By Janet Raloff, Science News
Two new analyses report that huge plumes of oil generated by the BP spill continue to roam deep within the Gulf of Mexico and appear disturbingly stable. Although some natural breakdown of hydrocarbons in the oil is underway, both new analyses report evidence that this biodegradation is very slow.
These findings contradict an Aug. 2 report by the National Incident Command, a largely federal group that has been coordinating management and cleanup of the BP Deepwater Horizon blowout. An estimated 4.9 million barrels of oil (almost 206 million gallons) escaped from the damaged well before it was successfully capped on July 15. Some 17 percent was captured before it fouled the water, the NIC reports.
But in testimony August 19 before the House Subcommittee on Energy and Environment, Ian MacDonald of Florida State University in Tallahassee cited data from the National Oceanic and Atmospheric Administration that he said showed only 10 percent of the spewed BP oil “was actually removed from the ocean” and that only “a fraction, perhaps 10 percent, will have evaporated.” The remaining oil still fouls the Gulf, he said.
New analyses by the Woods Hole Oceanographic Institution in Massachusetts and the Georgia Sea Grant at the University of Georgia in Athens also suggest that much of the BP oil has not been behaving as scientists would have expected.
For instance, in a paper online in Science August 19 the Woods Hole team describes data from a June cruise that mapped a huge plume of diffuse hydrocarbons more than 35 kilometers long. As this cloud of oil flowed roughly 1,100 meters below the surface, it maintained a configuration that was roughly 200 meters high, up to 2 kilometers wide and traveling at about 6.5 kilometers per day. Over the entire span of the plume studied, the cloud’s height “only varied by tens of meters,” notes Woods Hole team leader Richard Camilli.
Researchers cruising the Gulf in early June aboard a NOAA ship, the Gordon Gunter, also found evidence of that hydrocarbon plume. Like the Woods Hole team, these scientists collected much of their data using an autonomous underwater vehicle, in this case a torpedo-like chemical sensing laboratory developed by the Monterey Bay Aquarium Research Institute.
“We were in the same area as the Woods Hole group,” notes John Ryan of MBARI, and similarly found a hydrocarbon plume below 1,000 meters.
“We’re not sure why this plume set up at this depth,” says Camilli, “but it appears to have persisted for at least several weeks or months. And it appears very stable, but we really don’t know why yet.”
“I’m not shocked, but actually pleasantly surprised [by these data],” says Roberto Camassa, who directs the Center for Interdisciplinary Applied Mathematics at the University of North Carolina at Chapel Hill. In the laboratory, this fluid dynamicist and his colleagues have been studying oil plume formation. He says the buildup of stable deep plumes make sense, based on the evolving science of interactions between high-velocity oil and cold, slow-moving waters.
“In our lab experiments, things mainly get trapped based on their density,” Camassa observes. “So I would expect to find a somewhat sharp transition in density down there, and with such stratification the oil could persist for a long, long time.”
Oil in the plume hasn’t ascended to the surface, he explains, because if droplets are small enough they become neutrally buoyant and move with the water. Camassa’s lab studies suggest that the high-velocity spray of oil from the BP blowout would essentially have atomized the crude oil into microdroplets.
New modeling analyses of the BP oil spill by researchers at NOAA’s Geophysical Fluid Dynamics Laboratory at Princeton University also largely predict what the WHOI team has just reported, notes Robert Hallberg of NOAA.
His group’s findings — due to be published soon in Geophysical Research Letters — forecast not only that much of the oil spewed at great depth will break up into small particles that quickly become neutrally buoyant, but also that the breakdown of oil by microbes will proceed very slowly. The bugs will eventually eat the hydrocarbons, but temperature can dictate how quickly they scarf oil down. “It’s analogous to leaving a sandwich on the counter versus putting it in the refrigerator,” Halberg says.
The NOAA-Princeton team’s computer analyses also suggest why deep-sea plumes can hang around for months or longer. Currents at great depths, two-thirds of a mile below the surface, move far more slowly than those near shorelines or the surface. So don’t expect deep hydrocarbon plumes to swoosh rapidly out into the Atlantic, Hallberg says. They’re more likely to slosh back and forth with the tides and in response to local eddies. Indeed, his group’s modeling data suggest “they will stay very much confined — within, say, about 100 kilometers of the spill site.”
He predicts that if the Woods Hole team resurveyed the plume site three to nine months from now, it would likely still find much of the oil there. By then, microbes may be dining on the hydrocarbons in earnest, locally drawing down oxygen levels. In the deep ocean, Hallberg notes, oxygen isn’t replenished quickly, so any losses tend to accumulate over time. "According to our simulations, these [very low oxygen] areas will be peaking in October," he says, potentially making some portions of the northern Gulf inhospitable to sea life.
The Woods Hole team wouldn’t speculate about how much of BP’s oil and methane has ended up in the plume they measured, or how many similar plumes might be snaking along the Gulf’s seafloor. But an August 17 report by the University of Georgia in Athens and Georgia Sea Grant attempts just that. The Georgia analysis estimates that between 70 and 79 percent of the BP oil is still in the water.
A panel of experts convened by the Georgia team calculated what share of subsurface BP oil has likely degraded and now estimates it could be just “8 percent of the total oil released into the water.”
Oil that has resisted dispersion and evaporation likely will “remain potentially harmful for decades,” MacDonald said at the congressional hearing, adding: “I expect the hydrocarbon imprint of the BP discharge will be detectable in the marine environment for the rest of my life.”
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