Scientists Map Origin of Large, Underwater Hydrocarbon Plume in Gulf

Plume detected 22 miles long and more than 3,000 feet below surface

August 19, 2010

Sentry criss-crossed the plume in deep Gulf waters 19 times to help determine its size and shape.

Sentry criss-crossed the plume in deep Gulf waters 19 times to help determine its size and shape.

High resolution (Credit: WHOI/Dan Torres)

Scientists funded by the National Science Foundation (NSF) and affiliated with the Woods Hole Oceanographic Institution (WHOI) have detected a plume of hydrocarbons 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 was detected during a ten day subsurface sampling effort from June 19-28, 2010 near the wellhead. The results provide a snapshot of where the oil has gone as surface slicks shrink and disappear.

"These results create a clearer picture of where the oil is in the Gulf," said Christopher Reddy, a WHOI marine geochemist and one of the authors of a paper on the results that appears in this week's issue of the journal Science.

The study--which was enabled by three rapid response grants from NSF's chemical oceanography program, with additional funding from the U.S. Coast Guard and NOAA through the Natural Resource Damage Assessment Program--confirms once again that a continuous plume was found "at petroleum hydrocarbon levels that are noteworthy and detectable," Reddy said.

The researchers measured 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 Deepwater Horizon blowout at the Macondo well.

They reported that deep-sea microbes were degrading the plume relatively slowly, and that it was possible that the plume had and could persist for some time if the rate of microbial degradation or the dilution of the plume does not accelerate.

"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. "Our collaborations with Woods Hole and other academic and private research institutions are critical to the ongoing response and recovery efforts."

"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.

"Similarly, the NSF RAPID award program enables scientists to quickly arrive on the scene and begin rigorous study of episodic events like this oil spill."

NSF has so far issued a total of 90 RAPID grant awards to investigators; the grants to date are worth $10.2 million for study of the spill. NSF has invested an additional $3 million in ship-related operating costs.

"The payoff occurs when peer-reviewed results like these reported today are made public," said Conover.

The vessel Endeavor, operated by URI, embarking on a 12-day research expedition.

The vessel Endeavor, operated by URI, embarking on a 12-day research expedition.

High resolution (Credit: University of Rhode Island)

The research team based its findings on some 57,000 discrete chemical analyses measured in real time during a June 19-28, 2010, scientific cruise aboard the R/V Endeavor, which is owned by NSF and operated by the University of Rhode Island.

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

The scientists accomplished the feat using two advanced technologies: the autonomous underwater vehicle (AUV) Sentry and a type of underwater mass spectrometer known as TETHYS (Tethered Yearlong Spectrometer).

"We've shown conclusively not only that a plume developed, 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.

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

The levels and distributions of the petroleum hydrocarbons show that "the plume is not caused by natural [oil] seeps" in the Gulf of Mexico, Camilli said.

The plume has shown that the oil "was persisting for longer periods than we would have expected," Camilli said.

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."

The key to the discovery and mapping of the plume was the use of the mass spectrometer TETHYS integrated into the Sentry AUV.

Camilli developed the mass spectrometer in close industrial partnership with Monitor Instruments Co. in Cheswick, Pa., through a grant from the National Ocean Partnership Program.

The TETHYS--which is small enough to fit within a shoebox--is capable of identifying minute quantities of petroleum and other chemical compounds in seawater instantly.

Sentry, funded by NSF and developed and operated by WHOI, is capable of exploring the ocean down to 14,764 feet (4,500 meters) depth.

Equipped with its advanced analytical systems, it was able to criss-cross plume boundaries continuously 19 times to help determine the trapped plume's size, shape, and composition.

A cable-lowered sampling system was used to collect samples for lab analysis of the plume.

A cable-lowered sampling system was used to collect samples for lab analysis of the plume.

High resolution (Credit: WHOI/Dan Torres)

This knowledge of the plume structure guided the team in collecting physical samples for further laboratory analyses using a traditional oceanographic tool, a cable-lowered water sampling system that measures conductivity, temperature, and depth (CTD).

This CTD, however, was instrumented with a TETHYS. In each case, the mass spectrometer was used to positively identify areas containing petroleum hydrocarbons.

"We achieved our results because we had a unique combination of scientific and technological skills," said Dana Yoerger, a co-principal investigator and WHOI senior scientist.

In previous research, Yoerger said, "investigators relied mostly on a conventional technique: vertical profiling.

"We used Sentry and TETHYS to scan large areas horizontally, which enabled us to target our vertical profiles more effectively.  Our methods provide much better information about the size and shape of the plume."

The researchers detected a class of petroleum hydrocarbons at concentrations of more than 50 micrograms per liter.

The water samples collected at these depths had no odor of oil and were clear. "But that's not to say it isn't harmful to the environment," said Reddy.

The scientists benefited not only from new technology but older methods as well.

Contrary to previous predictions by other scientists, they found no "dead zones," regions of significant oxygen depletion within the plume where almost no fish or other marine animals could survive.

They attributed the discrepancy to a potential problem with more modern measuring devices that can give artificially low oxygen readings when coated by oil.

The team on Endeavor used an established chemical test developed in the 1880s to check the concentration of dissolved oxygen in water samples, called a Winkler titration.

Of the dozens of samples analyzed for oxygen only a few from the plume layer were below expected levels, and even these samples were only slightly depleted.

WHOI geochemist Benjamin Van Mooy, also a principal investigator of the research team, said this finding could have significant implications.

"If the oxygen data from the plume layer are telling us it isn't being rapidly consumed by microbes near the well," he said, "the hydrocarbons could persist for some time. So it is possible that oil could be transported considerable distances from the well before being degraded."

The NSF RAPID program, which provides grants for projects having a severe urgency and requiring quick-response research on natural disasters or other unanticipated events, significantly speeded up the acceptance of the WHOI scientific proposals.

Scientists Rich Camilli and Chris Reddy aboard the Endeavor in the Gulf of Mexico.

Scientists Rich Camilli and Chris Reddy aboard the Endeavor in the Gulf of Mexico.

High resolution (Credit: WHOI/Cameron McIntyre)

"In contrast to the usual six-to-eighteen-month lead time for standard proposals, our plume study was funded two days after the concept was proposed to NSF, and went from notification of the proposal's acceptance to boarding the Endeavor in two-and-a-half weeks," Reddy said.

"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."

Reddy said the results from this study and more samples yet to be analyzed eventually could refine recent estimates about the amount of the spilled oil that remains in the Gulf.

Camilli said he and his WHOI colleagues are considering a new research proposal to look for more plumes.

Reddy said the WHOI team members know the chemical makeup of some of the plume, but not all of it.

Gas chromatographic analyses of plume samples confirm the existence of benzene, toluene, ethybenzene, and total xylenes, together called BTEX, at concentrations in excess of 50 micrograms per liter.

"The plume is not pure oil," Camilli said. "But there are oil compounds in there."

It may be "a few months of laboratory analysis and validation," Reddy said, before they know the entire inventory of chemicals in the plume.

Other WHOI members of study team included Assistant Scientist James C. Kinsey and Research Associates Cameron P. McIntyre and Sean P. Sylva.  The research team also included Michael V. Jakuba of the University of Sydney, Australia, and a graduate of the MIT/WHOI joint program in Oceanographic Engineering, and James V. Maloney of Monitor Instruments Co.