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Crews work on a broken section of pipeline northeast of Peace River, Alta.Ian Jackson/The Canadian Press

Operators of an oil pipeline whose rupture created the largest Alberta spill in 36 years detected a potential problem nearly eight hours before halting the flow of crude.

In that time, workers with the Canadian subsidiary of Plains All American Pipeline L.P. attempted to restart the line several times, company executives said a week after 28,000 barrels spilled near the Cree community of Little Buffalo, Alta.

The leak, which the company said was related to an improper repair job and not a corrosion problem, has caused health concerns in Little Buffalo. The school has been closed amid complaints of a strong hydrocarbon odour that has triggered nausea, burning eyes and other symptoms. A few animals (seven beavers, seven ducks and a migratory bird) have died, and the spill has drawn criticism from both opposition politicians and Alberta Premier Ed Stelmach, who accused the company of not doing enough and pledged to "hold their feet to the fire."

On Friday, the company apologized for its communication lapses, even as more troubling details emerged about public disclosure of the spill's size and whether the company could have stopped it earlier. An apparent delay in releasing some details of the leak has raised questions about the conduct of Alberta's energy regulator during the last days of the federal election campaign.

Operators first noticed a problem on the 770-kilometre Rainbow line, which had been carrying 75,000 barrels a day of light sweet crude, at about 7 p.m. April 28.

"The control centre detected an unstable condition. It appeared to be an equipment malfunction," said Stephen Bart, vice-president of operations for Plains Midstream Canada, the pipeline's operator.

The pipeline's operation was temporarily halted and workers were dispatched to pipeline terminals "to confirm that the equipment was operating correctly. And from the information available to the operators in the control centre, it appeared that the pipeline was secure and it was appropriate to restart the pipeline," Mr. Bart said.

The pipe was restarted several times before, at 2:30 a.m. on April 29, operators "detected a release on the pipeline," he said. Twenty minutes later, the line was shut down.

Even then, the shutoff valves nearest the rupture were 137 km apart. Between those two points, the pipe followed a geographic dip that allowed crude to flow down to the site of the leak, near the bottom of the dip. A helicopter was dispatched at first light to examine the pipe.

When the spill was discovered just before 8 a.m. April 29, workers closed another set of valves, located 72 km apart. That spacing, Mr. Bart said, was in compliance with industry standards in 1966 when the pipeline was built, and still complies with standards today, although the company said it has been working to space shutoff valves more closely together.

Mr. Bart declined comment on whether the rupture occurred the evening of April 28, and whether the restarts pumped more oil through the broken pipe, saying: "We will be reviewing this in much greater detail in coming weeks."

After initially reporting the spill's size as 6,300 barrels of crude, the company raised its estimate to 28,000 barrels on April 30, Mr. Bart said.

But it took until nearly 4 p.m. on May 3 for the Alberta Energy Resources Conservation Board to release that number to the public. That was a day after the federal election, and the lengthy delay has raised questions about why the updated figure was not released earlier. An ERCB spokesman said he was given confirmation of the new information Tuesday.

Others aren't convinced. "I've always thought it very odd that it would take that long to come up with a reasonable estimate," said Chris Severson-Baker, managing director of the Pembina Institute, an environmental group.

Critics also question whether the age of the Rainbow line, which is more than 40 years old, contributed to the problem, especially because investigators pointed to corrosion and other issues as causes of a 7,500-barrel Rainbow spill in 2006.

But Mr. Bart said a crack-detection tool used in mid-April showed no problems at the spill site, which had been the site of a maintenance repair in 2010. Excavation of the rupture, however, has uncovered evidence that the repair job was done incorrectly.

"It appears the soil underneath the pipeline was inadequately compacted, resulting in stress which resulted in the break of the pipeline," said Mr. Bart, calling it a "singular" event. About two-thirds of the circumference of the pipe split, he said.

The pipe has been repaired, although it is not known yet when the flow of crude might be restarted.

The company has 300 people working to clean up the oil, some of which is being contained by a beaver dam. The cleanup will take two to four months; the company declined comment on how much it could cost, but noted it's likely to be less than the $550-million incurred by Enbridge Inc. after a Michigan spill of crude last summer fouled an important river. The Rainbow spill has so far been kept away from running water.

"We are committed to recover the vast majority of this oil and restore this area," Mr. Bart said. "We're doing everything possible to get this done as quickly as possible."

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