Trace amounts of cocaine, oxycodone and morphine have been detected in surface water in southern Ontario rivers, among other illicit and prescription drugs, a new study says.

The drugs originate in wastewater discharged into the Grand River watershed, according to a McGill University report published last week in the journal Environmental Toxicology and Chemistry.

Limited quantities of certain drugs also remained in Ontario's drinking water, even after passing through a drinking water treatment plant, researchers said.

"If the wastewater treatment plant - which is kind of an intense treatment that we do to remove these contaminants - is not able to remove fully the compound, we shouldn't expect the environment to remove it fully," said Viviane Yargeau, a chemical engineering professor at McGill and one of the study's authors.

"That's what we observed; we do detect (the drugs) in the drinking water."

A total of 17 substances were observed in small quantities in the wastewater, including cocaine, amphetamines, opioid drugs, and metabolites. Researchers said cocaine, ephedrine and prescription opioids were also not effectively removed at the drinking water treatment plant.

Yargeau told the Star that while the drugs do not pose a serious health risk to humans, they could have a negative impact on the environment.

"I do drink tap water even though I've measured these concentrations in tap water," she said. "It's a bit more of a concern that we discharge them in the river knowing that the river might accumulate some of these drugs, that the fish will be exposed to it."

The study did not measure the drugs' impact on the ecosystem.

Yargeau said that based on previous studies that detailed the effects of pharmaceuticals on aquatic life, however, the drugs could potentially alter fish behaviour and even threaten some species' survival.

"We can assume that the fish might have a different behaviour when exposed to the drug, and maybe that would have a decline on the survival rate of a fish population," she said.

The Grand River begins in Dufferin County and ends 300 kilometres later at Lake Erie. Its watershed covers several municipalities, including Waterloo, Kitchener, Guelph, Brantford, and parts of Hamilton.

Yargeau said the study was unique in that it followed the flow of water from the wastewater treatment plant, through the river, and after it was processed at a drinking water plant.

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She said she could not specify where exactly the water samples were taken, but that she hoped the study would put pressure on decision makers to invest in better wastewater treatment facilities.

"If we improve the wastewater treatment plant, then we will protect the environment at the same time and protect the source of drinking water."