"The big story here is the problems that many of the older cities are having with their aged infrastructure," said Elaine MacDonald, staff scientist and author of the report.

"Sewage treatment plants are overloaded and you end up with these overflows of raw sewage into the environment."

The report said that the 20 cities studied -- or about one-third of the 35 million people living in the heavily industrialized Great Lakes basin -- discharge more than 90 billion litres (24 billion gallons) of untreated sewage into the Great Lakes each year. That's equivalent of more than 100 Olympic swimming pools full of raw sewage each day, it said.

The Great Lakes -- Superior, Huron, Michigan, Erie and Ontario -- are the world's second-largest supply of fresh water after Siberia's Lake Baikal. Despite the huge size of the system, outflows are relatively slow and it takes 100 years to completely flush out into the Atlantic Ocean.

The problem is older municipal infrastructure that combines sanitary sewage systems with storm sewers, the report says. In rainy weather, the system is designed to overflow into local waterways instead of backing up into home basements.

"But it's not really an emergency (overflow) because you look at some cities and it's happening hundreds of times a year," MacDonald said in an interview.

Detroit, Cleveland and Windsor scored the poorest of the cities studied. Toronto, Canada's biggest city, was in the middle of the pack and Green Bay, Wisconsin, topped the list with no overflows.

The sewage outflows have resulted in beaches that have to be closed to swimming because of bacterial contamination such as E. coli, and an overabundance of nutrients in the lakes that lead to algae blooms and lost biodiversity.

Chemicals from commercial and industrial operations, as well as household cleaning products and pharmaceuticals, all make their way into the sewage system and ultimately end up in the lakes, MacDonald said.