Photo by Joseph Gruber

Update: The Coast Guard has determined that the mysterious substance in the Potomac River is fuel oil through samples taken last week. It has “been significantly degraded because of weathering,” said Michael Keane, chief of response at Coast Guard Sector Baltimore, at a press conference this afternoon.

The Coast Guard is expected to receive details from samples taken today by Wednesday, Keane said. Crews are still working to find the oil’s source.

Original:

If you’re riding past the Potomac River on your evening commute and find it to be a yellow-ish color, do not be alarmed. It’s just part of testing—though for a rather alarming reason.

The National Coast Guard is conducting a dye test this morning in the Potomac River and Roaches Run Waterfowl Sanctuary to find the source of an oily substance in the river.

The oil sheen was spotted last Wednesday. As of Friday, 18 geese were found covered in oil and sent to be treated by wildlife experts. While Coast Guard officials have been monitoring the waters, as well as deploying cleanup crews and using other oil-removal methods, they still aren’t able to identify the substance.

The dye that’s being used today “can be seen as a bright yellow or green and is non-toxic, non-staining and water-soluble,” Commander Michael Keane of the Coast Guard said in a statement. The discolored water will naturally go away, he added.

Amid a water crisis in Flint Michigan where dozens of children suffered lead poisoning and 10 died of Legionnaire’s disease after the city switched its water supply from Lake Huron to the Flint River, anxiety around water supplies has increased across the country.

“We want to inform people in an attempt to mitigate concern by the public, who may see the dye in the water and mistake it as something else,” Keane said.