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  • Dick writes about Earth and planetary science for Science magazine.
 

Gulf Spill Big But Not Enormous, Yet

27 May 2010 1:41 pm
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A federally convened expert team has estimated that oil has been gushing from the wrecked Gulf of Mexico well two to four times faster than first guessed. At 12,000 to 19,000 barrels per day, the spill has so far totaled between roughly 400,000 and 650,000 barrels. That puts it well over the 257,000 barrels spilled from the Exxon Valdez into an enclosed, high-latitude Alaskan bay. But it falls far short of the 3,400,000 barrels spewed into the southern gulf over a year following the IXTOC 1 offshore accident in 1979.

The first "independent and scientifically grounded" estimate of the spill's flow rate comes from a team of federal scientists, independent experts, and university representatives, Marcia McNutt, director of the U.S. Geological Survey and chair of the Flow Rate Technical Group, said during a telephone press conference this morning. The group used three independent methods to estimate the flow rate. One team used airborne infrared imaging to estimate the volume of oil on the surface on 17 May. Adding in the volume already burned, skimmed, dispersed, or evaporated, the team's initial estimate came to 12,000 to 19,000 barrels per day.

A second team analyzed videos of the plume of oil and gas gushing from the broken end of the riser pipe not far from the wellhead. Deriving a velocity and volume from its video analysis and a ratio of gas to oil of 3 to 1 from the pipe inserted into the riser end, the team came up with a flow of 12,000 to 25,000 barrels per day. A third group estimated a minimum flow of 11,000 barrels per day from the volume of oil recovered through the inserted pipe.

"Three methods are seeing a lower bound that is 12,000 barrels per day, and two methods suggest it could be as much as 19,000 barrels per day," McNutt said. Although the estimates are still preliminary, she said, she found it "remarkable that from entirely independent methods you'd get similar results."

The range clearly exceeds the much-quoted 5000 barrels per day, McNUtt noted. That estimate came from "very limited" National Oceanic and Atmospheric Administration surface observations, she said, and led to a range of 1000 to 13,000 barrels per day. So a then-informal group of federal employees chose "a number somewhere in the middle that was conservative but defensible." The new range falls well below some estimates--which ranged up to 100,000 barrels per day—offered by private citizens through the media. Even at the new rates, another half-year of unabated flow will have to pass to break IXTOC 1's world record total.

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