Production at the world’s third largest source of oil has polluted surrounding waters with toxic substances, according to a new study. The findings add fuel to a fiery debate over a proposed pipeline connecting Canadian oil sands with US refineries.

Lakes as far as 56 miles away from production facilities near Fort McMurray, Alberta, show unnaturally high levels of substances linked to cancer. Researchers say they are the result of roughly half a century of development at the Athabasca oil sands.

While concentrations of carcinogens remain low compared with those found in urban lakes, scientists at Queen’s University in Kingston, Ontario, called the findings, released Monday, ‘worrying’ and warned of future effects from the spread of oil sands contaminants.

‘We’re not saying these are poisonous ponds,