The Macondo well is still tightly capped; the “static kill” effort to shut it down by pumping mud, then cement, into the well through the containment cap may begin as soon as tonight. This would be followed up by a “bottom kill” procedure, hopefully in August, that would use a relief well to pump more cement into the well at a point about 13,000′ below the seafloor.

So, folks are asking – just how big was this spill?

Big enough to – at one time or another – cover 68,000 square miles of Gulf waters with oil slick or sheen, based on our ongoing analysis of satellite images. But that’s just the part we could see at the surface. Lots of oil remained in the water column, beneath the surface, out of sight on the satellite images we’ve been able to acquire.

The Washington Post published a calculation on July 29 (article and useful graphic) that as much as 218 million gallons (5.2 million barrels) leaked out of the well over the duration of the spill from April 20 – July 15. That assumes the government team’s high-end leak rate estimate of 2.52 million gallons (60,000 barrels) per day for 87 days. Subtracting 33.6 million gallons (800,000 barrels) the Coast Guard and BP claim to have kept out of the water — by siphoning oil directly from the leaking well — yields a high-end spill estimate of 184.4 million gallons (4.4 million barrels).

SkyTruth’s estimate on May 1 that the well was gushing at a rate of at least 1.1 million gallons (26,500 barrels) per day turned out to be on the low end of the later scientific estimates made by the government-assembled Flow Rate Technical Group. Our conservative number generates a total flow of 96.8 million gallons (2.3 million barrels) from the leaking well over 87 days. Subtracting the 33.6 million gallons supposedly diverted from the leaking well – we have no way to confirm that number – yields a low-end total spill estimate of 63.2 million gallons (1.5 million barrels) directly into Gulf waters.

How does this compare with our previous sad benchmark, the Exxon Valdez spill in 1989? The official estimate — a number that, we should note, is disputed as being far too small — is that 11 million gallons were spilled when the Exxon Valdez supertanker ran aground in Prince William Sound, Alaska.

That would make the BP / Deepwater Horizon oil spill in the Gulf anywhere from 6 to 17 times as large as the official Exxon Valdez spill. But don’t lock these numbers in just yet: they may change as the investigation proceeds and new information is brought to light in coming months.

We’ll have to work hard, and stay on guard, to make sure this is one sorry new record that will never be broken.