Gulf Oil Spill Plume Stretches 21 Miles, Not Breaking Down Much

A plume of oil some 700 feet thick and at least 21 miles long has been detected deep beneath the Gulf of Mexico. It originated at the Deepwater Horizon blowout and consists of hydrocarbons from the well, according to measurements released Thursday.

The survey, conducted by US and Australian scientists during a 10-day research cruise in late June, represents the most detailed picture yet of undersea plumes of oil and methane from the Gulf oil spill. The researchers were surprised by the plume’s relative stability as well as by an apparent lack of activity on the part of microbes to break down the oil.

The continuous plume appeared some 3,000 feet below the surface, according to data published Thursday afternoon in the journal Science’s online outlet, Science Express.

“This is the first chapter, and an important chapter in understanding and recognizing the size, shape, distribution, and behavior of this rather surprising phenomenon of a plume,” said Christopher Reddy, a lead investigator on the project, at a briefing Thursday. Dr. Reddy is a marine chemist at the Woods Hole Oceanographic Institution in Massachusetts.

The Deepwater Horizon well has been capped since July 15, and BP is close to finishing a relief well to help seal the blowout for good. But the timing for the final plugging of the well hasn’t been determined yet, said retired Coast Guard Adm. Thad Allen on Wednesday.

The research team investigating the plume has yet to perform detailed chemical tests on the hydrocarbons it found there. As a result, “we can’t say anything about its toxicity or bioactivity,” says Richard Camilli, the lead author for the study.

But the team, he says, was intrigued by how stable the plume appeared at that depth.

“That’s something that was a little bit unexpected,” he said. “We don’t have any clear indication as to why it set up at that depth.”

The team says its data suggest that the plume extends much farther than the 21 miles it measured, but the research cruise was cut short by bad conditions kicked up by hurricane Alex. In places, the plume was more than a mile wide, and it apparently was drifting with deep-sea currents to the southwest of the Deepwater Horizon well at a pace of about four miles a day.

Undersea plumes of oil and methane from the blowout continue to raise concerns among many marine scientists. The plumes, they say, threaten a range of marine organisms directly through a plume’s toxicity and indirectly as bacteria consume the oil and methane, drawing down oxygen levels at those depths.

Earlier this week, scientists at the University of South Florida returned from a cruise with preliminary evidence indicating that oil droplets were moving up the undersea DeSoto Canyon and onto the continental shelf off Florida’s west coast. The toxic effects of oil and dispersants appeared in collected samples of bacteria and phytoplankton, the scientists said. The at-sea analyses need to be backed up with more-rigorous lab work, the team acknowledged.

Scientists measuring oxygen in other hydrocarbon plumes found that oxygen levels were much lower within a plume than outside it, indicating that microbes were hard at work. But subsequent analysis suggested that the sensors may have been thrown off by the presence of oil, triggering the lower oxygen readings.

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On the 21-mile stretch of plume, scientists generally found very little change in oxygen levels during the period they took their measurements, Reddy says, suggesting moribund microbes.

The team used three independent approaches to take oxygen readings, each providing a reality check on the others. Indeed, the way the team made its oxygen measurements may be “one of the most substantial contributions of this paper” as researchers plan future measurements, writes Vernon Asper in an e-mail. Dr. Asper is marine scientist at the University of Southern Mississippi and one of a small army of researchers tracking the blowout and its effects.

The work being reported Thursday is expected to provide the kind of information that will help refine estimates of the oil’s fate in the environment. That has become the subject of some debate with dueling analyses from the unified command and from a team of scientists affiliated with the Georgia Sea Grant program.

The debate broke into the open this week when the Sea Grant team released a short report estimating that as much as 79 percent of the oil that the Deepwater Horizon blowout ejected into the Gulf remains in the environment. By contrast, the unified command’s report suggested that no more than half the oil is still in the environment. Some administration officials went further to say that only about 25 percent remained.

Thursday’s study in Science Express represents “a pretty clear documentation that there is, in fact, a large amount of oil compounds and methane, especially in that deep plume,” says Robert Hallberg, an oceanographer who focuses on modeling marine processes at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

Moreover, “what they’re showing with their very careful measurements is that the oxygen drawdown is something that will be happening very slowly,” he says. “It may take many months or longer before the oxygen drawdown reaches levels that are significant.”

The results broadly mirror those of modeling studies that Dr. Hallberg and colleagues are publishing in an upcoming issue of Geophysical Research Letters. The project couples a simple model of how bacteria degrade oil with a sophisticated ocean-circulation model.

The effort wasn’t designed to predict where a plume will be at a given time, he cautions. And it doesn’t deal with floating oil or tar balls. Instead, it tries to give a general picture of what could happen to the oil and how much of a given quantity might be available in some form for marine life to encounter.

One of the model’s results: Peak oxygen depletion in deep plumes may not appear for several months after oil stopped flowing into the Gulf. That oxygen depletion will be considerable locally, the results suggest. But depletion, as well as any toxic effects from the oil and dispersants, will be limited to the northern Gulf region.