By Janet Raloff, Science News
Most of BP’s spilled oil remains in the Gulf—with little sign of degrading, according to Ian MacDonald of Florida State University. And much of this surviving oil could be in sediment or on its way there, the scientist reported at a September 27 meeting in Washington, D.C.
This sediment link is hardly idle speculation. In her recent month-long cruise in the Gulf of Mexico, University of Georgia oceanographer Samantha Joye identified areas of the seafloor hosting what looks to be substantial amounts of BP oil. Photos on her blog graphically depict the pollution.
MacDonald was one of a panel of scientists who testified before the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling. “In my scientific opinion,” he said, “the bulk of [the BP oil] was dispersed in surface layers, from which about one-third evaporated and 10 percent was removed by burning or skimming. An additional 10 percent was chemically dispersed.
“The remaining fraction—over 50 percent of the total discharge—is a highly durable material that resists further dissipation. Much of it is now buried in marine and coastal sediments.” And the bad news, MacDonald added, is that “there is scant evidence for bacterial degradation of this material prior to burial.”
MacDonald didn’t explain how he arrived at the percentages that he reported. At one point in his testimony he referred to making, early on, back-of-the-envelope calculations of spill-flow-rate estimates for the accident. Not clear was whether the numbers he gave on oil surviving in the Gulf were similarly suggestive versus well-researched.
But the biological oceanographer did raise an interesting challenge to arguments that natural hydrocarbon seeps in the Gulf make the likelihood of seafloor biodegradation of BP oil likely. “Animals that feed or burrow into deep-sea sediments are not adapted to oil,” he notes. Moreover, he points out, “Burrowing organisms that are common in non-seep areas are completely absent in natural seeps, where oil saturates the sediments and oxygen is depleted immediately below the sediment-water interface.”
So the existence of oil-loving bugs somewhere in the Gulf does not necessarily mean that they’ll migrate to chow down on BP’s oil if it lands long distances from their home ecosystem. And, MacDonald adds, the deeper that oil becomes buried, the less oxygen that’s available for the microbes that ordinarily break hydrocarbons down. Another problem with buried oil: “Any storm event tends to resuspend them,” MacDonald says, something that appears to explain fresh oilings of Florida beaches after turbulent weather.
When oil falls out of the water, its seafloor deposition can be quite variable, noted Richard Camilli of the Woods Hole Oceanographic Institution in Massachusetts. He reported at the Commission meeting that heavy oil—a particularly viscous type of petroleum—“tends to pool in low-lying areas” on the seafloor, forming what looks like mud puddles.” They form here and there, not in a continuous stain.
If the same holds for the light crude spewed by the BP well, sediment pollution might prove quite patchy. But a technology that his team recently developed could map such dispersed deposits fairly quickly, he said.
Ordinarily, scientists sample for oiled sediment by taking cores and pulling the captured mud, sand and clay on board their ship for analysis. With a skilled team of researchers, “you may be able to collect a dozen or so [core] samples a day,” Camilli says. But using the WHOI technology that his team published in the October 2009 Marine Pollution Bulletin, “we could make several thousand measurements per day,” he noted, precisely pinpointing seafloor contamination to centimeter-scale precision.
Key to its speed: It doesn’t get bogged down sifting through seafloor muck. A remotely operated vehicle—a robotic research sub—sails about a meter above the seafloor. During field testing in the Gulf, its mass spectrometer essentially sniffed out a broad range of individual petroleum hydrocarbons. And it could detect them at concentrations of below one part per billion.
“The technology is actually not that different from what we used to map the [BP subsea] plume."
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