Treating the water at Giant Mine
Managing water is an important part of the Giant Mine Remediation Project, both now and in the future.
How does the Project treat water?
All water that enters the underground mine is collected so it can be treated. This contaminated water is pumped to the surface, where it is temporarily stored in one of the four tailings ponds (known as the Northwest Pond). This tailings pond is large enough to store all water collected from the underground mine throughout the year. During the summer, the water in the Northwest Pond is pumped to an on-site water treatment plant. The water treatment plant removes arsenic and other contaminants.
Once treated, water goes into the settling pond. There, most of the remaining contaminants settle to the bottom and form a sludge. The water then passes into the polishing pond, while the contaminants stay at the bottom of the settling pond. This treatment helps reduce the contaminants released into the environment.
When water quality in the polishing pond meets or exceeds the regulatory requirements set out in the mine's former Water Licence and in the federal Metal Mining Effluent Regulations, it is discharged into Baker Creek. Section 89 of the Mackenzie Valley Resource Management Act allows the current discharge.
Once a new water treatment plant is complete, the settling and polishing ponds will no longer serve a purpose. Upon the removal of the contaminated sediments, the area will be capped. This will keep any remaining contamination from getting into the environment.
Giant's treated mine water does not get into Yellowknife's water supply in the Yellowknife River. The treated mine water releases into Baker Creek. Though City of Yellowknife testing may show trace amounts of arsenic, these are well below the Guidelines for Canadian Drinking Water Quality. They are similar to levels found in many other communities such as Gameti, Edmonton, and Toronto. Arsenic is naturally-occurring in the area and commonly found in the water of many rivers and lakes, including the Yellowknife River.
What are the plans to treat water after remediation?
The Project team proposes to treat mine water to meet the federal Guideline for Canadian Drinking Water Quality for arsenic. The drinking water quality guideline is 10 micrograms of arsenic per litre. This directly responds to the Yellowknives Dene First Nations' requests that discharged water meet drinking water standards for arsenic. It also addresses concerns about potential locations for a new City of Yellowknife drinking water intake. Also, the new discharge will not use a diffuser, but instead will be a near-shore outfall. This will address concerns about ice thickness in Back Bay. Further details on this treatment method is available on the Mackenzie Valley Environmental Impact Review Board's public registry.
To achieve these standards for discharged water, the Project needs to build a new water treatment plant, slated for construction during remediation. The Project team held engagement sessions in the fall of 2016 to gather input on potential outfall locations (discharge point for treated water) for the new plant. Discussions focused on:
- where the stakeholders would like to see the outfall located
- the impacts of the outfall
- what the design could potentially look like
The input from these sessions is being used by the Project team to identify outfall location options.