The explosion and sinking of the Deepwater Horizon rig spilled more oil from the Macondo well than anyone guessed, the Washington Post reports.  Initially, the Coast Guard estimated a flow rate of 5,000 barrels a day, but the actual rate was at least twelve times that.  The well released almost five million barrels through its broken riser pipe, turning it into the worst unintentional oil spill in history:

The blown-out well in the Gulf of Mexico gushed 12 times faster than the government and BP estimated in the early weeks of the crisis and has spilled a whopping 4.9 million barrels, or 205.8 million gallons, according to a more detailed analysis announced late Monday.

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 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.

The new numbers once again have nudged upward the statistical 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 course of 10 months. That had been the largest unintentional oil spill in history, surpassed only by the intentional spills in 1991 during the Persian Gulf War.

BP captured over 800,000 barrels of oil in its “top cap” operation from last month, about a sixth of the total amount spewed.  Another 1.2 million barrels have either been skimmed, burned, or recaptured by other means.  Some of the remaining three million barrels of oil has dissipated, probably through the Gulf’s natural ability to consume oil through microbes in the water, but the rest of it probably lurks below the surface and will remain there for a long while.

To put this in another perspective, the total amount lost in the Gulf represents what the US normally consumes — in six hours.  The US consumes just under 20 million barrels of oil a day.  That’s less than a couple of years ago, before the economic collapse curtailed energy demand, but still puts us in the number 1 position, almost three times more than China, our closest competitor for the resource (the latter data as of January 2009).

As the Post reports, the calculation of the spill has legal ramifications.  At the least, BP will pay $1100 for every barrel spilled, assuming it didn’t act with negligence.  If a court finds “gross negligence,” the fine ascends t0 $4300 per barrel.  It’s the difference between $4.5 billion and $17.6 billion, and that is separate from the $20 billion fund to cover damage claims from the spill, which may or may not cover all of the claims.

BP has a big bill coming, and we have a lot of questions to ask about the quality of information coming from the administration’s response team in the first weeks of the disaster.

Update: HA reader GulfCoastTider reminds me via Twitter that BP didn’t make any estimates of oil flow during the crisis.  I’ve corrected the lead paragraph to reflect that.