DEVELOPMENT OF THE ISLAND COPPER MINE PIT LAKE FOR THE TREATMENT OF ACID ROCK DRAINAGE

The Island Copper Mine pit near Port Hardy, Vancouver Island, BC, Canada, was flooded in 1996 with seawater and capped with fresh water to form a meromictic (permanently stratified) pit lake of maximum depth 350 m and surface area 1.71 km^2. The pit lake is being developed as a passive treatment system for acid rock drainage (ARD). The physical structure and water quality has developed into three distinct layers: a brackish and well mixed upper layer; a plume stirred intermediate layer; and a thermally convecting lower layer. To date the upper halocline has risen due to the injection of buoyant acid rock drainage (ARD) into the base of the intermediate layer. An upper layer “equilibrium” depth is expected in the near future reflecting a balance between meteorological forcing from above and rising ARD plumes from below. The lower halocline depth fluctuates seasonally: deepening in the winter due to high ARD flows, which erode the lower halocline; and rising in the summer when thermal convection in the lower layer dominates over reduced intermediate layer stirring. The proposed treatment of acid rock drainage by metal-sulfide precipitation using sulfate-reducing bacteria is dependent on anoxia developing in the intermediate and lower layers. By March 2001, the lower layer was close to anoxia, whereas, the intermediate layer was not with DO=2.5 mg/L. The major issues facing the passive treatment system are developing and maintaining anoxia in the intermediate layer and maintaining meromixis.