Study Confirms Estimates of Gulf Oil Spill Rate

— Coral Gables — By combining detailed chemical measurements in the ocean, oil slick, and air, scientists at the National Oceanic and Atmospheric Administration (NOAA), the University of Miami, and elsewhere have independently estimated how fast gases and oil were leaking during the BP Deepwater Horizon Gulf oil spill in 2010.

The new study, "Chemical data quantify Deepwater Horizon hydrocarbon flow rate and environmental distribution," is published online in the Proceedings of the National Academy of Sciences . The improved chemistry-based estimate--an average of 11,130 tons of gas and oil compounds per day--is close to the official average leak rate estimate of about 11,350 tons of gas and oil per day (equal to 59,200 barrels of liquid oil).

"This study uses the available chemical data to give a better understanding of what went where and why," said Thomas Ryerson, a research chemist at NOAA and lead author of the study. "The surface and subsurface measurements and analysis provided by our university colleagues were key to this unprecedented approach to understanding an oil spill."

Elliot Atlas, professor of marine and atmospheric chemistry at UM’s Rosenstiel School of Marine and Atmospheric Science and co-author of the article, was among those providing data to NOAA. Along with colleague Donald Blake and his research team at the University of California-Irvine, Atlas collected air samples, which were then analyzed for the chemical composition of organic trace gases in the Gulf of Mexico atmosphere.

"A very distinct pattern of trace gases was observed over the spill area that provided a fingerprint of oil components that escaped from the Gulf waters to the atmosphere," Atlas said. "Combined with other measurements of the same components in the bulk oil, the air measurements could put some constraint on the rate of oil flow to the surface."

The underwater plume was enhanced with gases known to dissolve readily in water, the team found. This includes essentially lightweight methane (natural gas) and benzene present in the spilling reservoir fluid. The surface oil slick was dominated by the heaviest and stickiest components, which neither dissolved in seawater nor evaporated into the air.

The team also found that the leaking gas and oil quickly separated into three major pools: the well-known underwater plume about 3,300 to 4,300 feet below the surface, the visible surface slick, and an airborne plume. Each pool had a very different chemical composition.

The analysis further showed a majority of the leaking gas and oil, by weight, was retained in the deep subsurface plume. The visible surface slick represented about 15 percent of the total leaked gas and oil; the airborne plume accounted for about another 7 percent.

The chemical measurements made from mid-May through June showed that the composition of the atmospheric plume changed very little, suggesting little change in the makeup of the leaking gas and oil.

The new chemistry-based estimate of about 11,130 tons per day (8,900 to 13,300 tons, including uncertainty) compares well with the official estimate of 11,350 tons per day (10,000 to 12,700 tons, including uncertainty). This information about the transport and fate of different components of the spilled gas and oil mixture could help resource managers and others trying to understand environmental exposure levels.

The new analysis follows another NOAA-led study in which scientists estimated a lower limit to the Deepwater Horizon leak rate based on two days of airborne data and the chemical makeup of the reservoir gas and oil determined before the spill. This updated analysis adds many other sources of data, including subsurface and surface samples taken over six weeks during the spill and a direct measure of the makeup of the gas and oil actually leaking into the Gulf.