A one-billiion-litre spill from a waste water pond at an Alberta coal mine continues to cause concern as the effluent makes it way down the Athabasca River.

The spill the size of 400 Olympic-sized swimming pools was released by the Obed Coal Mine near Hinton when a retaining wall collapsed on Oct. 31.

The released sediment, along with organic debris it collected in its path, entered the Athabasca River, creating a brownish-coloured stretch of water. 

The 150-kilometre stretch of brownish-coloured slurry is now about halfway between the town of Athabasca and Fort McMurray, as it travels at a rate of two to five kilometres per hour.

While mine officials said the release poses no threat to public health, internationally respected water expert David Schindler said he's worried about mercury levels in the effluent getting into fish eaten by First Nation residents along the river.

"Mercury is particularly bad for the development of the brains of fetuses and young children and also if it's in the bodies of women of child bearing age, it tends to be transferred," he said.

The spill should be a wake up call about what could happen to the more toxic oilsands tailings ponds along the Athabasca River, Schindler said.

He advises oilsands mines stop to building them and find another way to deal with the waste.

"This should be a wake-up call to stop what we're doing before we have a huge accident," he said. "You have to wonder what would have happened if a downpour the size of the one that the Bow River catchment got in June was to hit the oilsands."

The Obed mine, located 30 kilometres east of Hinton, Alta., began producing export-quality coal in 1984, but is not currently operating.

The mine was acquired by Sherritt International Corporation as part of purchase of Luscar Ltd. in 2001.

With files from CBC's James Hees