Gulf Oil Plume Map Adds to Debate Over Spill’s Undersea Impact

Months after the Deepwater Horizon oil leak began, a 22-mile-long, 1.2-mile-wide and 650-foot-high plume of microscopic oil compounds floated 3,000 feet below the surface of the Gulf of Mexico near the well, according to a study published Thursday in the journal Science.

The research is the most precise mapping yet of an oil plume, and confirms what many scientists have suspected for months — that much of the oil that leaked from the Macondo well stayed below the surface of the ocean rather than floating to the top. That behavior has been one of the most singular and unexpected aspects of the spill, because oil is lighter than water and generally floats on the surface.

But in the case of the Deepwater Horizon leak, several factors — including the fact that the leak took place so far below the surface and the fact that BP sprayed 1.8 million gallon of dispersants at its source — have meant that some microscopic oil droplets remained suspended in the water instead. That has complicated scientists’ efforts to understand how much oil leaked from the well, where it ended up, and how it will affect the local environment.

The research is the most precise mapping yet of an oil plume, and confirms what many scientists have suspected for months — that much of the oil that leaked from the Macondo well stayed below the surface of the ocean rather than floating to the top.

“This evidence shows conclusively that a plume existed at this depth,” Richard Camilli, a researcher at the Woods Hole Oceanographic Institute and lead author of the paper, told reporters. “Furthermore the chemistry that we analyzed shows fairly clearly that this plume was created by the Macondo well site […] not natural seeps”

The plume the researchers mapped was made up of microscopic hydrocarbons — molecules of oil — that are invisible to the naked eye: A scuba diver wouldn’t see it.

“You would be hard-pressed to tell right in the middle of the plume whether it was spring water or plume water,” says Christopher Reddy, a marine geochemist at Woods Hole and a study author. “It did not smell of oil, it had no visible oil droplets […] readers might think that there’s a Hershey’s syrup of oil. There are certainly hydrocarbons in there, but not at the level of a ‘brown wall of death’ or anything like that.”

Instead, the researchers could identify the oil plume using high-tech underwater devices equipped with sensors to sample the water, getting detailed chemical analyses at thousands of sample sites. They found traces of the oil compounds such as benzene, toluene and ethybenzene at levels more than double what would be expected from natural oil seeps in the area.

The researchers took their samples between June 19 and June 28, so their study shows a snapshot of the plume as it existed nearly two months ago. Reddy says that it’s impossible to know for certain how it would have changed since then. But given that it was migrating southwest at about 6.5 kilometers per day, and that the “faucet was shut off” when BP capped the well in July, it would no longer be in the same place.

It is also possible that the plume has been degraded over time as microbes in the water break down the oil compounds. But the researchers’ work suggests that that breakdown is happening relatively slowly. When microbes break down oil they use oxygen, leading to oxygen depletion in the surrounding water. But the researchers’ sampling didn’t find much evidence of that depletion, suggesting that the oil breakdown is happening relatively slowly and the plume could travel far from the well.

The study comes during continued debate over the fate of the oil from the well. Earlier this month, a report from the National Oceanic and Atmospheric Association said that almost 75 percent of oil from the well had been captured, burned, naturally biodegraded or dispersed in the water, and that the dispersed oil was biodegrading quickly.

But this week, two reports from outside scientists have taken issue with the government’s conclusions. On Tuesday, a report by researchers at the University of Georgia estimated that 79 percent of the oil from the spill remains unaccounted for, and is biodegrading more slowly than the government’s estimates suggest.

And also on Tuesday, scientists at the University of South Florida released preliminary findings of oil droplets sitting on the sediments on the sea floor at the DeSoto Canyon, an underwater canyon farther east of the wellhead. The researchers also found that the oil droplets were toxic to the phytoplankton that make up the base of the area’s food chain.

David Hollander, one of the researchers who worked on the University of South Florida study, says that the oil plume research published Thursday in Science fits well with what he’s observed — particularly the fact that the Woods Hole scientists found evidence that the oil in the plume was not biodegrading quickly.

“The deep sea hydrocarbons are degrading at a very low rate, and consequently will have a persistence in the environment for years to decades,” he says.

But Woods Hole researcher Reddy says that it will take more research before scientists can draw any firm conclusions about the fate of the oil. Thursday’s report mapped the shape of a plume, but, he says, more analysis needs to be done on the concentrations and distribution of particular oil compounds. Because the study only looks at some oil compounds, not all of them, the researchers can’t say definitively the amount of oil in the plume.

“Our objective was to understand how the petroleum hydrocarbons behaved when they were released,” Reddy says. “With time we’ll be able to fill in how much was in plume. Science is incremental, a lot of samples are being analyzed, and with time they’ll be able to help fill in these questions of where the oil is and where did it all go.”

Watch Thursday’s NewsHour for more on oil plume study.