Arsenic and mercury are present in surprisingly and sometimes dangerously high levels in numerous small lakes surrounding Yellowknife, NWT, scientists have found.

The contaminants form a toxic footprint around the city that covers roughly one thousand square kilometres, a legacy of the now-shuttered Giant Mine, which is already the focus of a long and costly remediation effort.

The findings raise questions about the potential impact on human health across a region that is home to half the population of the Northwest Territories.

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In a study published on Wednesday in the journal PLOS ONE, a University of Ottawa team makes the case that the contaminants in the lakes are linked to industrial emissions from decades past when gold-bearing ore was heated in a "roaster" at the Giant Mine, releasing tonnes of arsenic-laden dust and other pollutants.

"We believe that the bulk of the contamination that we're seeing is airborne," said Jules Blais, a professor of environmental toxicology and senior researcher on the study.

Parallel investigations by scientists working for the territorial government have confirmed and augmented the team's findings.

The Ottawa study includes data from 25 lakes within a 25-kilometre radius of Yellowknife. In some cases, it found arsenic concentrations in the water as high as 136 micrograms per litre – more than 13 times the recommended limit for drinking water and 27 times the level deemed adequate for the protection of aquatic life. The highest concentrations were found in lakes within four kilometres of the Giant Mine site.

The study rules out natural sources of arsenic.

None of the affected lakes are part of the watershed that supplies Yellowknife with its drinking water. The lakes are mostly small and isolated, receiving the majority of their water through precipitation and losing it through evaporation, providing no opportunity for lingering toxic chemicals to be flushed out.

André Corriveau, the territory's chief public health officer, said he first learned of the findings last fall and has been gathering information for a public-health advisory, which he hopes to issue before the lakes thaw.

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Dr. Corriveau added that his concern about the potential health risk is "low but not zero," since it is unlikely that anyone would be drinking from the lakes on a regular basis. The lakes are not widely used for recreation, he said, and have only recently become more accessible because of a new road. But he said anyone who may be camping or fishing should be made aware of the contamination.

Researchers were startled after they began to sample the lakes, initially as part of a climate-change study. However, once the results came in, the focus of the investigation quickly shifted. "We saw arsenic concentrations that were higher than any we had seen anywhere," Dr. Blais said.

About 237,000 tonnes of arsenic trioxide dust are known to have been produced and captured during roasting operations at the Giant Mine before it ceased operation in 1999. That material remains at the site, where a solution for its containment or removal remains an ongoing issue.

However, researchers estimate that 20,000 additional tonnes of arsenic trioxide escaped into the air, the bulk of it before capture methods were put in place starting in 1958. Most of that dust would have settled out within the 25-kilometre radius that defined the limits of the Ottawa study. Sediment cores taken from the lakes show that arsenic deposition was highest around the early 1960s.

But if the "aquatic" arsenic seen in the lakes today comes from the dust that was released so many years ago, researchers have yet to completely understand how it is transported through the environment. For example, it is not clear whether the lakes are currently acting as "sinks" where arsenic is pooling from the landscape or as active sources from which arsenic is continually escaping.

Another surprising find relates to the elevated mercury levels in the lakes. The chief concern is not the total amount of mercury found, but the unusually high proportion that occurs as methyl mercury, a compound that is more deadly to humans than mercury alone. In lakes nearest the mines roaster, up to 44 per cent of the total mercury is in this form. The Ottawa researchers posit that bacteria living off sulphur emissions from the mine are responsible for converting mercury into methyl mercury.

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While environmental contamination from the Giant Mine, along with a smaller mine that closed in 1971, is not a new theme, most of the attention until now has been directed toward the mine sites rather than the surrounding region.

Mike Palmer, a scientist with the territory's Department of Environment and Natural Resources, said that further work was under way to better understand the wide swath of historic pollution from mining activities in the region, including studies to test for the accumulation of toxins in fish and other species.

Data from ongoing monitoring of 98 lakes by the territory has now been posted as an open file by the Northwest Territories Geological Survey.

"There's really been a ramping-up of research efforts," Dr. Palmer said, adding that a clearer picture of the extent and activity of the contaminants would be essential for determining what kind of risk they may pose.

Adam Houben, a PhD student who is lead author on the study and who did much of the sampling and analysis of the lakes, said the findings offer an example of the unforeseen consequences that can arise from resource development, much of which takes place outside of the public's awareness.

"There are real impacts on communities and on the greater landscape as well," Mr. Houben said.