Gulf Oil Plume Deep Under Surface

Friday, August 20, 2010 @ 03:08 PM gHale


While the debate rages over the environmental damage to the Gulf of Mexico and what has happened to all the oil, there now seems to be an answer.
There is a plume of hydrocarbons at least 22 miles long and more than 3,000 feet below the surface of the Gulf of Mexico, which scientists say is a residue of the BP Deepwater Horizon oil spill.
As a part of a study, researchers from the Woods Hole Oceanographic Institution (WHOI) measured distinguishing petroleum hydrocarbons in the plume and, using them as an investigative tool, determined the source of the plume had to come from the blown out well. The ruled out any kind of natural oil seeps.
Deep-sea microbes were degrading the plume relatively slowly, but it seems the 1.2-mile-wide, 650-foot-high plume had and will persist for some time, the researchers said.
This all comes as a result of 57,000 discrete chemical analyses measured in real time during a June 19-28 scientific cruise aboard the R/V Endeavor, owned by the National Science Foundation (NSF) and operated by the University of Rhode Island. Researchers used 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 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. “Until now, these have been treated as a theoretical matter in the literature.
“In June, we observed the plume migrating slowly [at about 0.17 miles per hour] southwest of the source of the blowout,” Camilli said. 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 study confirms a continuous plume exists “at petroleum hydrocarbon levels that are noteworthy and detectable,” said Christopher Reddy, a WHOI marine geochemist and oil spill expert and one of the authors of study on the subject. The levels and distributions of the petroleum hydrocarbons show “the plume is not caused by natural [oil] seeps” in the Gulf of Mexico, Camilli said.
The plume has shown 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 said. “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. 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 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 crisscross plume boundaries continuously 19 times to help determine the trapped plume’s size, shape, and composition. 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, also had a TETHYS. In each case, the mass spectrometers positively identified areas containing petroleum hydrocarbons.
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. “The plume was not a river of Hershey’s Syrup,” said Reddy. “But that’s not to say it isn’t harmful to the environment.”
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 problem with the 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 were below what they expected, and even those lower levels were not that low.
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.”
Reddy said the WHOI team members know the chemical makeup of some of the plume, but not all of it. Gas chromatographic analysis 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.



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