Some of the oil released in the Deepwater Horizon disaster was not cleaned up, but settled on the floor of the Gulf of Mexico.

A new study led by Florida State University Professor of Oceanography Jeff Chanton has confirmed that oil caused particles in the Gulf of Mexico to clump together and sink to the ocean floor. Some 6 million to 10 million gallons are buried in the sediment on the floor of the Gulf, about 62 miles southeast of the Mississippi Delta.

In total, more than 200 million gallons of crude oil spilled into the Gulf of Mexico in April 2010 when the Deepwater Horizon oil rig exploded. The accident killed 11 people and caused extensive damage to marine and wildlife habitats in the region.

The U.S. government and BP cleanup crews were unable to locate all of the spilled oil. But now, almost five years later, natural abundance radiocarbon measurements on surface sediment organic matter have shown where oil is distributed on the ocean floor.

Chanton noted that in the short term, the oil sinking to the sea floor might have seemed like a good thing because the water was clarified, and the oil was removed from the water. But it's a problem in the longer term.

Less oxygen exists on the sea floor relative to the water column, so the oiled particles are more likely to become hypoxic. That means it's more difficult for bacteria to attack the oil and cause it to decompose.

If the sediment is stirred up in the future, the oil could enter the water again. And the contamination will also enter the food chain, because worms ingest the sediment, and fish eat the worms.

The findings of the study have been published in the latest edition of the journal Environmental Science & Technology.