How long does it take oil from an underwater spill to reach the surface?

It depends on the composition of the oil — and on how we are attempting to clean it up…

By Lori Baker

How quickly oil from an underwater well rises to the surface depends on the size of the individual oil droplets. Larger, more buoyant droplets rise more quickly — in some cases, 100 or more times faster, according to Eric Adams, senior lecturer of civil and environmental engineering who helped analyze a study on underwater oil jets and plumes in 2000. A spill consisting of many small droplets, he says, may take months or even years to reach the surface.

Adams explains that an oil spill like the one in the Gulf of Mexico starts off on the ocean floor as a plume consisting of buoyant droplets of oil mixed with bubbles of methane (natural gas). The oil initially shoots out at a high velocity and pulls in a lot of cold ocean water as it goes. The plume behaves, in effect, like an elevator — the oil rises rapidly at first, then slows down as it approaches the surface, where warmer water causes the plume of oil, gas, and water to become neutrally buoyant.

At some point the contents of the plume achieve the same density as the surrounding water, Adams says, and the smaller oil droplets simply “step off the elevator.” Ambient currents can also cause oil droplets to separate from the rest of the plume. At this point, the small droplets begin to rise individually, and much more slowly, in proportion to their diameter, squared. A droplet that is one-third of a millimeter in diameter might rise to the surface in a few days. But a droplet with a diameter ten times smaller would rise a hundred times more slowly, Adams says. “It can take months or even years before they all reach the surface.”

Nobody knows the average size of the oil droplets being released into the Gulf of Mexico, but whatever their size, chemical dispersants being used by BP are certainly making them smaller, likely “significantly smaller,” Adams says. But, he adds, that’s the intention. By creating smaller oil droplets, dispersants spread the oil over the water column so they will do less damage to beaches, wetlands, birds, and other life forms that live at the ocean’s surface. Smaller droplets are also more likely to be dissolved or degraded by Mother Nature over time, he says.

In the case of the Gulf, however, nobody knows what affect the dispersants will have, or even how long it will take for all of the oil to turn up, Adams says. “We have a lot of experience using chemical dispersants with surface oil spills, but very little with using them in deep water.” Likely there is a fair amount of oil beneath the surface of the Gulf in the form of small droplets, he says. Still, he adds, in a situation where there’s no happy answer, it’s assumed that using dispersants is the lesser evil. “If all of this oil were to wash ashore,” Adams says, “that would be very damaging as well.”

Posted: September 17, 2010

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