Health officials downplay high arsenic concentrations in Yellowknife-area lakes
Highest contamination in lakes southwest of Giant Mine, near Fred Henne park
Research identifying high arsenic levels in lakes in and around Yellowknife has prompted a health advisory cautioning people to limit their exposure to the lakes.
But in issuing the advisory, health officials emphasized that the only health standard for arsenic concentrations — the one set for drinking water — is largely irrelevant when considering the health effects of swimming in water with higher arsenic concentrations.
"The set level at 10 parts per billion is very strict," said N.W.T.'s Chief Public Health officer Andre Corriveau.
"It includes a margin of safety. It assumes you're drinking the water daily for 30 years in terms of your risk for cancer. To be slightly above drinking water guidelines for swimming, even for wading and the odd gulp of water, would be entirely safe."
The higher concentrations in the lakes are the result of historic emissions from the Giant Mine roaster stack.
The mine operated for 56 years, finally shutting down in 2004. During its run, the stack atop its gold roaster emitted hundreds of thousands of tons of arsenic dust which settled on the land and water around it. Over the years, levels in lakes have increased as arsenic dust has washed into lakes with each spring melt.
Prevailing winds carried the dust mainly south-southwest of the roaster stack, which is reflected by higher levels in lakes in that area. The highest concentrations — up to 60 times the arsenic limit set for drinking water — are in small lakes with little drainage southwest of the mine.
Some of them flank a new bypass road linking the city to recreational areas on the Ingraham Trail. Those lakes are also adjacent to the Prospector Trail, a hiking trail that begins in Fred Henne Territorial Park, the busiest park in the Northwest Territories.
An official said signs will be posted along the trail this summer cautioning people about high arsenic concentrations in the water.
Similar signs will be posted on some lakes within the city, such as Frame Lake, the official said.
Health officials recommend not drinking untreated water from any lake.
The advisory says lakes with arsenic concentrations less than 52 parts per billion are considered safe for swimming and fishing. Lakes with concentrations between 10 and 52 parts per billion are above Health Canada's drinking water guidelines but the advisory says occasional exposure does not pose a significant risk for arsenic-related health effects.
The advisory says water should not be consumed from lakes with concentrations at 52 parts per billion and above. It also recommends avoiding fishing and harvesting berries, mushrooms and other edible plants in the area.
Arsenic concentrations in lakes in and around Yellowknife (ppb)
- Frame Lake 343
- Long Lake 39.7
- Jackfish Lake 88.3
- Grace Lake 9.8
- Mason Lake 2.7
- Hay Lake 6.5
- Pontoon Lake 6.5
- Vee Lake 24.7
- Walsh Lake 8.9
- Bighill Lake 1
- Martin Lake 20.4
- Lower Martin Lake 40.6
- Handle Lake 140
- Landing Lake 10.5
- Ryan Lake 20
- Duckfish Lake 2.2
Source: Department of Environment and Natural Resources