USCG: GoM oil slick likely just silt from dredging [Photos]

What was reported as a miles-long oil slick in the Gulf of Mexico is likely a plume of silt emanating from a dredging operation on the Mississippi River, a U.S. Coast Guard spokesman said Sunday.

Still, the Coast Guard is testing the substance, which has an associated rainbow sheen similar to that resulting from oil spills, to determine exactly what it is, said spokesman Henry Cambre.

It is possible, Mr. Cambre said, that whatever is causing the sheen was trapped in river bed sediment and was released by the dredging work. The Coast Guard said it expects results from those tests to be available later Sunday.

The Coast Guard station in New Orleans received a report Saturday morning of a three-mile-long patch of rainbow sheen south of Grand Isle, La. Two subsequent sightings that were relayed to the Coast Guard had the slick growing; the last caller said it stretched from six miles south of the coast to 100 miles offshore.

Officers confirmed the existence of a substance near the water’s surface, but that initial investigation was cut short when their helicopter was diverted to a separate search and rescue mission. The Coast Guard has since dispatched additional aircraft and boats from Mobile, Ala., and New Orleans to the scene.

-by Ryan Dezember, Dow Jones Newswires