Substance in Gulf of Mexico likely sediment, not oil

ST. PETERSBURG, Fla A reported "sheen" in the Gulf of Mexico off the coast of Louisiana is likely the result of large amounts of sediment, not spilled oil, the U.S. Coast Guard said on Sunday.

The Coast Guard collected and tested samples of a dark substance after receiving varying reports on Saturday of possible pollution floating on and beneath the water's surface. The largest sighting described the substance as stretching 100 miles into the Gulf, south of Grand Isle, Louisiana.

An analysis found only trace amounts of petroleum hydrocarbons, oil and grease -- all at levels well within the Louisiana Department of Environmental Quality's clean water standard.

Sediment carried down the Mississippi River and possibly agitated by dredging is believed to have caused the dark substance, according to a Coast Guard news release.

The Coast Guard also investigated on Sunday an oily substance washing up along the Louisiana shoreline.

Samples of that substance will be tested. It is not suspected to be residual oil from the Deepwater Horizon spill in April.

(Editing by Greg McCune)