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A water plant operator checks the clarity of a sample of treated water. Environmental groups in Ontario are critical of plans for a New York State facility to treat 'fracking water' due to concerns that toxic outflow could contaminate Lake Ontario. (Keith Srakocic/Associated Press)

Environmentalists worry that Toronto's drinking water could be threatened by the nearby treatment of toxic wastewater in New York State.

Residents on both sides of the border worry about the potential for a spill if 'fracking fluid' — the liquid byproduct from the process of extracting natural gas from the ground — could flow into the Great Lakes.

The Niagara Falls Water Board in New York is mulling plans to treat fracking water at its wastewater plant on the U.S. side of the border.

What is fracking?

The controversial practice, also called hydraulic fracking or hydro-fracking, is used to extract natural gas from shale rock formations and coal beds.

Fracking involves injecting a pressurized mix of water and other substances into the rock to release the trapped natural gas.

The process has been banned in several jurisdictions, but in British Columbia — where huge deposits of shale gas are found in the northeastern part of the province — the practice is moving full steam ahead.

But Mark Mattson, president of the non-profit group Lake Ontario Waterkeeper, said those plans could put the primary source of drinking water for nine million people at risk.

"The idea of treating fracking fluid at sewage treatment plants and discharging it into the Great Lakes, that's a new issue," he said.

"Lake Ontario, which is already a threatened lake, needs more protection, not more pollution. So in this case, if they're going to ask for the privilege to put their pollution into our lake, they better be held to the highest standard to make sure that it doesn't affect our drinking water now or in the future."

Mattson also worries for the billions of fish swimming in the Great Lakes.

The Niagara Falls Water Board has hired a consultant to figure out whether its facility could treat the fracking fluid.

The Council of Canadians has written to the board, expressing concerns that fracking water will end up flowing into Lake Ontario via the Niagara River.

The board says it's only in the early stages of its plausibility investigation.

Daniel Heath, with the Great Lakes Institute for Environmental Research at the University of Windsor, said contaminants and salt may leak into the water system.

"A spill now and then is dramatic and scary and certainly serious," he said. "But if there was ongoing leakage, that would not be good."