We all saw the images of oil-coated birds and shorelines in the wake of the 2010 Deepwater Horizon spill. These were the most visible impacts of the catastrophe, but much of the oil that gushed from the busted Macondo wellhead 5,000 feet underwater never made it to the surface. Of the estimated 5 million barrels that spilled, approximately 2 million stayed trapped in the deep ocean. And up to 31 percent of that oil is now lying on the ocean floor, according to a new study.
Based on an analysis of sea-floor sediment samples collected from the the Gulf of Mexico, geochemists at the University of California-Santa Barbara were able to offer the first clues about the final resting place of hundreds of thousands of barrels of oil. Their results were published today in the Proceedings of the National Academy of Sciences.
The data, which was gathered as part of the ongoing federal damage assessment, shows "a smokingly clear signal, like a bulls-eye" around the Macondo well, said lead author David Valentine.
When oil first began to shoot out of the broken well, some 2 million barrels' worth broke up into microscopic droplets before reaching the surface and became suspended in the deep ocean, Valentine said. His goal was to discover the fate of that oil, beyond the reach of any cleanup efforts, four years after the spill. The researchers combed through the sediment samples for traces of hopane, a chemical compound found in crude oil that doesn't break down over time. Hopane was also used as a indicator of oil distribution following the Exxon-Valdez spill in 1989.
To test whether traces of hopane originated from the Macondo blowout—rather than from a natural seep or some other well—Valentine scrutinized both where they appeared in individual sediment cores and how concentrations changed at varying distances from the well. Both indicators strongly implicate the Macondo well, the study found. Close to the well, hopane concentrations were very high in the top half-inch of sediment, a sign that the chemical had been deposited recently and in great volumes. Even more telling was the spacial distribution: Within 25 miles of the well, hopane concentrations were 10 times higher than outside that boundary, Valentine said. A further clue was the distinctive splatter pattern in which hopane concentrations were found, which matched the pattern that would be expected from oil leaking from a well.
Add it all up, the study finds, and between 4 and 31 percent of the oil that originally was suspended in the deep ocean (roughly 80,000 to 620,000 barrels) has now come to rest on the ocean floor. The remainder, Valentine said, is still unaccounted for: It could still be suspended in the water column; it could have risen to the surface; it could have been eaten by bacteria, etc.