The Alberta Energy Regulator (AER) is investigating after a mine pit near Hinton gave way on Oct. 31, releasing a large volume of water and mine sediment into two tributaries of the Athabasca River.

The leak happened at the Obed Mountain Coal Mine, about 30 kilometres east of Hinton.

“What we are solely concerned with at this point is what happened,” said Darin Barter, who speaks for the AER.

This type of breach is very rare, he said, adding this is the first leak of this type he has seen in his 10 years with the AER.

“Right now, our investigation is focusing on what happened why did this pit give way? And we will work backwards from there.”

The pit is no longer leaking liquid or sediment.

The AER is investigating to determine whether the company was in compliance with provincial requirements.

Alberta Environment and Sustainable Resource Development is involved in the investigation.