Subsea Oil Plume 22 Miles Long Found in Gulf of Mexico

FALMOUTH, Massachusetts, August 20, 2010 (ENS) – Scientists from the Woods Hole Oceanographic Institution have detected a plume of hydrocarbons that is at least 22 miles long and more than 3,000 feet below the surface of the Gulf of Mexico, a residue of the BP Deepwater Horizon oil spill.

The 1.2-mile-wide, 650-foot-high plume of trapped hydrocarbons provides at least a partial answer to recent questions asking where all the oil has gone as surface slicks shrink and disappear.

WHOI researchers Rich Camilli, left, and Christopher Reddy at work (Photo by Cameron McIntyre courtesy WHOI)

“These results indicate that efforts to book keep where the oil went must now include this plume” in the Gulf, said Christopher Reddy, a WHOI marine geochemist and oil spill expert and one of the authors of the study, which appears in the August 19 issue of the journal “Science.”

The researchers measured distinguishing petroleum hydrocarbons in the plume and, using them as an investigative tool, determined that the source of the plume could not have been natural oil seeps but had to have come from the blown out well.

Moreover, they reported that deep-sea microbes were degrading the plume relatively slowly, and that it was possible that the plume had and will persist for some time.

The WHOI team based its findings on some 57,000 separate chemical analyses measured in real time during a June 19-28 scientific cruise aboard the R/V Endeavor, which is owned by the National Science Foundation and operated by the University of Rhode Island.

They accomplished their research using two advanced technologies: the autonomous underwater vehicle Sentry that can explore the ocean down to a depth of 14,760 feet; and an underwater mass spectrometer known as TETHYS, small enough to fit in a shoebox, that can instantly identify minute quantities of petroleum and other chemical compounds in seawater.

“We’ve shown conclusively not only that a plume exists, but also defined its origin and near-field structure,” said Richard Camilli of WHOI’s Applied Ocean Physics and Engineering Department, chief scientist of the cruise and lead author of the paper.

“In June, we observed the plume migrating slowly [at about 0.17 miles per hour] southwest of the source of the blowout,” said Camilli.

A cable-lowered sampling system was used to collect samples for lab analysis of the plume. (Photo by Dan Torres courtesy WHOI)

The researchers began tracking it about three miles from the wellhead and out to about 22 miles (35 kilometers) until the approach of Hurricane Alex forced them away from the study area.

The study confirms that a continuous plume exists “at petroleum hydrocarbon levels that are noteworthy and detectable,” Reddy said.

The levels and distributions of the petroleum hydrocarbons show that “the plume is not caused by natural seeps” in the Gulf of Mexico, Camilli added.

WHOI President and Director Susan Avery praised the WHOI scientists for their “prudence and thoroughness, as they conducted an important, elegant study under difficult conditions in a timely manner.”

“Very good science was done that will make a big difference,” Avery said. “This cruise represents an excellent example of how non-federal research organizations can work with federal agencies and how federal agencies can work together to respond to national disasters.”

“These findings confirm what NOAA and our federal partners have reported about the presence and concentration of subsurface oil, and provide an additional piece of the puzzle as we continue to aggressively monitor the fate of the oil in the Gulf,” said Steve Murawski, NOAA’s chief scientist.

“This research illustrates the value of NSF’s long-term investment in state-of-the-art technology like Sentry so that it can be deployed not only to advance basic knowledge but also in national emergencies,” said David Conover, director of NSF’s Division of Ocean Sciences. The research was funded with an NSF RAPID grant and addition funding from the U.S. Coast Guard.

The plume has shown that the oil already “is persisting for longer periods than we would have expected,” Camilli said. “Many people speculated that subsurface oil droplets were being easily biodegraded. Well, we didn’t find that. We found it was still there.”

Whether the plume’s existence poses a significant threat to the Gulf is not yet clear, the researchers say. “We don’t know how toxic it is,” said Reddy, “and we don’t know how it formed, or why. But knowing the size, shape, depth, and heading of this plume will be vital for answering many of these questions.”

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