The are two analyses of where the oil from the MC252 well has gone:
NIC - "In summary, it is estimated that burning, skimming and direct recovery from the wellhead removed one quarter (25%) of the oil released from the wellhead. One quarter (25%) of the total oil naturally evaporated or dissolved, and just less than one quarter (24%) was dispersed (either naturally or as a result of operations) as microscopic droplets into Gulf waters. The residual amount — just over one quarter (26%) — is either on or just below the surface as light sheen and weathered tar balls, has washed ashore or been collected from the shore, or is buried in sand and sediments."
GSG - "However, after accounting for oil that has been skimmed and burned (10% collectively), evaporated (8-12%) and degraded (4-8%), we estimate that the oil remaining at or below the surface is between 70 and 79% or between 2.9 and 3.2 million barrels.
We note that this does not account for oil that we know has washed into coastal wetlands. This is a particularly difficult form to quantify, since much of it has settled in tidal creek and bay bottoms or has been buried in salt marsh and creek bottom sediments."
Why don't the two reports agree?
There are three primary reasons for the differences. Additional confusion arises from the common, but flawed, interpretation that the NIC reports states that only 25% of the oil remains. As GSG states, "The news media’s tendency to interpret
“dispersed” and “dissolved” as “gone”is wrong."
Different amounts of oil considered. The two reports consider different total amounts of oil. NIC uses a figure of 4.9 million barrels - the estimate for the total amount of oil released from the well, including the 0.8 million barrels which was recovered to ships. GSG works with a figure of 4.1 million barrels - the amount of oil that was spilled into the Gulf of Mexico. In assessing the effects of the oil, the GSG approach is more appropriate. NIC's use of the larger figure reduces the percentages of each figure describing where the oil went. For example, had NIC based their numbers on the 4.1 million barrel spilled oil figure their residual oil figure would be 31% rather than 25%.
Dispersed oil is not "gone." The NIC reports estimates that 24% of the 4.9 million barrels was dispersed, either naturally or as a result of chemical dispersants. Based on the 4.1 million barrel figure the dispersed oil fraction would be 29%. This dispersed oil is not included in their residual oil figure, even though they state that it is still a hazard in the marine environment - "Until it is biodegraded, naturally or chemically dispersed oil, even in dilute amounts, can be toxic to vulnerable species." Unfortunately, many have interpreted the "residual oil" figure as the net amount of oil remaining from the spill. Such an interpretation is incorrect and misleading.
Considering that the dispersed oil and residual oil remain using the 4.1 million barrel basis, the NIC figures indicate that the remaining oil is 60% of the total spilled. This figure more accurately represents the oil remaining in the marine environment and compares more closely with the GSG range of 70 to 79%.
GSG considered dispersed oil to be part of the residual oil remaining in the environment. However, the GSG report estimates that 4 to 8% of the spilled oil has been degraded and no longer remains as a threat.
Estimates of evaporated and dissolved oil differ. Most of the remaining difference between the two reports is due to differences in the estimates of evaporated and dissolved oil. NIC estimates evaporation and dissolution as 30% of the 4.1 million barrel spilled oil total. GSG estimates evaporation alone at 8 to 12% of the spilled oil. The GSG report points out that oil can evaporate only when it is at the surface of the water and that the distribution of the oil in the 5000-foot deep water column near the wellhead is not well understood. GSG uses a range of evaporation based on a low and high figure for the fraction of the oil that reached the surface.
The NIC estimates lump evaporation and dissolution together. Since dissolved oil remains in the water, it would have been more appropriate to address it separately instead of considering it to be gone from the marine environment like the evaporated oil. The GSG report took this more appropriate approach.
So what's the "right" answer?
Based on the findings of the two reports it is reasonable to estimate that about 2/3 (60% to 79%) of the oil spilled from the MC252 well remains in the marine environment. The amount of residual oil is then 2.4 to 2.9 million barrels. This is approximately 9 to 11 times the size of the Exxon Valdez spill. Note that the lower range does not include the unknown quantity of dissolved oil.
When the Deepwater Horizon rig sank, approximately 700,000 gallons of Diesel fuel were released. It is not known what fraction of that fuel burned immediately or what happened to the remainder.
Note: Due to round-off errors, insignificant differences may appear in summed figures.