Millions of gallons of oil released following the Deepwater Horizon explosion on April 20, 2010 have settled at the bottom of the Gulf of Mexico, covering at least 1,250 square miles — an area nearly the size of Rhode Island. That is the conclusion of a new study led by University of California researcher David Valentine and published Monday in the Proceedings of the National Academy of Sciences.
As a result of the explosion and subsequent leak, nearly five million barrels of crude oil gushed into the sea. About one million barrels were recovered, and another two million floated to the sea surface, says Valentine. That left another two million barrels, or about 84 million gallons, which was believed to be trapped in the Gulf's deep water currents, the oil breaking into discrete particles too small to float to the surface.
"This is a phenomenon that had been theorized before, but this really came to the forefront during this event," Valentine told VICE News.
Those tiny bits of oil, through a process that's still unknown, clumped together into bits that were more dense than water and fell to the seafloor, Valentine said, accumulating in a pattern that forms a giant "bathtub ring" on the ocean floor.
Using federal data from the Natural Resource Damage Assessment Process, Valentine and his colleagues mapped out concentrations of hopane, a chemical component of crude oil that does not degrade over time, in sediment cores taken around the well. They found that the top half-inch of sediment contained hopane concentrations up to thousands of times higher than concentrations found from long-term natural build up, enough to account for up to 31 percent of the two million missing barrels.
More of it, Valentine said, is likely spread out in a similar pattern, with decreasing concentrations, beyond the area his team investigated.
"It's undoubtedly there," Valentine told VICE News. "There's no invisible wall that separates the area that we studied from the area outside it."
Most of the oil concentrated at depths of about 4,000 to 5,000 feet, or just under a mile below the sea surface. At those depths, the water is full of "ocean snow," small particles that drift downward and provide food for deep-sea creatures, said Erik Cordes, a Temple University biologist who has studied corals in the wake of the Deepwater Horizon disaster. The Gulf of Mexico contains hundreds of miles of coral reefs, including the most extensive living reef in the United States.
'It all basically points right back to the Deepwater Horizon as the source.'
The scattered, rainfall-like pattern identified by Valentine and his team is consistent with the damage previously seen to corals in the Gulf, Cordes said. Some corals were hit by only a small amount of oil and are largely recovering, while others were covered in the stuff.
"The ones that were really heavily impacted, their health has continued to decline," Cordes told VICE News. "Branches are falling off. They're really not doing well."
Coral is a "foundation species" in the Gulf, Cordes said, providing shelter for small fish and sea stars that then provide a basis for the larger ecosystem. Some bottom-dwelling creatures, like red crabs, have been found with oil on and inside of their bodies, a danger to other species that feed on them.
"That'll just work its way through the food chain for sure," Cordes told VICE News.
BP has disputed the results of the study, saying the researchers failed to undertake a chemical analysis that proved the oil found in the sediment came from the Macondo Well. In an email to the Associated Press, BP spokesperson Jason Ryan said the authors' methods led them to "grossly overstate" the level of oil from the well.
Valentine said such a chemical analyses isn't necessary to draw a link to the BP spill. The location of the concentrations, the significantly higher concentrations in only the top layer of the sediment, and other factors make it clear the oil came from the spill, he said.
"It's really a very clear signal," Valentine told VICE News. "We go through the arguments in the paper. It all basically points right back to the Deepwater Horizon as the source."
Follow Laura Dattaro on Twitter: @ldattaro