Burying the past
Threat to city water supply contained
Yellowknife ( Mar 01/00) - The land and water around Discovery mine is not mended yet, not by a long shot, but the spread of environmental damage from the mine has stopped.
"The plug has been put back in the bottle," said Scott Mitchell, head of waste management for the contaminated sites office of the Department of Indian Affairs and Northern Development. Located 96 kilometres north-northwest of Yellowknife, the mine produced 1.1 million tons of tailings during its 25 years of operation, from 1944-69.
The tailings contained very high levels of mercury and other heavy metals, byproducts of the process used to refine the million ounces of gold the mine produced.
For most of the life of the mine, a series of dykes channelled the tailings to a small lake nearby. In 1965, one of the dykes developed a breach in the worst possible place, at the top of a steep slope leading down to Giauque (pronounced Jakeway) Lake.
Giauque, a large lake on the shores of which Discovery was perched, is part of the same watershed that Yellowknife draws its drinking water from.
Tailings continued to pour into the lake for the next four years. A 1976-77 Environment Canada study concluded 75 per cent of the lake bottom was contaminated.
Today, work at the mine is limited mainly to quarrying the rock and clay that will be used this summer to extend the cap over the tailings pond.
From the start of the cleanup, in 1997, it was understood that the final part of the work would be left to time and nature.
It was estimated it would cost $30-$40 million to dredge the bottom of Giauque, a process that could do as much harm -- by stirring up metals on the lake bottom -- as good.
Another scenario that was briefly explored was reprocessing the tailings left on the land to remove gold and mercury.
"For $13 million we could get $2 million worth of gold and, maybe, get the mercury out," said Mitchell of the rejected plan.
Ultimately, it was decided to simply contain the worst of the contamination, starting with Giauque.
A sheet of containment material was spread over the most contaminated part of the lake bottom, with crushed rocks piled on top. From there the cap was extended along the path of the breach leading back to the main tailings pond, which covers an area 350 metres wide by 1,750 metres long.
On land, the cap consists of a 30-centimetre layer of clay under a 30- centimetre 'armour' of crushed rocks.
Fortunately, there is an abundant supply of clay and crushed rock in the area. The
material being gathered this winter will be used to cap the remaining half of the tailings pond. That will bring the cost of the cleanup to $8.5 million.
The job is complicated by GMD Resources' claim to mineral rights at the site. GMD has been doing exploratory work in the same area that is being remediated.
The remediation effort, said Mitchell, is tempered by the realities of costs and benefits that apply to all such jobs.
"Our priority right now is to stop any further contamination," said Mitchell. "In remediation of industrial sites, mine sites or military sites, you never achieve total restoration."
No decision has been made on what will happen with the roughly 60 buildings that once formed a small but thriving mining camp that included a curling rink, school, tennis court and homes.
The cost of levelling the buildings is an estimated $3.5 million. Restoring it or moving the buildings would cost significantly more.