MONTREAL -- A new study says drinking water in parts of southern Ontario contains traces of several illegal drugs -- including cocaine.

Researchers at McGill University found water discharged from waste-water treatment plants in the Grand River watershed has the potential to contaminate sources of drinking water with drugs such as morphine, cocaine and oxycodone.

The study -- published in the journal Environmental Toxicology & Chemistry -- says the drugs are found only in relatively limited quantities in the river water.

However, it notes their concentration did not decline with distance downstream from the waste-water treatment plant and says many of the drugs were not removed completely during drinking-water treatment.

Lead author Prof. Viviane Yargeau of McGill's Department of Chemical Engineering says improving waste-water treatment processes can help clean up drinking water.

Yargeau says the results of the study demonstrate a link between waste-water plant discharges and quality of drinking-water sources.

"While previous studies have shown that there are trace elements of various chemicals that remain in our drinking water, what is novel about this research is that we looked at the chemicals that are found in the water course between the waste-water treatment plant and the drinking-water treatment plant," Yargeau said.

The next stage in Yargeau's research will be a five-year project to look into how improvements of waste-water treatment and natural processes along rivers impact the presence of contaminants in drinking water.