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Environmental officials are citing natural causes for an oily sheen on a stretch of the Athabasca River, but a Northern Alberta First Nation remains concerned about a possible oil facility leak.

Chief Allan Adam of the Athabasca Chipewyan First Nation told Â鶹ӰÊÓ the oily sheen was spotted on Saturday along a stretch of the river approximately 60 kilometres north of Fort McMurray.

"We followed the Athabasca River just above Poplar Point and we could see the oil residue from a plane up above that stretched over 100 kilometres long," he said.

Concern about a possible oil breach led Chief Adam to notify Alberta Environment and Sustainable Resources Development (ESRD). Ultimately, oil companies in the area were asked to check for a leak, but none were found according to ESRD spokesperson Robyn Cochrane.

"Industry completed a thorough check of their facilities and no sources were found," she said.

Cochrane added that officials conducted their own aerial search along the length of the river from Fort Chipewyan to Fort McMurray, resulting in no signs of an oil spill. Officials believe a natural cause is the likely answer.

"What they saw with their expertise, it appears to be natural," she said. "Any sheen or stain appeared to be natural."

During the surveillance, officials landed on a sand bar that Chief Adam believes is oil stained. However, Cochrane assured him that the dark residue is simply silt, and no hydrocarbons were present.

Despite the lack of a leak source and the officials' findings, Chief Adam is not convinced and wants answers.

"I really don't know what it's from. I really can't determine anything at this point in time," he said. "But I guarantee you, somewhere down that, somewhere along that corridor there has been a breach and [an] oil, petroleum product is making its way to the Athabasca River."

Although the ESRD says it will continue to monitor the river, Chief Adam says his community is taking matters into their own hands. They have taken their own water samples for testing.

"Once we've sent out those samples to get it analyzed, we will determine who is responsible for this oil spill," he said, explaining that oil samples can be identified through DNA-type characteristics.

"You'd be able to locate who were the producer of it and which line it was sent out."

Cochrane says their experts would also take water samples for testing if additional sheen on the river was discovered.

Oil-related pollution has already occurred on the Athabasca River earlier this year. Back in March, industrial waste spilled into the river from a broken pipe at Suncor Energy Inc.’s oil-sand project near Fort McMurray.