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The Suncor tar sands mining operation north of Fort McMurray, Alberta, November 3, 2011.Todd Korol/Reuters

In 2010, a single oil-sands operation run by Suncor Energy released into the atmosphere 28,940 tonnes of volatile organic compounds, 22,210 tonnes of sulphur dioxide and 14,011 tonnes of particulate matter.

With those three types of substances combined, Suncor emitted into the air pollutants equivalent in weight to nearly 4,800 city buses – and the company operates just one of several mines sprawling across the landscape north of Fort McMurray, Alta.

Yet when scientists drilled into lake bottoms 200 kilometres from those oil-sands mines, they discovered something surprising: At that distance, levels of those pollutants were negligible. In fact, the lake sediments, whose layers opened a window onto hundreds of years of air and water quality, showed that in many ways those lakes are cleaner today than they were decades, and even centuries, ago.

"It's still, by and large, a natural landscape," said Roland Hall, a University of Waterloo professor of biology and one of the lead researchers on the new study.

Communities downstream of the oil sands, such as Fort Chipewyan, have long pointed the finger at oil-sands operations for sullying water, fish and people with toxins.

But, Dr. Hall said, the level of pollutants travelling by air and water to a place like Fort Chipewyan are not high enough to merit the concern that has been expressed. Instead, he said, "the conclusion in our paper is that natural sources of PACs" – polycyclic aromatic compounds – "via the water continue to be a main process that's delivering them."

In other words, toxins well downstream of the oil sands come from the environment, not big steel stacks – a finding that suggests the industrial impact on the environment is largely local, rather than broadly regional.

The research is controversial. Three researchers – David Schindler, from the University of Alberta, Kevin Timoney, a water expert, and Peter Lee, executive director of Global Forest Watch Canada – argued against its conclusions. Dr. Timoney and Mr. Lee posted an online commentary calling its methods "problematic." Dr. Schindler said the lake sampling was conducted "well outside the range where atmospheric transport would reach it, so tells nothing about that source. In short, it adds very little to the controversy over the amount that industry is contributing to the river."

It doesn't help that the work was funded by Suncor.

But the scientists behind the new research, who spent years trying and failing to attract funding from non-industry sources, have sterling credentials. Dr. Hall was associate dean of graduate studies at the University of Waterloo; he is currently on sabbatical. Brent Wolfe holds a northern research chair from the Natural Sciences and Engineering Research Council of Canada. George Dixon is Waterloo's vice-president of research, a scientist whose expertise in aquatic toxicology has been called upon by governments across the world.

To do their tests, they dug into sediments from three northeastern Alberta lakes near the Peace-Athabasca Delta, a UNESCO World Heritage site important for fish and migratory birds.

The researchers found levels of airborne metals peaked in the late 1950s and 1960s – before the advent of the oil sands – while arsenic has now fallen to pre-industrial levels and lead is now 10 per cent above "background" concentrations. The researchers attributed some of the pollutant declines to the switch away from leaded fuels and to the end of smelting operations at Yellowknife's Giant Mine, which, according to Dr. Hall, vented as much arsenic in one 1950s day as the entire oil-sands industry in 45 years.

Perhaps most intriguing: They found levels of polycyclic aromatic compounds, which can cause cataracts, organ damage and cancer in humans, were at their lowest between 1975 and 1995, a period of oil-sands growth. The most PACs came between the mid-1700s and mid-1800s, a dry pre-settlement period on the Prairies that saw substantial forest-fire activity.

Dr. Hall said, however, that his research does not negate findings by Dr. Schindler and others, who showed that pollutants contaminate an area within 50 kilometres of oil-sands sites. That's less than the 200-kilometre distance Dr. Hall studied, but still a huge area, given that various extraction operations extend over a broad swath of the Alberta landscape.

A 2009 paper Dr. Schindler co-authored, for instance, showed summer levels of PACs in the Athabasca River were 22 times higher in waters downstream of oil-sands development.

What's clear, Dr. Hall said, is that more study is needed. Measuring contaminants is an important first step. The next is to "know about what effect they're exerting on the organisms that live in the water."