The Sentry autonomous underwater vehicle (AUV) aboard the research vessel Endeavor at the Deepwater Horizon oil spill site.
The Sentry autonomous underwater vehicle (AUV) aboard the research vessel Endeavor at the Deepwater Horizon oil spill site.

A new report finds that the oil in the Gulf of Mexico still persists in the form of a massive plume that, according to scientists is not going away anytime soon.

More than a month after BP capped its broken oil well in the Gulf of Mexico, experts have differed on how much of the more than four million barrels of spilled crude remains in Gulf waters and where that oil is going.

The U.S. government agencies involved in the cleanup estimated recently that roughly 75 percent of the oil from the leaking well had been skimmed, burned, evaporated or naturally broken down.

Massive plume

The Woods Hole Oceanographic Institution report, released on-line Thursday in the journal Science, describes the mammoth plume. 

Over a 10-day period in late June - two weeks before the damaged seafloor wellhead was successfully capped the researchers measured a massive plume near the wellhead site.

The oil plume was approximately 1,000 meters deep, 35 kilometers long, 200 meters high and 2 kilometers wide two months after the well explosion.  

The team deployed an autonomous underwater vehicle, or AUV, and a sampling device tethered by cable to their ship on the surface.  Researcher Chris Reddy says what they saw was a forensic snapshot showing that hydrocarbons can move into deep marine ecosystems.

Sampling device being lowered from the Endeavor re
Sampling device being lowered from the Endeavor research vessel.

"Our goal was to document a plume and see what its size, shape, distribution and eventually its chemical composition. This is an important aspect because there is very little known about oil in the sub-surface."

The AUV zigzagged its way through the oil plume to measure chemicals and biological activity at various depths.

The chemical analysis reported so far showed no substantial decay in the plume, which suggests that petroleum-eating microbes were not significantly breaking down the oil. Researcher Benjamin Van Mooy says that is in part because of water temperature.

"If everything else were equal same bugs, same oil, same nutrients, same oxygen. We would expect that the bacteria in the deep waters would be 10 times slower, just because it's cooler."    

Going nowhere fast

Florida State University oceanographer Ian MacDonald has studied the natural breakdown of spilled crude oil.  He says that isn't happening as fast as had been hoped in the Gulf.

In a hearing before Congress Thursday, he disputed government claims that the oil in all its forms in the ocean is biodegrading quickly.

Chief Scientist Rich Camilli uses underwater mass
Chief Scientist Rich Camilli uses underwater mass spectrometers which are capable of identifying minute quantities of petroleum and other chemical compounds in seawater instantly.

"This oil has already degraded, evaporated and emulsified," said MacDonald. "It is going to be very resistant to further biodegradation. This oil is going to be in the environment for a long time."

Oceanographer MacDonald's testimony is in line with a report released this week by University of Georgia researchers finds that up to 79 percent of the oil that spewed from the broken well remains in the Gulf.

Scientists from Woods Hole hope that their data will help compile a more accurate picture of the deep ocean. They call for a sustained and coordinated research effort to better understand the impact of the spill and how best to respond to world's worst-ever maritime oil disaster.