Days after industrial waste spilled into the Athabasca River from an oil-sands project, the Alberta government has revealed toxic water flowed into the river from the same site for three days in 2011.

Alberta's Department of Environment and Sustainable Resource Development on Thursday issued an environmental order against Suncor Energy Inc. for an industrial waste-water release in March, 2011, discovered after fish died in a monthly experiment that uses them to test the toxicity of industrial waste water from the oil-sands site. It is unclear why the investigation took two years.

The company never issued a press release regarding the 2011 incident and the Fort McKay First Nation just downstream from Suncor said it has tried to get details from the Alberta government about which chemicals were released, but has yet to find out even two years after the test fish died.

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"We've asked for data. It hasn't materialized," said Daniel Stuckless, manager of environmental and regulatory affairs for the first nation.

The incident comes in a week already heavy with criticism from those who say major oil-sands projects risk the health of northern Alberta's rivers and lakes, and communities. A critical U.S. decision on the proposed Keystone XL pipeline, which would bring heavy Canadian crude to lucrative Gulf Coast markets, is mere weeks away.

On Thursday, the Alberta government confirmed the 2011 incident happened at the same Suncor site north of Fort McMurray as Monday's spill, but insisted the timing of this enforcement order on Thursday is a coincidence.

"Absolutely not related," Alberta Environment spokeswoman Jessica Potter said.

The enforcement order from the government said the release into the Athabasca River two years ago was not within allowable limits of "acute lethality toxicity."

At least 50 per cent of rainbow trout used in the test had to survive being placed in a sample of treated industrial water taken from a pond that discharges into the river.

But many of the test fish did not survive. The document said Suncor discovered a "failure" in a March 21, 2011, test fish sample on March 24. It then closed the pond to the river, and began diverting the industrial waste-water to a tailings pond.

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Even though the company has continued to hold the pond water back from the Athabasca River in the two years since, the pond has failed an additional 39 fish tests. The source of toxicity is still unknown, but the government order said it could include naphthenic acids, which are often found in tailings ponds and are toxic to aquatic animals.

The order issued on Thursday said Suncor must continue to keep the pond closed off from the Athabasca River, identify the source of the toxic elements in the water, conduct an engineering audit of the waste-water treatment process, and more regularly report to government. In future, the government also said the company must carry out necropsies on fish that die in the water test.

Suncor spokeswoman Sneh Seetal said an analysis carried out on the company's behalf showed the risk that the river from the 2011 incident is "limited." She also said Environment Canada closed their investigation of the incident in late 2011, stating that the company couldn't have foreseen it. She noted that before 2011, the oil sands giant had no fish test failure in recent memory.

"Notwithstanding that, the release of the water that does not meet the regulated standards is unacceptable," Ms. Seetal said.

On Monday, the leak from a water pipe at the Suncor oil-sands site saw an estimated 350,000 litres of industrial waste water pour into the Athabasca over a 10-hour period, causing "a short-term, negligible impact on the river," according to the company.

Suncor has provided few details about which chemicals and substances were involved, but said in a statement "tests confirm the process affected water was a combination of water with suspended solids (clays and fine particulates) and inorganic and organic compounds. It does not contain bitumen."

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In Fort McKay – the small, aboriginal community downstream from Suncor's mine and base plant – the first nation has hired an independent environmental consultant to test the Athabasca waters this week. The information provided by Suncor about the makeup of the industrial waste water release has not answered all of the community's questions, said communications director Dayle Hyde.

"The one thing is we are still concerned about is whether or not the water that was released contained hydrocarbons," Ms. Hyde said.

Suncor and Alberta government officials continue to investigate the latest incident. The province says the results of water tests should be available next week.

"It's important that we have all the facts," Environment Minister Diana McQueen said. "Our officials will be working through the weekend to determine what impacts, if any, have occurred."