This is an explanation of how we determined what number to use to count the total amount of oil spilled from the Deepwater Horizon spill in the Gulf of Mexico.
We based our calculation on the approach used by SkyTruth, which is explained below.
Using their method it is estimated that 5,000 barrels or 210,000 gallons of oil has been spilling each day since the incident began.
U.S. Coast Guard estimates:
When the Deepwater Horizon initially caught fire at 11 PM on Thursday, April 20th and burned until it sank mid morning on Thursday, April 22nd, the U.S. Coast Guard estimated that approximately 8,000 (or 336,000 gallons) barrels of oil was leaking per day.
On early Friday, April 23rd, the U.S. Coast Guard reported that the leak appeared to have stopped.
Sometime on Saturday, April 24th, the U.S. Coast Guard reported that oil was leaking from the sea floor (5000 feet below the surface) at 1,000 barrels (or 42,000 gallons) per day. Until today, that was the number they were reporting.
To our knowledge, the U.S. Coast Guard has not provided any specific justification for how these numbers were estimated.
The folks at SkyTruth took a different approach. On April 27th (approximately Day 6 of the spill) they calculated the total volume of the spill based on satellite observations and divided by the amount of time that oil had occurred. This approach yields a conservative estimate that the spill is leaking approximately 5,000 barrels (or 210,000 gallons) of oil each day. It does not account for the oil that burned or has been cleaned up to that point, which, if accounted, for would make the daily estimate higher. This estimate is based on empirical evidence - the area and minimum thickness of the spill to be visible.