Toronto residents are being warned against swimming at Sunnyside beach due to high E. coli counts found in water samples taken over the past 24 hours.

According to the city’s website, high E. coli levels emerge likely as the result of severe thunderstorms. On Monday afternoon, the Greater Toronto Area experienced heavy rain and thunderstorms. The heavy rainfall triggered a warning from the Toronto and Region Conservation Authority that said the storm could cause water levels to rise, especially rivers and streams.

Sunnyside is the only beach in the GTA that is being labelled as “unsafe to swim” after the storm. However, the City of Toronto is warning swimmers to wait before venturing into any other body of water.

“It can be unsafe to swim, even at designated swimming beaches, for 48 hours after a rainfall due to the possible presence of high levels of bacteria that could pose a risk to human health,” the city website indicates. “The bacteria could be a result from water pollution, which is a complex issue and can come from many sources.”

The bacteria sources include storm water runoff, which is when rain water picks up a number of pollutants from roofs, roads, cars and sidewalks before entering bodies of water. This bacteria impacts the water quality in Ontario’s lakes as well as in rivers and streams.

Heavy rainfall can also cause sewage overflow, which can also lead to water contamination.

Exposure to E. coli can cause diarrhea, abdominal cramping, and nausea.