Track methane to gauge size of BP spill: scientist

WASHINGTON (Reuters) - To figure out how much oil has spilled into the Gulf of Mexico, experts should measure the plumes of dissolved methane coming from the wrecked BP offshore rig, a marine scientist said on Sunday.

Methane gas is the most abundant compound coming from the April 20 blowout and spill, making up about 40 percent of the leaking petroleum by mass, David Valentine of the University of California-Santa Barbara wrote in an opinion article in the journal Nature.

"Although methane from surface-vessel spills or shallow-water blowouts escapes into the air, I expect that the vast majority of methane making the long trip to the sea surface from a deep-water spill would dissolve," he wrote.

Unlike oil, methane dissolves uniformly in seawater so it could be measured accurately and scientists could use that measurement to calculate the amount of the spill, Valentine said.

Estimates of how much oil and other compounds are coming from the ruptured BP wellhead have varied widely. The company initially put the leak at 5,000 barrels (210,000 gallons/795,000 liters) a day but some published estimates have ranged to 70,000 barrels (2.9 million gallons/11 million liters) or more daily.

To determine how much methane -- and by extension, how much oil -- is leaking, Valentine suggested dispatching floats that would track water flow and any plumes of methane in the water.

This, in addition to spot analyses and followed by a two-vessel expedition to the area, would be able to set a lower limit on the total amount of spilled oil, Valentine wrote.

Measuring how the methane plumes move could help estimate the rate of the spill, he said.

Valentine said the U.S. academic research fleet has a dozen vessels that could do this job "at costs of probably a few million dollars or less."

"Capitalizing on this idea requires immediate action," he wrote. "I am calling for a concerted community effort, with appropriate commitment from the U.S. government, the trustees of the Deepwater Horizon incident and BP. The likely rewards far exceed the costs."

(Editing by Bill Trott)