NEW ORLEANS — The BP spill is by far the world’s largest accidental release of oil into marine waters, according to the most precise estimates yet of the well’s flow rate, announced by federal scientists on Monday.
Nearly five million barrels of oil have gushed from BP’s well since the Deepwater Horizon rig exploded on April 20, according to the latest data. That amount outstrips the estimated 3.3 million barrels spilled into the Bay of Campeche by the Mexican rig Ixtoc I in 1979, previously believed to be the world’s largest accidental release.
The BP spill was already thought to be the largest spill in American waters, but it was unclear whether it had eclipsed Ixtoc.
“We’ve never had a spill of this magnitude in the deep ocean,” said Ian R. MacDonald, a professor of oceanography at Florida State University.
“These things reverberate through the ecosystem,” he said. “It is an ecological echo chamber, and I think we’ll be hearing the echoes of this, ecologically, for the rest of my life.”
Federal science and engineering teams, citing data that are “the most accurate to date,” estimated that 53,000 barrels of oil a day were pouring from the well just before BP was able to cap it on July 15. They also estimated that the daily flow rate had diminished over time, starting at about 62,000 barrels a day and decreasing as the reservoir of hydrocarbons feeding the gusher was gradually depleted. Before Monday’s announcement, federal scientific teams had estimated the spill in a range from 35,000 to 60,000 barrels a day.
The teams believe that the current estimates are accurate to within 10 percent. They also reported that of the roughly 4.9 million barrels that had been released from the well, about 800,000 had been captured by BP’s containment efforts. That leaves over four million barrels that gushed into the Gulf of Mexico from April 20 to July 15.
As the estimates of the number of barrels spilled increases, so, too, do the penalties under the Clean Water Act, which calls for fines of $1,100 per barrel, or $4,300 per barrel if the government finds that gross negligence led to the spill.
At 4.9 million barrels, that means that the total fine could be $5.4 billion — and, if gross negligence led to the spill, $21 billion. If BP successfully argues that the 800,000 barrels it has recovered should mitigate the penalty, then the figure drops to $4.5 billion and $17.6 billion, respectively.
The amount of oil estimated to be pouring from the well has been a matter of dispute from the earliest days of the spill. Federal and BP officials initially announced that no oil appeared to be leaking, then 1,000 barrels a day, then 5,000 a day, frequently repeating that spill estimates are rough at best and that the main goal was to stop the well. But criticism mounted that no effort was being made to measure the leak with more certainty.
The Obama administration announced the creation of a scientific group dedicated to analyzing the flow rate, which came up with a new estimate of 12,000 to 19,000 barrels a day in late May, a figure that was met with skepticism. That, too, was later revised upward several times before Monday’s announcement. Previous estimates came from analysis of videos from remote-controlled vehicles at the wellhead, modeling of the reservoir and measurements of the oil that was collected by surface ships in the response effort.
After BP capped the well, these measurements could be reinforced by pressure readings within the well. Those pressure readings were compared with pressure estimates when the well was first drilled to determine whether the rate had changed over time, which it apparently had.
The government is continuing to study the data and may refine the estimate.
Meanwhile, BP continued efforts Monday to permanently seal the well. It said it was preparing to conduct final testing on Tuesday to determine whether to go ahead with a plan to pump heavy drilling mud into the runaway Macondo well, in hopes of permanently sealing it by the end of the week.
During the tests, a surface ship will slowly inject small amounts of mud into the well to make sure the mud will reach the oil reservoir from the column of pipes and valves that sit atop it. If that is accomplished, BP will pump higher volumes of mud, and possibly cement, into the well, in an operation known as a static kill or bullheading.
BP executives said Monday that they expected positive results from the tests, which will also check the pressure of the well to ensure that it is safe to pump the mud.
The efforts come 18 days after BP placed a tight-fitting cap on the well that put a temporary end to months of leaking. Engineers had planned to begin the tests on Monday but had to delay when they found a small hydraulic leak in the capping control system above the well.
Kent Wells, senior vice president for exploration and production at BP, said on Monday that a day or two after the pumping of mud began, engineers would consider pumping cement into the well, which could permanently plug it. Engineers might also decide to wait for a relief well to be completed before pumping cement in. There is also a chance that they will pump cement during the static kill and later through the relief well, to make sure the runaway well is sealed.
“We want to end up with cement in the bottom of the hole, completely filling the entire Macondo well,” Mr. Wells said Monday. “Whether that comes from the top or whether it comes from the relief well, those will be decisions made along the way.”
An estimated 2,000 pounds of mud is to be flooded into the well this week.
Thad W. Allen, the retired Coast Guard admiral who is leading the federal response to the spill, cautioned against rushing to declare the static kill a final victory over the well. “I don’t think we can see this as the end-all, be-all, until we actually get the relief wells done,” he said.
Mr. Wells said the last 100 feet of the first of two relief wells should be completed by Aug. 15. A final killing of the well by pouring mud and cement just above the reservoir could take a few days or as much as a few weeks. If the first relief well somehow misses its target, a second one is being drilled for insurance.