2 million barrels of oil from BP spill may be on ocean floor

A new study now has been able to describe the path the oil followed to create a footprint on the deep ocean floor

Almost two million barrels of oil from the disastrous 2010 BP oil spill in the Gulf of Mexico may be trapped in the ocean floor, a new study suggests.

Due to the environmental disaster's unprecedented scope, assessing the damage caused by the April 2010 explosion on BP's Deepwater Horizon oil rig has been a challenge, according to researchers.

One unsolved puzzle is the location of two million barrels of submerged oil thought to be trapped in the deep ocean, researchers said.

University of California - Santa Barbara's David Valentine and colleagues from the Woods Hole Oceanographic Institute (WHOI) and the University of California, Irvine have now been able to describe the path the oil followed to create a footprint on the deep ocean floor.

The study used data from the Natural Resource Damage Assessment process conducted by the National Oceanic and Atmospheric Administration.

The US government estimates the Macondo well's total discharge - from the spill in April 2010 until the well was capped that July - to be 5 million barrels.

By analysing data from more than 3,000 samples collected at 534 locations over 12 expeditions, they identified a 1,250-square-mile (3,237-square-km) patch of the deep sea floor upon which 2 to 16 per cent of the discharged oil was deposited.

The fallout of oil to the sea floor created thin deposits most intensive to the southwest of the Macondo well. The oil was most concentrated within the top half inch of the sea floor and was patchy even at the scale of a few feet.

The investigation focused primarily on hopane, a nonreactive hydrocarbon that served as a proxy for the discharged oil.

Researchers analysed the spatial distribution of hopane in the northern Gulf of Mexico and found it was most concentrated in a thin layer at the sea floor within 25 miles (40 km) of the ruptured well, clearly implicating Deepwater Horizon as the source.

"Based on the evidence, our findings suggest that these deposits come from Macondo oil that was first suspended in the deep ocean and then settled to the sea floor without ever reaching the ocean surface," said Valentine, a professor of earth science and biology at UCSB.

"The pattern is like a shadow of the tiny oil droplets that were initially trapped at ocean depths around 3,500 feet and pushed around by the deep currents.

"Some combination of chemistry, biology and physics ultimately caused those droplets to rain down another 1,000 feet to rest on the sea floor," Valentine said.

While the study examined a specified area, the scientists argued that the observed oil represents a minimum value. They purport that oil deposition likely occurred outside the study area but so far has largely evaded detection because of its patchiness.

The study was published in the Proceedings of the National Academy of Sciences.