The Dona Lake Mine, an underground gold mine that ceased operation in 1994, is located near Pickle Lake, in northern Ontario. The nominal production rate during operation was approximately 500 tonnes per day.
The tailings impoundment is situated in a low lying area and is surrounded by natural topographic highs to the east and west. A dyke, Dyke 3, was constructed at the south end during operation of the tailings impoundment, and a dam was constructed to the north after production ceased. The physical characteristics of the tailings impoundment are not conducive to underwater tailings disposal.
Acidic drainage prediction tests were conducted by Placer Dome Inc. As the prediction tests indicated that the tailings were potentially acid generating, site rehabilitation focused on maintaining the highest possible degree of saturation in the tailings material. The elevated water table, maintained by the site topography, minimizes the ingress of oxygen in the saturated tailings and ultimately reduces their oxidation. The Dona Lake Mine provides an example of a current application of the elevated water table concept to the decommissioning of a tailings management facility.
Field monitoring involving piezometers, neutron probe measurements of moisture within the tailings, and matrix suction profile measurements were conducted by the University of Saskatchewan. Predictive modeling was conducted to couple the depth to phreatic surface with the climatic conditions.
To improve the infiltration rate into the tailings beach, where tailings were exposed and oxidized to a 1 to 2 m depth, a series of soil cover models were analyzed for different cover options. A rockfill cover appeared to be the most effective option among the cases analyzed. The rockfill material promotes infiltration due to its high permeability at lower suction and lower permeability at moderate suction range. It allows the water to infiltrate easily through the cover during rainfall events when suction decreases to zero.
The tailings impoundment area was partly flooded and a rockfill cover was installed over the exposed tailings to reduce the area of tailings potentially exposed to oxidation to <1% during normal years and approximately 5% under drought conditions.
Key words: Elevated water table, Dona Lake, acid mine drainage, neutron probes and rockfill cover.
