Members of the Athabasca Chipewyan First Nation said they were concerned about a large sheen visible on the Athabasca River.

They said the sheen was first seen on Saturday morning about 60 kilometres north of Fort McMurray.

“We could see the oil residue from a plane up above on that stretch over 100 kilometres long,” Chief Allan Adam told CTV News.

“Along the stretch there was oil residue that covered the whole river. The oil was about five kilometres in length.”

Chief Adam said they contacted oil companies in the area but none reported any leaks, something Alberta Environment and Sustainable Resource Development (ESRD) confirmed.

“Industry completed a thorough check of their facilities and no sources were found,” Robyn Cochrane explained.

She also said they did an aerial surveillance of the river.

“Environment staff working in the area flew the length of the Athabasca River from Fort McMurray to Fort Chipewyan and found no source of the hydrocarbon leak.”

However, she added that ESRD officials determined the cause of the sheen was likely natural.

“I know in one instance there was a sandbar that appeared to be dark but they landed and they inspected it and they verified it to be silt.”

Despite no source of a leak being found, Chief Adam said they were still concerned about the sheen.

“It’s become a serious situation here in Fort Chip because all of that pollution is going to end up on Lake Athabasca, where we reside.”

He said First Nations officials were still investigating and would be taking 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,” Chief Adam said, explaining that oil samples have a DNA-type of identifier.

“You’d be able to locate who were the producers of it and which line it was sent out.”

Cochrane said their experts would also be following up next week and would take water samples if more sheen was discovered.

With files from Amanda Anderson