Yellowknife should spend an extra $15 million to keep taking its drinking water from the Yellowknife River, not from Yellowknife Bay, according to a new report.
The two choices carry significantly different price tags. Replacing the ageing underwater pipeline to the Yellowknife River, eight kilometres away, is estimated to cost up to $33 million over a 25-year working life.
Drawing water from Yellowknife Bay would cost only $18.2 million over that time, but runs a greater risk of arsenic contaminating the water if something goes wrong with the clean-up of the nearby toxic Giant Mine site.
With time running out to make a decision on replacing the pipeline, the City asked engineering firm AECOM to study its water supply options in March 2017.
AECOM's report will be discussed by city councillors at a meeting on Monday. The full report appears in an agenda for that meeting published on Friday.
"Overall, the Yellowknife River source with a new submarine pipeline has a higher capital cost, but has less risk of arsenic contamination," the report's executive summary concludes.
"Arsenic contamination of the Yellowknife Bay source water due to a major failure at Giant Mine has a low probability of occurring but is considered plausible."
The report's summary adds that even with an arsenic treatment facility in place, "the arsenic removal treatment process may not be able to consistently meet the drinking water quality standards" if contaminated material is released from Giant Mine downstream into the bay.
However, the report says this risk only exists until the remediation – or active clean-up – phase of work at Giant Mine is completed. Once that work is done and the mine site enters long-term care and maintenance, the report's authors believe "plausible failures ... would only release a small amount of waste" without impacting water quality.
Under a scoring system devised by AECOM, which takes into account contamination risk among a host of other factors, continuing to draw water from the Yellowknife River emerged as the "preferred option."
Giant Mine, a former gold mine which ceased production in 1999, is home to 237,000 tonnes of a highly toxic mining byproduct named arsenic trioxide. A federal clean-up project to freeze the arsenic trioxide safely within underground chambers has begun, but a Government of Canada webpage last updated in 2014 estimated that work may only reach completion by 2029.
The City's current pipeline to the Yellowknife River, built in the late 1960s, is due for replacement before 2020.
The report sets out a potential timeline in which pipeline replacement can be comfortably completed by April 2020, assuming council reaches a decision in the near future. The pipeline would be put together on frozen lake ice above its intended route, then dropped through.
AECOM found both water sources have good water quality under normal conditions. The pipeline's annual operating costs are estimated at $300,000, compared to $510,000 for the bay.
If councillors agree to pursue replacing the pipeline, Yellowknife Bay would continue to act as the city's emergency water source should the pipeline fail. That isn't without precedent: in 2015, the city took water from the bay for a couple of hours while its new $30 million water treatment plant underwent commissioning.