Why is Wissahickon Creek suddenly fluorescent green?

If you were out trekking in Wissahickon Valley Park as it runs through Chestnut Hill earlier this week, you might have noticed the water had turned a bright, fluorescent green.

Not to worry, says the Philadelphia Water Department (PWD).

The green came from a harmless dye PWD used to trace the source of an issue on the 200 block of Rex Avenue.

Brian Rademaekers, a PWD spokesperson, said a street had caved in May 10. Workers used fluorescein, a nontoxic fluorescent dye, to investigate the source of the leak.

Some of the diluted dye made its way to a tributary of Wissahickon Creek.

“The public should know that this dye is harmless,” Rademaekers said. “There are no impacts on wildlife or drinking water quality.”

Rademaekers said crews will be going back to Rex Avenue to investigate what’s causing the stretch of street to sink. They will be using a camera and dye, “so there is a chance people may see this again in the next week or so.”

Friends of the Wissahickon, the nonprofit that helps the city manage Wissahickon Valley Park, said in a Facebook post Monday that it had received many reports of green water near Rex Avenue in the Rex Avenue Tributary and Wissahickon Creek.

FOW said the water department may be conducting more testing and confirmed that the dye is nontoxic as it makes its way into stormwater retention basins around the park.