28 agosto 2018
In British Columbia, Canada, the Island Copper site has demonstrated innovative design in treating acidic drainage to reclaim disturbed lands.
Located on the northern coast of Vancouver Island, the former Island Copper open-cut mine operated from 1971 to 1995, producing copper and molybdenum concentrates, as well as gold, silver and rhenium as by-products. After 24 years of ore extraction, the site was closed, with a large volume of water requiring long-term management and treatment.
Upon closure, the Island Copper open pit was flooded with seawater and then capped with a layer of fresh water from a nearby river. This resulted in the formation of three layers of water of different salinities within the pit, which are maintained due to their unique densities. This layered structure is used to manage and remove metals from acid rock drainage that occurs when rainwater drains off and through the nearby waste rock dumps into the pit lake. Treated water that complies with regulatory standards then flows from the top layer to the adjacent marine environment.
Treatment of the water in the pit lake is achieved through a biological process called phytoremediation. A low-cost liquid fertiliser is added to the top layer of the lake year-round. The fertiliser grows phytoplankton cells, which bond to the metals and then sink to the bottom of the lake. As they’re sinking, the phytoplankton also provide a carbon source for oxidation in the middle seawater level, which creates low-oxygen conditions in this layer that further reduce metal concentrations.
In 2004, an issue emerged that had the potential to jeopardise water quality; the boundary between the top and middle layers was rising due to the continued addition of the acid rock drainage into the middle layer. If the boundary had continued to rise, a risk of release of middle layer water (not always compliant with regulatory standards for discharge) to the environment would have arisen.
The solution was an innovative in-lake water management system, called the Middle Layer Lifting System. This system harnesses the energy of downhill drainage from one of the waste rock dumps to draw or ‘lift’ water from the middle layer to the top layer in the pit. This has proven to be an effective way to maintain the boundary elevation and ensure water discharged from the site meets our environmental obligations.
The surrounding landscape was also rehabilitated to help manage mine drainage. Waste rock dumps were re-contoured and capped with soil, and more than 500,000 trees were planted. This has encouraged the return of wildlife to the area, including elk and deer.
Island Copper’s engineered pit lake and overall site closure has significantly reduced the cost of treatment and proved effective in reducing the risk of environmental impact. It has also highlighted the importance of the interrelationship between the physical and biological characteristics of a pit lake.
The lessons from this experience are now being applied in the design and management of other mining pit lakes more broadly and the site has been recognised publicly for its environmental management and innovation.