Why Isn't the Oil Floating to the Surface?

Controversy continues to swirl over the size of the Gulf oil spill, with one estimate suggesting as much as 100,000 barrels of oil could be spewing into the water daily. Modest amounts of oil have begun washing over coastal marshes. But if the higher estimate is accurate, most of the oil remains out of sight. So why isn't it behaving like oil in your bottle of salad dressing and floating to the surface?

Part of the answer might simply be time, says Edward Overton, an environmental chemist at Louisiana State University, Baton Rouge. It takes a while for the oil to migrate a mile upwards from the sea floor. But because the oil is a light crude that is much less dense than the surrounding water, "by every law of physics it has got to come up," Overton says. Alternatively, say Overton and others, the oil may also be forming droplets smaller than 100 micrometers across. This would increase the oil's surface to volume ratio, making it no more buoyant than water.

Two factors could be encouraging the formation of such microdroplets, say Overton and others. First, along with oil, large amounts of methane gas are also jetting out of the ocean floor. As both emerge at high pressure, the gas effervesces out of the solution, much as bubbles of carbon dioxide form when the cap is removed from a bottle of soda. "That could break the oil into very small particles," Overton says. The chemical dispersants being released at the leak site would also tend to break the oil into smaller droplets, adds Jeffrey Short, an environmental chemist who spearheaded cleanup efforts following the Exxon Valdez accident for the National Marine Fisheries Service and who now works with Oceana, a conservation group in Juneau, Alaska. If the oil is forming microdroplets, that could help explain reports that the oil has formed an underwater plume as much as 16 kilometers long, 5 kilometers wide, and 100 meters thick, although the existence of this plume also remains controversial.

Over time, Short says, he suspects that if the oil has formed microdroplets they will coalesce and form larger, more buoyant drops that should rise to the surface. However, he adds that if those droplets combine with particulates in the water, that could in turn make them more dense and cause them to sink. Just how these potentially countervailing forces will interact, "we just don't know," Short says.

Overton adds that much of the current debate over what's happening to the oil could be settled by the remotely operated vehicles (ROVs) currently taking video of the oil emerging from the broken pipe. "The first thing I would do is use one of those ROVs to look up and see what the plume looks like," Overton says. That, he adds, would reveal how much of the oil is remaining below the surface. "To me, that is the big question," he says.

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