In the plume. Inside the plume half a kilometer from the well (right), oil droplets glow in a submersible's lights but not outside the plume (left).

R. Camilli et al., Science

Report Paints New Picture of Gulf Oil

As millions of barrels of oil gushed into the Gulf of Mexico this summer from a blown-out BP well, the media painted a portrait of dark, massive plumes billowing into the sea. Meanwhile, a recent U.S. government report has stated that microbes are breaking down the oil quickly. Neither picture is correct, at least in the case of a plume described in the first peer-reviewed publication of oil-spill observations.

Oceanographers from the Woods Hole Oceanographic Institution (WHOI) in Massachusetts surveyed the gulf around the BP well from the research ship Endeavor from 19 to 28 June, a period of heavy flow. Led by oceanographer Richard Camilli, the team deployed an array of instruments on both a cable-lowered water sampler and an autonomous underwater vehicle. All told, the instrumentation made more than 57,000 separate chemical analyses of a plume southwest of the well.

The first thing that the researchers noticed was that the plume wasn't quite as “massive” as many news reports had made out. The plume surveyed by Endeavor was only 200 meters thick and about 2 kilometers wide. Although plenty of oil was flowing from the ruptured well, it didn't look much like an underwater oil slick. The team's camera picked up a yellowish fog half a kilometer from the well (see figure), and water samples farther from the well did not look or smell like oil. “The plume was not a river of Hershey’s syrup,” says marine geochemist Christopher Reddy of the WHOI group.

The plume did, however, contain more than 50 micrograms per liter (about 0.05 parts per million) of a group of particularly toxic petroleum compounds that includes benzene, the team reports online today in Science. That amount of benzene-related petroleum compounds is roughly consistent with the 1 to 2 parts per million of total oil reported in plumes by some other researchers.

On the microbe front, the WHOI team also found differences. A report released last week by a group of federal agencies led by the National Oceanic and Atmospheric Administration stated—without documentation—that early signs show the oil is “biodegrading quickly.” Not so in the southwest plume in late June, the WHOI researchers found. Their measurements of oxygen dissolved in seawater, which bacteria consume as they feed, showed that microbes had not appreciably degraded the oil during its first 5 days out of the well.

Despite the new findings, oceanographers don't yet have a complete picture of subsurface oil. The mass of oil in the southwest plume surveyed in late June “doesn’t hold a candle to the plume we saw” to the southwest in May, says biogeochemist Samantha Joye of the University of Georgia, Athens. And then there’s the plume to the northeast, toward the Florida panhandle. In a close-in survey, the WHOI group found it to be the lesser of the two plumes. But Joye says that at other times researchers have found the northeast plume to be five times as massive. And this week, researchers from the University of South Florida in St. Petersburg led by chemical oceanographer David Hollander announced the first observation of oil droplets from a plume settling to the bottom of the gulf. Apparently, the northeast plume was massive enough to lay down a carpet of oil droplets off of West Florida.

Painting a realistic picture of the oil’s ultimate fate will take a while longer, researchers say. “Maybe we’re a little impatient,” says Reddy. “We’re just in round one.” Environmental engineer Nancy Kinner of the University of New Hampshire, Durham, agrees. “We have snapshots of data from different time periods” and different places, she says. “We have yet to get all the data together in one place where we can look at it as a time series to understand what’s going on.”

For more on the gulf oil spill, see our full coverage.