"Environment Canada's enforcement branch conducted a thorough review of the circumstances surrounding the Suncor spill," said spokeswoman Jirina Vlk in a three-sentence email.
"There was not sufficient evidence to indicate a violation of the Fisheries Act. The file has been closed."
Her statement came in response to a letter from an environmental law firm asking what actions the federal government was taking over the spill. The letter to Environment Canada came after a provincial investigation concluded that the spilled wastewater was toxic to fish.
"It indicates that a violation of the Fisheries Act has occurred," said Melissa Gorrie of Ecojustice, who wrote the letter on behalf of Greenpeace, Keepers of the Athabasca, the Council of Canadians, Public Interest Alberta and the Sierra Club.
On March 25, a wastewater pipe at the Suncor plant near Fort McMurray froze and burst open, sending process-affected water into a partially frozen outfall pond containing treated water.
An investigation by Alberta Environment found undiluted samples from the spill killed rainbow trout fingerlings exposed to it. The investigation concluded that the fish were probably killed by high levels of naphthenic acids, chemicals that occur naturally in bitumen.
The undiluted samples were also found to contain levels of salts and ammonia above provincial guidelines. Metals including selenium, boron and arsenic were found at twice the recommended levels for long-term exposure.
The provincial investigation concluded that by the time the water spilled from the outfall pond into the Athabasca River, it was probably too diluted to pose a health threat to humans.
In her letter, Gorrie points out the Fisheries Act specifically forbids releasing any "deleterious substance" into fish-bearing waters. She writes that the offence exists whether or not waters become toxic as a result.
"It is our position that the substances released into the Athabasca River as a result of the spill are clearly deleterious," the letter says.
Gorrie pointed out Alberta continues to investigate the spill and said it was "amazing" that Environment Canada reached its conclusions so quickly.
"I'd be interested to know a bit more about their rationale," she said. "It's interesting Environment Canada has shut the door on the possibility of prosecuting."
The department has not responded to her letter and made its decision known in response to a question from The Canadian Press.
Gorrie said the groups are considering a private prosecution under the Fisheries Act.
"If the federal government doesn't act, we would consider our options in moving this forward through a different avenue."
The full lab results from Alberta's investigation into the spill have not yet been released.
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.
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