High arsenic levels found in some Yellowknife-area lakes

Some lakes around Yellowknife have levels of arsenic trioxide above the recommended limit for drinking water, a researcher has found.

During the 50 years the Giant gold mine operated, 20,000 tonnes of the highly toxic dust went up the roaster stack and settled on the ground and lakes around Yellowknife.

In the early days at Giant, with almost no pollution controls, the mine was pumping an average of about six tonnes of arsenic trioxide dust into the air each day.​

Over the last few years, researchers checked arsenic levels in 100 lakes within a 30 kilometre radius of the mine.

Mike Palmer, an environmental monitoring specialist, says arsenic concentrations ranged from 6.5 parts per billion up to 1,800 parts per billion in a small lake near the mine. 

The recommended limit for drinking water is 10 parts per billion. 

The level in Long Lake, which includes the popular Fred Henne Park beach, was 39.7.

"There's an area within 10 kilometres to the west and northwest of the mine site that has generally higher levels of arsenic than other areas around town," Palmer said.

"That's where the prevailing winds blew the dust coming out of the stack."​

The poisonous dust, dissolved, is carried into lakes and rivers when the snow melts each spring.

"It tended to be the small, organic rich lakes that had higher arsenic levels than the larger lakes with flow-through that are connected to other bigger lakes," Palmer said.

Palmer cautions against looking at the numbers alone. 

"One of the questions and one of the important things to recognize is that the bioavailability and toxicity of arsenic really depends on its speciation," he said.

Speciation means distinguishing between different kinds of arsenic. In addition to the highly toxic, highly soluble arsenic that went up the stack at Giant mine, there are high levels of far less toxic forms of arsenic in the rock around the city.

Palmer says that work will be the focus of future studies.