BP / Gulf Oil Spill – How Much Oil Underwater?

The new spill-rate estimate provided by government scientists yesterday got us thinking:

  1. How much of the oil, spilled 5,000′ down at the seafloor from BP’s Macondo well, never floated up to the surface, or sank back below the surface after being sprayed with dispersant?
  2. How much oil may remain in Gulf waters, possibly thousands of feet below the surface?
  3. What are the impacts of that “unseen” oil, and how long will it have toxic effects?

Time for some math: SkyTruth’s estimate of the spill rate (26,500 barrels per day), calculated way back at the end of April with help from Dr. Ian MacDonald at Florida State University, was based entirely on the oil we could see and measure at the ocean’s surface (8.9 million gallons, based on a Coast Guard map of the oil slick for April 28, 8 days into the spill).

Our spill-rate estimate is 43% of the 62,000 barrels-per-day rate that scientists now claim for the early days of the spill. But surely some of the “missing oil” in our estimate was consumed in the fire that raged on the Deepwater Horizon rig before it sank on April 22; so let’s optimistically say 2 days worth of flow from the well was totally burned up, bumping SkyTruth’s estimated daily flow rate up to 35,317 barrels per day – 57% of the new government rate. No oil was being diverted from the well at that time, and skimming operations probably weren’t collecting much at that point either, but dispersants were being used to break up the slick.

This suggests that at least 43% of the oil that leaked from the well remained under water or was driven back under water by dispersants, out of sight to satellite images and Coast Guard observers. Given a total spill of 172.2 million gallons, if we extrapolate from that first week we can conclude that at least 43% of that oil – 74 million gallons – may still be lurking beneath the surface in the Gulf.

That’s our attempt at answering the first question. As for the second question, biodegradation should be steadily breaking down this oil, but we don’t know at what rate, or what the byproducts of that breakdown might be. And we have no idea how to answer the third question.

Getting these answers will require a concerted, sustained, publicly transparent science effort. So we can be better prepared the next time something like this happens, and make better-informed plans for how we use our nation’s ocean resources in the future.