Concern is growing in British Columbia after a tailings pond used to dispose of heavy metals from a mine suffered a breach, sending massive amounts of wastewater and mining debris into surrounding lakes and rivers.

Spill estimates have doubled since Monday, after an earthen dam at one end of a four-kilometre long pond breached, forcing waste from the Mount Polley Mine into the pristine lakes and rivers south of Prince George, B.C.

Ten million cubic metres of water – enough to fill 4,000 Olympic-sized swimming pools – and 4.5 million cubic metres of silt from the Mount Polley Mine, operated by mining firm Imperial Metals Corp., prompted drinking water warnings in some areas, and has helped to wash out waterways like Hazeltine Creek and Polley Lake, which rose more than a metre. The mine is an open pit gold and copper mine located about 140 kilometres southeast of Quesnel, B.C. The Cariboo Regional District issued a water ban warning people not to drink or bathe in the water, or allow animals to drink the water.

The area is also one of B.C.’s largest salmon breeding grounds, and upstream from the Fraser River.

“It’s a very dangerous mix and there’s a really significant risk of these metals moving through the creek, the river, the lake,” Sierra Club’s Jens Wieting told CTV News.

The mine’s tailings pond is part of a system to dispose of heavy metals suspended in that water, such as cadmium, lead and mercury. Problems with that system surfaced as early as 2011.

Environmental consultant Brian Olding was commissioned by nearby First Nations groups to study the facility, and found there was more water going in than going out. He was also hired by Imperial Metals Corp. to review the company’s plans to treat and release water.

“You must discharge,” Olding said. “You can’t just keep building it up.”

Olding said he wanted a sedimentation pond to treat the water before releasing it. He says Imperial Metals didn’t build one. Instead, they built a bigger dam.

“It puts a pressure on the earthen dam, so how high are you going to build this thing?” Olding said. “The higher you go, the more risk you take should it ever fail.”

Company’s response

Imperial Metals said on Tuesday that it never expected a problem with the dam, and that the water is less toxic than feared. They also said the cause of the breach is unknown.

“Mercury has never been detected in our water and arsenic levels are about one-fifth of drinking water quality,” Brian Kynoch, president of Imperial Metals, said in a statement.

B.C. Energy and Mines Minister Bill Bennett said government, scientists and company officials are working to figure out the scope of the impact.

“We don’t know the quality of the water that was in the tailings pond,” Bennett said. “I am advised it was fairly high quality water and I hope that turns out to be the case.”

"We have to make sure the water the people are using is safe and we need to do some work to clean up whatever debris has come out of the dam," Bennett added.

With files from The Canadian Press and a report by CTV News’ Jon Woodward