WASHINGTON — Experts said Thursday they have mapped a 35-kilometer (22 mile) long underwater plume of oil that spewed from BP's ruptured Gulf of Mexico well, seeming to challenge US government assertions that most of the oil has disappeared.
The oily underwater cloud measured two kilometers wide and 200 meters (650 feet) thick and was drifting through the Gulf at a depth of at least 900 meters, according to the paper by Woods Hole Oceanographic Institution (WHOI) marine biologists, published in the journal Science.
The plume was seen as not dissipating as rapidly as experts had expected, despite widespread use of dispersants which the government has insisted have been vital to the breakdown of vast amounts of oil.
The observations were made in late June, several weeks before the ruptured wellhead was capped, and about two months after an explosion sank the BP-leased Deepwater Horizon rig, triggering the largest ever maritime oil spill.
Challenging US government estimates based on natural processes rapidly dissipating the toxic crude, the authors said deep-sea microbes were degrading the plume only slowly and predicted the oil would endure for some time.
"We've shown conclusively not only that a plume exists, but also defined its origin and near-field structure," said lead author Richard Camilli.
The oil already "is persisting for longer periods than we would have expected," he added.
"Many people speculated that the sub-surface oil droplets were being easily downgraded. Well, we didn't find that. We found that it was still there."
US and BP officials earlier this month proclaimed that about three-quarters of the oil which gushed into the Gulf had been cleaned up or dispersed through natural processes.
Around 4.9 million barrels of oil are believed to have spewed from the fractured wellhead before it was capped last month. US officials say that of that amount, 800,000 barrels were contained and funneled up to ships on the surface.
The leak not only threatened livelihoods of fishermen and tourism businesses along the US Gulf coast, but also stoked fears of long-term ecological damage.
On August 4, the US National Oceanic and Atmospheric Administration (NOAA) said the "vast majority" of oil had been evaporated, removed by cleanup teams or was dispersing naturally.
The remaining 26 percent -- or about 1.3 million barrels of oil -- was classified as "residual oil" and "is either on or just below the surface as residue and weathered tar balls, has washed ashore or been collected from the shore, or is buried in sand and sediments," the report said.
The Woods Hole team used a robotic submarine equipped with an underwater mass spectrometer to detect and analyze the plume, making repeated horizontal sweeps to ascertain its size and chemical composition.
They followed the "neutrally buoyant" cloud as it migrated slowly, at 0.27 kilometers per hour, southwest of the leaking well.
The plume was then tracked for a distance of about 35 kilometers before the approach of Hurricane Alex forced the scientists to turn back.
The spectrometer found petroleum hydrocarbons at concentrations of more than 50 micrograms per liter, a level that meant the samples had no smell or oil and were clear. The impacts on biodiversity remain uncertain, though.
"The plume was not a river of Hershey's Syrup," said Christopher Reddy, a marine biochemist. "But that's not to say it isn't harmful for the environment."
The damaged well was capped on July 15. Earlier this month BP engineers plugged the site with heavy drilling fluid and then sealed it with cement.
The company aims to permanently seal the well in the second week of September, a US official said on Thursday.
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