Alberta Environment says undiluted samples from the March 25 spill from a pipe at the Suncor plant near Fort McMurray killed rainbow trout fingerlings exposed to it. But the department says toxins were diluted by the time they reached the Athabasca River and the spill did not threaten humans.
"There is no concern to human health," said the report.Story continues after slideshow
The leak lasted for at least a few hours. It did not affect the plant's operations.
Alberta Environment said it is examining possible effects of the diluted water on the river.
"We are currently reviewing the effects of dilution of the process-affected water by both treated water in the combined outfall pipe and the river water. This will help determine what potential environmental impacts may have occurred."
Although it was originally unclear whether the water contained any oil, the investigation concluded the rainbow trout in the test were probably killed by naphthenic acids, chemicals that occur naturally in bitumen.
The undiluted samples were also found to contain levels of salts and ammonia that were above provincial guidelines. Metals including selenium, boron and arsenic were found at twice the recommended levels for long-term exposure.
"Trace element exceedances are not expected to cause harm to aquatic life over the short duration of the release," the government said.
Suncor was recently given until the end of April to fix a separate problem with one of its wastewater treatment ponds, which leaked toxins into the Athabasca in March 2011.