The Delaware County Hazmat Team and environmental officials are investigating a possible spill in the West Branch of the Chester Creek after 30-40 dead fish were found behind a pumping station on Friday.

The fish were found behind the pumping station on Mount Road in Aston Township, near Lewis Fisher Memorial Park.

A public works employee noticed the dead fish and a sheen on the water, and alerted authorities, Ed Beebe, the Delaware County director of emergency services, told CBS News Philadelphia.

Teams have blocked the flow of water behind the pump station and are investigating what the substance is. According to Delaware County Emergency Services, they've concluded that the substance was not petroleum-based and believed it was, in fact, chlorine-based. The substance started to dissipate as of 1 p.m. Friday.

"At this point, we don't see any reason for concern," said Larry Bak, chief officer of the Hazardous Materials Response Team. "Yes, there are some dead fish, but beyond that, we haven't really found anything in the water that would be of harm to the general public."

The Pennsylvania Department of Environmental Protection, Aqua PA and the state Fish and Boat Commission also responded to the scene.