The “unprecedented quantity of oil” that surged from 2010’s blown Macondo well has contaminated a wide swath of the ocean floor, settling into a ring encompassing 3200sq km, according to recently-published research by the Proceedings of the National Academy of Sciences of the United States of America (PNAS).
By proxy, the scientists said, this area represents 4-31% of the oil estimated to be trapped in the deepwater Gulf of Mexico.
Co-authored by a team of scientists from University of California, Santa Barbara and the Woods Hole Oceanographic Institution (WHOI), the report investigates a “fallout plume” of hydrocarbons that was initially suspended in deep water and then depressed downward, eventually settling into the sea floor.
Overseen by the National Oceanic and Atmospheric Administration, researchers focused on 17α(H),21β(H)-hopane as a tracer for crude oil sediment. Scientists embarked on 12 expeditions to collect more than 3000 samples at 534 stations around the well site for analysis.
“[W]e treat hopane as a degradation-resistant proxy for Macondo’s liquid-phase oil. Analysis of the spatial distribution of hopane allows us to define both a regional background level and a depositional footprint of oil from the Deepwater Horizon event,” the paper’s authors wrote. “In combination with other lines of evidence, this analysis leads us to conclude that significant quantities of particulate oil sank from the intrusion layers to rest on the underlying sea floor.”
However, BP does not agree with the methodology used in the study, and issued a statement saying, "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. Instead of using rigorous chemical fingerprinting to identify the oil, the authors used a single compound that is also found in every natural oil seep in the Gulf of Mexico, causing them to find false positives all over the sea floor.
"In addition, while the authors acknowledge the scattered nature of the impact to sediments, their mapping technique connects the sample locations as if the oiling were continuous between the sampling points. This dramatically overestimates the impacted area," BP said.
The supermajor also commented on the nature of seepage in the Gulf of Mexico, noting that "the multi-agency Operational Science Advisory Team Report (OSAT-1) released in December 2010 found that only about 1% of sediment samples taken after 3 August 2010 exceeded EPA (US Environmental Protection Agency) aquatic life benchmarks for polycyclic aromatic hydrocarbons, and only those within 3km of the wellhead were consistent with Macondo oil.”
The PNAS report could be another blow dealt to BP following the September ruling by US Judge Carl Barbier that charged the supermajor with gross negligence in the 20 April 2010 disaster. Being ruled grossly negligent opens the door for US prosecutors to seek up to US$4300 per barrel spilled under the Clean Water Act.
How much oil was spilled is currently under dispute. US Department of Justice attorneys assert that 4.2MMbo poured into the Gulf of Mexico. BP insists the quantity is substantially lower at 2.45MMbo.
Scientists are still trying to determine five years later where all of the oil went, WHOI said in a statement.
“Among the pressing uncertainties surrounding this event is the fate of ~2MMbo of submerged oil thought to have been trapped in deep-ocean intrusion layers at depths of ~1000–1300m,” researchers wrote in the report, entitled, “Fallout plume of submerged oil from Deepwater Horizon.”
This assertion could prove costly for BP, as penalties for all parties involved in the incident will be decided upon in a third trial planned for 20 January 2015.
Researchers said that the “novelty of this event makes the oil’s subsequent fate in the deep ocean difficult to predict.”
Map of the northern Gulf of Mexico showing the sampling locations (dots) and the Macondo well (star) overlaid on the National Geophysical Data Center Coastal Relief Model bathymetry from PNAS.
Read more:
Court rules BP showed gross negligence in Macondo spill
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