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When the Deepwater Horizon oil rig operated by British Petroleum caught fire and sank on April 22, 2010, it caused the largest offshore oil spill in U.S. history. The immediately visible damage was already catastrophic, with at least 5 million gallons of oil spilled into the Gulf of Mexico, but now a new study released by the National Academy of Sciences indicates that a 1,200-square-mile ring of oil encircles the Macondo well on the seafloor of the Gulf.

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The study was headed by a team of scientists from the University of California, Santa Barbara; the University of California, Irvine; and the Woods Hole Oceanographic Institute. According to The Times-Picayune, the study tested more than 3,000 samples from 534 different locations in the Gulf for hepane, a chemical constituent of crude oil that was found in samples taken from the spill.

The paper found that the oil was distributed in two ways on the ocean floor: A “bathtub ring” that formed a heavy oil-rich layer, and a “fallout plume,” where oil particles sank into sediment up to a mile deep. The majority of the oil is dispersed around the BP Macondo well site, and overlaps with a range of deepwater coral reefs in the region.

The authors of the study believe that there is even more oil residue in the Gulf that has yet to be discovered, stating, “We also suggest that a significant quantity of oil was deposited on the ocean floor outside this area but so far has evaded detection because of its heterogeneous spatial distribution.”

The question of just what happened to the oil spilled at Deepwater Horizon has been a significant point of contention amongst federal officials, researchers and BP, with opinions split as to how much oil washed ashore, sank to the bottom or was broken down by microbes. The United States Department of Justice estimated 4.2 million barrels of oil were released in the spill, while BP estimates 2.5 million barrels were released (one barrel of oil is equivalent to 42 U.S. gallons).

The new study estimates that 31 percent of that oil is still on the floor of the Gulf. Lead author David Valentine of UC Santa Barbara told The Times-Picayune that while they do not know for sure what happened to make this oil sink to the bottom, “We do provide hypotheses, that a combination of coagulation and bacterial growth drove the oil into a floc form and facilitated particles or droplets sinking to the seafloor. Some of the oil was certainly eaten by bacteria, and other components dissolved to the water.”

While the study does not focus on the effects of this spill on wildlife in the Gulf Coast and at the lower depths of the Gulf, immediate reports following the spill were already showing significant damage to aquatic life at lower depths. As RT notes, the National Parks Service reported that more than 8,000 recorded wildlife deaths have been attributed to the spill. Meanwhile, a 2013 study led by researchers at the University of South Florida found that oil from the spill was washing up onto the western coast of Florida.

In September, a federal district court ruled that BP’s handling of the Deepwater Horizon spill was “grossly negligent,” and specifically liable under maritime law for the spill. BP is facing similar claims of negligence from Columbian farmers who filed a recent suit over damages caused by a mismanaged BP pipeline.

BP is still challenging notions of long term damage to the gulf, with a company spokesperson recently authoring a highly controversial editorial in Politico that claimed the damage to the Gulf from the spill was largely short term. On Monday, BP spokesman Jason Ryan challenged the findings of the recent study by UC Santa Barbara et al, claiming that “the authors failed to identify the source of the oil, leading them to grossly overstate the amount of residual Macondo oil on the sea floor and the geographic area in which it is found.”

The Deepwater Horizon spill is the only known oil spill of its scale in the region surrounding the Macondo well.

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