The cause of an oil spill in the Bronx River remains a mystery.

The Bronx River Alliance says it was collecting water samples on Monday along the Bronx River near Oak street in Mount Vernon when an oily sheen coating the surface was observed.

"We don't know where this material came from, it might have been a community member actually dumping material down a storm drain, which in their eyes, you know, it's out of sight, out of mind," said Christian Murphy, the ecology and education manager for the Bronx River Alliance.

The state Department of Environmental Conservation spills response experts responded to the report and visited several locations along the river to investigate the extent of the sheen.

The DEC has not yet determined the cause of the sheen but will continue to monitor the river to ensure the protection of public health and the environment.

It's the second spill within a month and the third in a year on the river. Sections of the river turned green in March when a Department of Environmental Protection tanker truck leaked chemicals that spilled into a storm drain that flows into the river.

In April 2024, 1,000 gallons of non-hazardous fluid from a Con Edison transmission feeder leaked into the river causing roughly a weeklong cleanup effort.

Murphy says oil spills are concerning as they threaten the health of the river and can be hazardous to both humans and wildlife.

"You could get reactions to your skin that can be toxic and then there's obviously major impacts to wildlife who can be harmed or killed by these spills. And so, we're very concerned whenever there's any chemical entering the river," said Murphy.

Murphy says they need more eyes and ears on the river to help maintain their efforts.

They are asking the public and ask anyone who witnesses contamination to call the NYS Spill hotline.