OTTAWA — The development of Alberta’s oil sands has increased levels of cancer-causing compounds in surrounding lakes well beyond natural levels, Canadian researchers reported in a study released on Monday. And they said the contamination covered a wider area than had previously been believed.

For the study, financed by the Canadian government, the researchers set out to develop a historical record of the contamination, analyzing sediment dating back about 50 years from six small and shallow lakes north of Fort McMurray, Alberta, the center of the oil sands industry. Layers of the sediment were tested for deposits of polycyclic aromatic hydrocarbons, or PAHs, groups of chemicals associated with oil that in many cases have been found to cause cancer in humans after long-term exposure.

“One of the biggest challenges is that we lacked long-term data,” said John P. Smol, the paper’s lead author and a professor of biology at Queen’s University in Kingston, Ontario. “So some in industry have been saying that the pollution in the tar sands is natural, it’s always been there.”

The researchers found that to the contrary, the levels of those deposits have been steadily rising since large-scale oil sands production began in 1978.