Seven residents of the southwestern Ontario town of Walkerton died and 2,500 were sickened in 2000 by tap water contaminated with E. coli. (CBC)

The federal government is ordering tests of Canada's drinking water over concerns it may contain contaminants thought to raise the risk of cancer and other health problems.

Health Canada is now seeking a contractor to determine if the contaminants — known as disinfection byproducts — flow from the country's taps.

Water-treatment plants have long used disinfectants such as chlorine and ozone to eliminate bacteria from drinking water.

But in the 1970s, scientists discovered the disinfectants react with organic materials in untreated water, such as decaying vegetation, to form the byproducts.

There are hundreds of byproducts. Some of the more common ones are trihalomethanes, haloacetic acids, bromate and chlorite.

The discovery of new byproducts, however, "challenges the basis of our current mitigating strategies," says a request for proposals issued Tuesday.

"[DBPs] migrate into drinking water sources and may not be eliminated by current water treatment processes," it says.

"Some substances in this category have been identified as either known or suspected carcinogens and endocrine or reproductive disruptors. Limited surveys have shown that many of these compounds, thought to have significant health effects, can be present in Canadian drinking water."

Tests a precaution, expert says

The contractor will collect samples from 60 water-treatment plants and distribution systems across the country, and report back to Health Canada.

Those findings will shape federal guidelines that set out the maximum concentrations of contaminants allowed in drinking water.

The contractor will also test the water for other contaminants, including pharmaceutical products washed down drains and bisphenol A, which Ottawa banned from baby bottles last year.

The request for proposals says the study will address concerns raised by the scientific community about the quality of Canada's drinking water.

Water expert Steve Hrudey of the University of Alberta said after more than three decades of study, scientists have yet to conclusively link the byproducts to cancer in humans.

He suspects the testing is being done as a precaution.

"There's hundreds of these things that have been identified," Hrudey said. "Doing individual tests on [each byproduct] to see whether they cause cancer in rodents is a multimillion-dollar expense, so most of them have never been tested."

Hundreds of boil water advisories

Health Canada did not immediately respond Tuesday to an email inquiry from the Canadian Press.

It took one of the worst public-health disasters in Canadian history to prompt sweeping changes to laws and practices surrounding municipal drinking water.

Seven residents of the southwestern Ontario town of Walkerton died and 2,500 were made ill in 2000 by tap water contaminated with E. coli.

But water-treatment systems still do not get rid of all harmful bodies.

Last May, the Canadian Medical Association Journal reported there were 1,760 boil-water advisories across the country — excluding those for 93 First Nations.

Boil-water advisories are public-health warnings that tap or well water is unsafe to drink — sometimes even to bathe in — due to bacterial or other contamination.