Researchers weighed in on the controversial question of how much oil poured into the Gulf of Mexico during the BP Plc spill and said Thursday the total amount was around 4.4 million barrels.
The figure, derived using a technique called optical plume velocimetry, represents the first independent peer-reviewed study of the worst oil spill in U.S. history and conforms closely to the most recent estimate by government scientists.
The spill began in April when a deepwater rig exploded and sank in the Gulf. The well was capped on July 15 and killed on Sunday after causing billions of dollars worth of environmental and economic damage to Gulf coast states.
Estimating the extent of the spill precisely is a crucial part of any environmental damage assessment and also has big legal implications for the energy giant.
BP faces potential fines under several anti-pollution laws and recent increases in the estimate of how much oil leaked from its well suggest this could amount to a major liability.
The company has already clocked up potential fines of up to $9 billion, based on a possible $4,300 per barrel fine for polluting major waterways.
"It's important to understand how much oil was put into the system as we work to understand the ecological impacts in the short term and long term," said Timothy Crone, professor at Columbia University's Lamont-Doherty Earth Observatory.
"People need to know how much oil came out," Crone said in an interview. The paper, co-authored with Maya Tolstoy, will be published in the journal Science.
BP initially said it had no estimate of the amount of oil flow but over a period of weeks growing fears of a catastrophic spill put pressure on the company and the U.S. government to produce more accurate estimates.
Officials put the figure at 1,000 barrels per day but the company also said it was collecting more than that amount and that far more was escaping.
The next estimate was 5,000 barrels, a figure disputed by scientists, then 12,000-19,000 barrels and later a government estimate using an analysis of video of the gushing well produced said 56,000-68,000 barrels a day.
One consequence of the progressive upward revisions was a loss of public trust in the company and the federal government, with critics on the Gulf coast arguing that it showed BP wanted to minimize the scale of the spill to limit its liability. BP denied this.
"The idea that it wasn't measurable was false. Almost any ocean scientist that was thinking about this would have agreed," said Crone, who has previously studied hydrothermal vent systems.
Researchers analyzed two high resolution videos 20-30 seconds long and estimated the average flow rate of oil using a technique that enabled them to make a two-dimensional model of the plume billows and then calculate volumetric flows.
Despite the fact that BP streamed live video of the flow for weeks, the quality was insufficient for Crone's analytical technique, forcing him to rely on short videos obtained from the government.
The average flow rate between April 22 June 3 was 56,000 barrels per day, with a formal uncertainty of 21 percent.
After a riser pipe was removed on June 3 as part of efforts to seal the well, 68,000 barrels of oil was flowing into the ocean each day with a formal uncertainty of 19 percent, the researchers said.