WASHINGTON - Oil that gushed out of BP's failed Gulf of Mexico well in 2010 may have left a 1,235-square-mile "footprint" on the deep-ocean floor, according to research released Monday.
The study, published in the Proceedings of the National Academy of Science, suggests that "significant quantities" of crude from the destroyed Macondo well may have migrated near deep-sea corals southwest of the site.
It also helps answer a question of scientific - if not legal - dispute since the Macondo well blew out on April 20, 2010: What was the fate of all that oil?
An estimated 4.9 million barrels, or more than 200 million gallons, are believed to have escaped from the well, with much of it burned off or broken up with chemical dispersants. Some of the crude washed ashore. An estimated 2 million barrels is believed to have gone unrecovered.
To find out where it went, a team of researchers, led by University of California Santa Barbara microbial geochemist David Valentine and including representatives from UC Irvine and the Woods Hole Oceanographic Institution, analyzed data from more than 3,000 samples collected at 534 locations around the ruptured BP well.
Their work focused on identifying hopane, a tracer hydrocarbon that indicated the presence of crude. While it was found throughout the northern Gulf of Mexico, the researchers discovered it was most concentrated in a thin layer on the seafloor within 25 miles of the Macondo well.
"Our findings suggest that these deposits come from Macondo oil that was first suspended in the deep ocean and then settled to the seafloor without ever reaching the ocean surface," Valentine said.
Light, freshly released oil normally is generally not expected to sink, and even dispersed oil is more likely to remain suspended in water.
Valentine described the footprint as a "shadow of the tiny oil droplets that were initially trapped" higher up, in the water above. "Some combination of chemistry, biology and physics ultimately caused those droplets to rain down another 1,000 feet to rest on the seafloor," he added.
The 1,235-square-mile area he identified is almost twice the size of the city of Houston.
BP says research flawed
BP suggested that the researchers inaccurately attributed the tracer hopane to Macondo crude.
"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," BP spokesman Jason Ryan said. "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 seafloor."
Although the PNAS study documents scattered, patchy deposits, the "mapping technique connects the sample locations as if the oiling were continuous between the sampling points," Ryan added. "This dramatically overestimates the impacted area."
Natural seeping of oil
Complicating any analysis of spilled crude are natural seeps of oil. According to the National Research Council, natural seeps release 560,000 to 1.4 million barrels of oil into the Gulf of Mexico each year.
But Valentine argues the pattern of contamination "is fully consistent with the Deepwater Horizon event" and "not with natural seeps." For instance, much higher concentrations of the tracer hopane were found at the of top superficial layers of sediment, suggesting they were dumped there recently, not as part of a natural accumulation over time. The higher concentrations close to the well were also markedly different from the "background" amounts documented farther from the site.
Valentine's footprint calculations suggest that once-suspended oily particles were sinking through the water near deep-sea corals about 7 miles from the well.
Unrelated research published in 2012 by the National Academy of Sciences linked the Gulf spill with damage to those corals, found months after the Deepwater Horizon blast.