The "major failure" of a pit at an Alberta coal mine has released one billion litres of contaminated water into the Athabasca River.

The breach at the Obed Mountain Coal Mine has resulted in murky water entering two tributaries, which carried the refuse into the Athabasca and is now visible in the river in the form of a muddy plume, states the Alberta Energy Regulator (AER.)

Coal mines typically have a pit where the waste and water – coal dust and water – gather. And that pit, the open pit that contains that mixture, failed,” Darin Barter, a spokesperson with AER, told Global Edmonton.

“It’s our understanding that the water has entered two tributaries in the Athabasca River.”

The pit, which is located approximately 30 kilometres east of Hinton, failed during Halloween, stated the AER.

These kinds of incidents are rare, Barter told the Edmonton Journal, adding he was surprised it happened.

An employee at Sherritt International, which owns the mine, told the Edmonton Journal in anonymity the material in the pit is inert and non-toxic to people and wildlife.

But the province isn't taking chances.

Water samples were taken from the plume and are being analyzed by independent labs and will make those results public as soon as they are available, Robyn Cochrane, Alberta Environment spokesperson, told Global Edmonton.

The leading edge of the plume is slowly dissipating and as of Sunday was located between Whitecourt and Athabasca, the Journal reports.

Investigators don’t know the actual length of the plume because aircraft needed to make the assessment were grounded due the weekend's heavy snowfall, added the Journal.

Operation at the mine was suspended last year, due to what's believed to be overwhelming economic and market pressures, and is currently undergoing reclamation.

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