WASHINGTON — The oily sheen detected in the Potomac River earlier this week is now so diluted it’s considered “unrecoverable,” local officials said Wednesday.

While federal investigators work to determine the source of the petroleum-based substance, it’s being diverted away from intake valves for water treatment plants operated by the U.S. Army Corps of Engineers and the Washington Suburban Sanitary Commission.

“Not because it’s a public health threat, but simply because it would cause a maintenance issue for the plants to change their treatment processes,” said John Emminizer, chief of emergency operations for the District’s Department of Energy and the Environment.

What’s left of the petroleum product in the river is so diluted, it won’t stick to absorbent pads that typically would be used to recover a spill. But it also won’t stick to plants, feathers or fur, Emminizer said.

The plume initially was spotted by a U.S. Park Police helicopter on Sunday near Point of Rocks, Maryland. Emminizer said the material might have escaped notice entirely if river water levels weren’t so low and river currents moving so slowly.