring, and the resultant disturbance or resuspension of deposited tails, is the most negative impact related to the application of water covers in the north.
In addition to dry barriers, permafrost offers another option for ARD management in northern Canada. Successful encapsulation of tailings in permafrost may ensure year round in situ temperatures at or below 0C with simultaneous reductions in chemical and bacterially assisted reaction rates. The characteristics of permafrost related to the active freeze-thaw layer, material transport, freeze-thaw cycles, potential for frost heave, and its use in tailings management are discussed. While acid generating reactions are slowed at temperatures approaching 0C, they are not stopped. Freeze/thaw cycles may also promote frost weathering and heaving. In addition, permafrost has a finite permeability, a small but significant percentage of porewater remaining unfrozen within permanently frozen ground.
Permafrost, while a promising factor in the management of sulphide oxidation and ARD migration, will not provide an absolute control to ARD production. It is recommended that future work quantify the impact of below freezing temperatures on the rates of acid generation and the longer-term performance of various cover scenarios.
