The company was prohibited from releasing water from the pond — the final step in Suncor's treatment process — into the Athabasca River in March 2011. Toxicity in the water was found to exceed allowable limits.
"They had to close the discharge outlet, which they did," said Alberta Environment spokeswoman Jessica Potter.
Under the department's enforcement framework, inspectors have been working since then with the company to try to fix the problem. But when water samples from the pond were recently tested, they again failed. The water was found to kill rainbow trout fingerlings.
Suncor officials have told government inspectors the toxicity could be dissolved organic material.
Alberta Environment has told the company it must determine the source of the toxins and come up with a plan to clean them up by April 30. No water from the pond will be allowed into the Athabasca River until that happens.
Potter said Thursday's order was not related to a pipeline spill of process water on the Suncor site earlier this week.
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