A breaking story at the Washington Post, based on a new analysis of the Deepwater Horizon oil spill, puts the total at 4.9 million barrels:
BP’s Macondo well spewed 62,000 barrels of oil a day initially, and as the reservoir gradually depleted itself the flow eased to 53,000 barrels a day until the well was finally capped and sealed on July 15, according to scientists in the Flow Rate Technical Group, supervised by the U.S. Geological Survey and the U.S. Department of Energy.
Note there is some dispute, given measurement difficulties, of the size of past oil leaks. The Post continues:
The new numbers, released by the government Monday night, once again nudge upward the scale of the disaster. If correct — the government allows for a margin of error of 10 percent — the flow rate would make this spill significantly larger than the Ixtoc I blowout of 1979, which polluted the southern Gulf of Mexico with 138 million gallons over the coursre of 10 months. That had been the record for the largest unintentional oil spill in the planet’s history, surpassed only by the intentional spills of the Persian Gulf War.
By contrast, Wikipedia shows only spill that is clearly worse as the Lakeview Gusher, in 1910 and 1911, which spewed an estimated 9 million barrels. Wikipedia’s sources come up with a wide range of estimates for the Gulf War leaks, of 2 million to 6 million barrels
This leak is well over the total for Ixtoc, heretofore the worst Gulf disaster, whose high estimate was roughly 3.5 million barrels. Exxon Valdez was a mere 260,000 to 750,000 barrels.