Researchers are still working to understand the dynamics of the Deepwater Horizon oil spill and its impact on the Gulf of Mexico. A paper published this week in the Proceedings of the National Academy of Science uses some unique data gathered from the leaking oil well to illuminate some of the reasons why the spill looked so much different from surface spills like the Exxon Valdez incident.

The group collected direct, in situ samples of the oil gushing out of the well using one of the submersible ROVs which were in service at the time. Other attempts to characterize the leaked petroleum were based on samples taken higher in the water column and at the surface, but at that point the oil had already undergone significant changes. The researchers were interested in the composition of the petroleum—what proportion was methane, longer-chain hydrocarbons, etc.—so they could understand why the petroleum behaved the way it did.

Together with the US government’s final volume estimate for the spill of 4.1 million barrels, the team calculated that 170 million kilograms of single-carbon to five-carbon-chain compounds were released.

Some of the petroleum made it up to the surface (the lighter, more volatile compounds), and some settled on the bottom (heavier, insoluble hydrocarbons), Characterizing the behavior of the surface and seafloor fractions of the spill is very difficult because of the large amount of chemical dispersants that were used, but the dissolved petroleum may tell the majority of the story. It appears that most of the spill's output dissolved in deep ocean water on the way up and was transported away with the current. That’s why the deep plume that was tracked following the spill was so large and the images of oil-slicked beaches were less overwhelming than many expected.

The deep-water dissolution had a big impact on the dynamics of the spill. It ensured that the petroleum was diluted as it moved with the current. While the amount of microbial degradation is unknown, it was likely enhanced by this dispersion—in a typical surface spill, the oil is less accessible to the microbes. The researchers were unable to detect any concentration trends along the plume, however, which indicates that microbial activity was fairly slow. The team estimates that the "half-life" for the plume was about one month for most of the petroleum compounds (methane, notably, was likely consumed more quickly).

The diffuse nature of the contamination has made it difficult to evaluate the impact on the Gulf. But researchers are learning things that will help them figure out where to look and what to monitor, and may improve the response to any future deep-water oil spills.

PNAS, 2011. DOI: 10.1073/pnas.1101242108 (About DOIs).