EDMONTON - Geotechnical engineers remained at the Obed Mountain coal mine Sunday trying to determine how one billion litres of murky water leaked from a containment pond into the Athabasca River.

A barrier gave way on Halloween, allowing liquid containing coal dust, sand and dirt to flow through two creeks into the Athabasca, said Darin Barter, a spokesman for the Alberta Energy Regulator.

“I haven’t seen this happen. Coal mine incidents and pit leak incidents are really rare,” he said. “I was surprised this could happen.”

The Obed Mountain mine, owned by Sherritt International and now undergoing reclamation since operations were suspended last November, is about 30 kilometres northeast of Hinton.

The dirty water travelled 25 kilometres to the Athabasca, forming a muddy plume now floating downstream, Alberta Environment spokeswoman Jessica Potter said.

Department staff and Alberta Health Services were analyzing water samples to determine if anything in the sediment could cause environmental or health problems, she said.

“There’s actually quite a noticeable change of colour (in the river),” she said. “It’s like muddy water … murky, muddy water.”

The leading edge of the plume, which is slowly dissipating, was between Whitecourt and Athabasca on Sunday.

Investigators don’t know how long the plume is because aircraft needed to monitor the situation were unable to fly in the snowy weekend weather.

However, most communities in the area don’t take their drinking water from the river, and municipal water systems are designed to filter out suspended solids anyway, Potter said.

Ten communities were notified about the release as a precaution, including Whitecourt, Hinton, Athabasca, Yellowhead and Woodland counties, and the Alexis Nakota Sioux and Alexander First Nations.

Generally, mines, oilsands and other energy sites are designed so water from operations as well as surface water from rain or melting snow flows into ponds, Potter said.

They hold the liquid until the sediment settles out and it’s safe to release.

The murky water passed through Whitecourt on Saturday night, Mayor Maryann Chichak said.

“The province and Sherritt Coal have both been very expedient in getting all their departments on board, ensuring any communities downstream were notified,” she said.

“It hasn’t had any impact on our community.”

Sherritt restarted mining at Obed Mountain in 2009 after the facility was shut down by Luscar six years earlier.

An estimated one million cubic metres (or one billion litres) of water containing fine clays, mudstone, sandstone, shale and fine coal flowed out of the breached pond, which is now empty, a Sherritt official said.

That’s roughly 500 times the amount of water in the Kinsmen Sports Centre’s five-metre-deep dive tank.

The materials in the pond are inert and aren’t toxic to humans or fish, the Sherritt official said in an email, asking not to be named because she’s not a company spokesperson.

The mine’s main pond has been inspected and is sound, she said. The company has dispatched an emergency response firm and staff from other operations to help with remediation.

Barter said the energy regulator sent engineers to the mine Friday, but couldn’t say how long it will take them to determine what happened.

The regulator approved and licensed the pond, he said, although he didn’t have information immediately available on how many similar structures there are in Alberta or how often they’re inspected.

In 2008, the RCMP deployed laser monitors to help crews successfully stabilize the slumping wall of an ash lagoon at TransAlta’s Keephills power plant, where 1.5 billion litres of toxic ash and water were stored.

gkent@edmontonjournal.com