Why are some bodies of water in Toronto turning fluorescent green?

A pond in the northeast corner of High Park in Toronto on Nov. 10, 2022. The city is performing dye testing. (Courtesy of Adam Bunch)

Toronto residents have been left perplexed after a number of small bodies of water in the city have turned fluorescent green.

Toronto resident Adam Bunch told CTV News Toronto he came across a pond in High Park, located at the very northeast corner of the park near Keele and Bloor streets at about 3:45 p.m. on Wednesday.

“One of High Park’s stormwater ponds has gone fluorescent green,” Bunch said. “I can’t imagine that’s a great sign.”

One of High Park’s storm water ponds has gone fluorescent green.

I can’t imagine that’s a great sign. pic.twitter.com/3w2JnQLteT

— Adam Bunch (@TODreamsProject) November 11, 2022

Another user took to Twitter after they noticed Yellow Creek in Rosedale had turned bright green to report the discolouration to the city.

@311Toronto someone has dumped something into the sewers which has drained into the Yellow Creek east of Mount Pleasant en route to the Don River. pic.twitter.com/qpb1YtGhHk

— Spaceport Toronto (@Spaceport_TO) November 10, 2022

When reached for comment, the City of Toronto confirmed to CTV News Toronto that the colouring can be attributed to dye testing being performed by city staff and that it’s “completely harmless.” They confirmed the dye is biodegradable.

Dye tracing is a practice that can be used to observe the flow of water or sewage, and detect any cracks in infrastructure.

For further assistance or questions about the dye testing, the city says to call 311, option 9.