Up to 80% of the Spilled Oil Remains in the Gulf

Equipment at the site of BP's well / U.S. Coast Guard photo

Two weeks ago, NOAA released a report claiming that most of the oil leaking from BP's blown-out well had disappeared. The government report was widely promoted in the syndicated media but was met with skepticism from environmental journalists and marine scientists. Since that time, scientists from the University of Georgia performed their own evaluation of the government's numbers and reached a much different result (pdf). UGA's report omits oil captured directly from the wellhead (since that oil never entered the water), so its numbers do not match the government's exactly, but it still offers a useful critique of the conventional wisdom.

The UGA scientists agree that burned or skimmed oil can be said to have left the Gulf. That accounts for 10% of the oil that entered the ecosystem. The rest is either dispersed (naturally or chemically), dissolved, or residual. "Dispersed" or "dissolved" may mean that some oil has been degraded or evaporated, but that should not be assumed.

On degradation:

There are no data available from the scientific literature or the National Incident Command on rates of decomposition or weathering of oil released from the BP spill. Because so much oil exists as micro-droplets in deep, very cold ocean waters, it is difficult to infer decomposition rates from studies of previous spills occurring closer to the surface. However, several scientific studies are currently underway to directly address this critical need.

We asked our scientific experts to estimate, as best they could, the percentage of subsurface oil that has degraded. They suggested a range of between 5% (see Figure 3) and 10% (see Figure 2). Given that estimate, we calculated that between 168,000 and 319,000 barrels have been removed from the Gulf through degradation. This is equivalent to 4-8% of the total oil released into the water. 

 On evaporation:

The NIC report estimates that 1.2 million barrels (30%) of oil released at the wellhead dissolved in the water and are, therefore, in a form that could evaporate. However, for oil to evaporate, it must come in contact with the atmosphere. Without knowing how much of the oil is at various depths, it is difficult to estimate how much oil could have reached the surface in order to evaporate. Our experts set the range of evaporation at 25% (see Figure 3) to 40% (see Figure 2). Based on this estimate, we calculated that between 306,000 and 490,000 barrels of oil have evaporated into the atmosphere and are no longer in the Gulf of Mexico. This amounts to 8-12% of the total oil spilled into the Gulf.

The scientists also note that the degraded and evaporated forms may not be environmentally harmless, either to humans or to other organisms.

The one piece of welcome news in this report is that the remaining oil is now unlikely to enter the Loop Current and make its way along the East Coast.

Update: The New York Times has its own take on the UGA report.