Enough waste water, oil and grease to fill an Olympic-sized swimming pool leaked into the Athabasca River in September. (CBC)

One million litres of waste water, including grease and oil, leaked into the Athabasca River last September, the Liberals revealed in the legislature on Thursday.

Liberal MLA David Swann released details from an internal Suncor Energy report that said the leak, equal to the volume required to fill an Olympic-sized swimming pool, occurred Sept. 8 from the company's Millennium API Upgrader in Fort McMurray, where crude oil is processed.

He said the grease left a sheen on the river, but residents downstream only heard of the incident at a public meeting in Fort Chipewyan eight months later, on May 6.

Swann said the issue in question was that of trust and whether members of the public were receiving all necessary information for their safety.

"A million litres of waste water with grease and oil in it is probably not very significant [given the] billions of litres that go by," he said.

"The big question now is public trust, how much do we not know about leeching and spills and inappropriate discharges? That is the basic scientific question, and a number of scientists have said we need a good epidemiological study to assess that."

A Suncor spokesperson said the communities and the province's environment department were notified through the normal reporting channels at the time of the leak.

Suncor said it tested the water at the time and found no risk for fish, wildlife or human health, upstream or down.

Premier Ed Stelmach says the government is "diligently" investigating what happened.

"Were the people notified? What was the level of toxicity, if there was any?" he asked. "That's a matter for the investigation, but we're pursuing that very diligently."

The leak announcement follows the recent move by Alberta Health and Health Canada to investigate reports of unusually high rates of cancers in Fort Chipewyan. The results of their analysis are expected in the fall.