Oil-sands development is polluting nearby remote Alberta lakes with rising levels of a toxic carcinogen, refuting long-standing claims that waterway pollution in the region is largely naturally occurring, a study has found.

The peer-reviewed study, published Monday by a research team including Environment Canada scientists, found levels of polycyclic aromatic hydrocarbons (PAHs) have risen roughly at the same pace as development in six nearby lakes. It's "well established" that some PAHs are carcinogenic, according to the American Agency for Toxic Substances and Disease Registry. Some types of PAHs have been linked to infertility, immune disorders and fish mutation.

The PAH pollution level remains low – on par, at worst, with an urban lake – but is rising. Results in one remote lake showed PAH levels 23 times higher than pre-development levels 50 years ago. It's the rate of growth that's most alarming, said John Smol, a Queen's University professor, Canada Research Chair in Environmental Change and study co-author. "You only have to start doing some back-of-the-envelope calculations of, in 15 years, where they might be," he said.

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Government and industry both accepted the findings, with the latter saying levels remain low, considering the scale of economic development in the region.

Even with higher present-day levels, researchers found no "observable negative effects" on a bellwether type of microcrustacean, the Daphnia. They attributed the higher PAH levels to "multiple environmental factors," including climate change, and not just nearby oil-sands development. But, as levels increase, "… there exists great potential for Athabasca oil-sands ecosystems to experience marked changes …" the study said.

The study also refutes a key argument long made by provincial officials, who have said that, because oil seeps naturally into rivers around the oil sands, contaminants are mostly natural.

"I think it's pretty convincing evidence," said David Schindler, a University of Alberta biologist who co-authored a 2010 study that revealed contaminants, such as mercury and lead, in the Athabasca River near the oil sands. "Hopefully, this will kill the all-the-pollutants-are-natural theory once and for all."

Monday's study raises further questions. In particular, it finds elevated and rising levels in a lake as far as 90 kilometres away from development sites. "The footprint is significantly different than what's been suggested," Prof. Smol said. But Derek Muir, an Environment Canada scientist of environmental chemistry and study co-author, said more work needs to be done. "I'm cautious in my interpretation of what we're seeing out there," said the Environment Canada researcher in an interview, adding Environment Canada has become "extremely active" in the region over the past two years.

Alberta's Ministry of Environment and Sustainable Resource Development welcomed the "very valuable research," spokeswoman Jessica Potter said. "Overall, we do know there is an impact from industry," she acknowledged. The province continues to implement, with Ottawa, a new environmental monitoring program in the oil sands.

In the context of massive production – about 1.5-million barrels a day, expected to more than double by 2025 – the PAH levels in the lakes remain relatively low, said Travis Davies, a spokesman for the Canadian Association of Petroleum Producers industry group. "They also suggest more research, and we absolutely back that," he said.

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The study, published in the Proceedings of the National Academy of Sciences journal, relied on core samples from the bottoms of lakes, where layers of sediment allow researchers to see year-to-year changes. It amounts to a snapshot, for lack of one. "We're trying to make up for missed monitoring, I guess," Prof. Smol said.

PAH levels in all six tested lakes were found to have increased, though only in one are contaminants at urban-lake levels. In that lake, seven of 13 PAHs tested are at a level considered to have the potential for "possible" but not "probable" impact. The other five are "generally comparable to other remote lakes and much lower than" urban lakes, the study concludes.