An environmental cleanup is underway in East Rockaway after hundreds of gallons of oil leaked into the Mill River on Monday night, leaving behind a shimmering, rainbow-colored sheen on the surface of the water.

Officials say the spill involved dielectric fluid — a type of mineral oil used to insulate electric transmission lines — and originated from a PSEG Long Island utility line. However, as of Tuesday, crews had not yet pinpointed the exact location of the leak.

The incident has raised concerns among residents in East Rockaway, Oceanside and Lynbrook, who reported both seeing the oil slick and smelling the chemical odor lingering in the air.

“We love our area here. We keep it clean. And look what it looks like now,” said East Rockaway resident Lorraine Keegan. “It’s so wrong.”

PSEG Long Island and the Nassau County Fire Marshal have both stated that the fluid poses no direct risk to human health. Abhinav Kumar, with PSEG, assured residents that the oil is non-toxic and comparable to vegetable oil in terms of safety.

“There is some smell to it, I can’t stress enough,” said Kumar. “It’s a dielectric fluid. It’s not toxic.”

Despite these reassurances, the National Oceanic and Atmospheric Administration (NOAA) has noted that dielectric oil can still be harmful to wildlife, particularly aquatic species and birds that rely on the waterway for food and habitat.

“There’s so many birds just sitting there, and who knows how it’s going to affect them,” said local resident Cara Filomio. “It’s where we do our local fishing. We come from a fishing community, and it’s going to affect us.”

Containment booms have been deployed to limit the spread of the spill, and crews are actively skimming the oil from the water’s surface. Workers are expected to remain on site around the clock as the cleanup continues.

A timeline for the full cleanup remains uncertain. A PSEG spokesperson said the process could take time.