the report’s authors started with an estimate from another government scientific team: how much oil spewed from the out-of-control BP well before it was capped on July 15. That calculation was itself the product of a drawn-out controversy in which the government was accused of deliberately playing down the size of the spill in the early days.
Starting with the latest estimate, 4.9 million barrels plus or minus 10 percent, a scientific team led by the National Oceanic and Atmospheric Administration incorporated various assumptions about the nature of the oil and the fates it could have encountered after hitting the water. (NOAA is the same agency that devised the early, now-discredited estimate that the well was leaking only 5,000 barrels a day, one reason some people distrust the new report.)
The firmest number in the report is that 17 percent of the oil emerging from the wellhead was captured by various containment devices. From there, the numbers got less certain.
The report estimated, for instance, that 25 percent of the oil either evaporated from the hot ocean surface or dissolved in the water into individual molecules of hydrocarbon. Some scientists, Dr. Joye among them, said they doubted that more than 10 percent or 15 percent of the BP oil had disappeared in this way.
Bill Lehr, a NOAA scientist in Seattle who was involved in creating the model, said the figure was based on both direct measurement and past scientific research about the fate of spilled oil. Efforts to refine the estimate, and the rest of the model, are continuing, he said.
Dr. Lehr said one difficulty was figuring out how much oil had dispersed naturally into tiny droplets. The accepted methodology for making that calculation is based on shallow spills. In this one, the oil shot out of the broken well at high speed a mile below the ocean surface, and some of it dispersed in the deep ocean. A new formula had to be created to take that factor into account.
When all the math was done, the government team concluded that about 16 percent of the oil had dispersed naturally. “We think it’s sound theory, but it’s new,” Dr. Lehr said. “You could say it’s an experiment in that respect. You do the best you can with what you’ve got.”
Similarly, the report offered calculations about how much oil had been burned or skimmed from the ocean surface, how much had been chemically dispersed, and so forth.
By a process of elimination, the researchers concluded that only 26 percent of the oil had come ashore or was still in the water in a form that could, in principle, do additional shoreline damage. And much of that was breaking down quickly in the warm waters of the gulf, the report said.
Of course, that 26 percent equals more than 53 million gallons of oil, five times the size of the Exxon Valdez spill in Alaska.
“One way of looking at it is to say that 26 percent of the world’s largest oil spill is still out there,” said Greg Butcher, director of bird conservation for the National Audubon Society. “And that is a lot of oil.”
http://www.nytimes.com/2010/08/05/us/05oil.html?src=mv