Traces of painkillers and other drugs can be found in the drinking water of 15 southern Ontario municipalities, a new study reports.

"This work demonstrates the potential of Ontario source waters, particularly river water sources, to contain trace levels of selected pharmaceuticals and personal-care products," says the study led by University of Waterloo biology professor Mark Servos, published in the March issue of the Water Quality Research Journal of Canada.

Researchers looked for eight types of pharmaceuticals — including ibuprofen — and the antibacterial agent triclosan in raw and treated water at 20 drinking-water treatment plants in southern Ontario.

The testing sites are not identified but all are said to be within easy reach of Environment Canada's National Water Research Institute in Burlington.

Scant trace of drugs in treated water

The researchers found that river-water samples taken downstream of sewage outfalls were the most contaminated, while raw water taken from large lakes also had low but detectable levels of several of the drugs. The study said this suggests "that these chemicals are widespread in the environment."

They did not find levels of the drugs in samples from wells.

"Most of the acidic drugs were not detectable in finished waters," the study said. It said that levels of the painkiller Naxproxen and triclosan "were detectable in finished water but were significantly reduced in concentration relative to the raw water."

Servos said the amounts of the drugs found were small, with most compounds reduced to trace or non-detectable levels after passing through water treatment plants.

"Our best scientific judgment right now is that they represent a minimal risk," he told CBC News.

Servos said people dumping medications down the toilet is only part of the problem.

"The majority of the drugs are taken by people and they're basically excreted into the toilet and they end up in the sewage treatment plant," he explained, adding the antibiotics are also leaching into the water from livestock manure.

Treatment plants not designed to remove drugs

He said sewage treatment plants are good at removing things like bacteria, but were never designed to get rid of compounds such as drugs.

He said a number of methods for removing the drugs are being explored, and that UV light, with peroxide, ozone and different kinds of carbon, can help reduce the presence.

Servos said two Ontario companies, in London and Mississauga, are on the verge of developing the technology to remove the drugs.

The study said further research is needed.

"There is a need to complete a more comprehensive assessment of these compounds in source waters and of the factors influencing their treatment and removal from finished drinking water."

With files from the Canadian Press