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Oil and gas that flowed into the Gulf of Mexico during the 2010 BP Deepwater Horizon disaster never reached the surface, according the first comprehensive report tracking the amount of oil and gas released and where it went.

More oil and gas — 36 percent — stayed in deep underwater plumes, the government-funded study said.

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“The visible surface slick that people were riveted by during the months of the spill was really only 15 percent of the total mass,” said Thomas Ryerson, a research chemist with the National Oceanic and Atmospheric Administration who led the study.

After a catastrophic explosion on April 20, 2010 that killed 11 workers, an estimated 206 million gallons of crude shot out from the Deepwater Horizon riser pipe over three months. The spill occurred a mile deep and about 60 miles off the coast of Louisiana.

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Nearly two years later, learning what material released, where it went and in what quantity is key for understanding the impact on ecosystems.

The study also provides critical benchmarks, such as the ratio of oil to gas, that will prove important as the federal government seeks money from BP to pay for the response and recovery.

Ryerson and 13 other scientists studied the chemical make-up of oil and gas sampled from the pipe, the deep plumes, the surface slick and the atmosphere. They collected the samples over six weeks during the disaster.

The findings support other reports made during and after the spill detailing vast plumes of microscopic oil droplets floating through the deep sea.

“It makes perfectly legitimate sense that a large portion of the deep sea blowout wouldn’t always go to the surface,” said David Hollander, a chemical oceanographer at University of South Florida, who was one of the first to discover the underwater plumes.

The Deepwater Horizon tapped a well of light crude and gas. The riser rupture released a mix of substances called hydrocarbons. Some of the substances were thick and bouyant; others readily dissolved.

Measured in tons, gas made up nearly a quarter of the total release of 879,300 tons. Most of the gas — methane and ethane — dissolved before reaching the surface. All of the benzene, a carcinogenic liquid, also dissolved before reaching the surface, the study showed.

Volatile compounds, such as propane and dozens of other substances, dissolved in the water or evaporated at the surface. Only the thick, more bouyant crude formed the surface slick.

The movement of the oil through the ecosystem is important for determining which creatures suffered exposure. For instance, dolphins and sea turtles — as well as response crews and people on shore — likely did not breathe benzene because it did not surface.

Instead, animals would have come in contact with benzene and other toxins deeper down.

Scientists were unable to determine whether chemical dispersants applied at the wellhead played a role in keeping most of the oil chemicals and gas from surfacing.

“We can’t tease out any evidence that dispersant had any effect, or if it did, what effect,” Ryerson said.

The plumes of oil are invisible, not a river of goo resembling anything like the surface slick.

“If you dredge up a sample within the heart of the plume, it looks like you can drink it,” Ryerson said.

That does not mean it is harmless. The chemicals could harm bottom-dwelling creatures or fish larvae, as other studies have suggested.

It is also possible that some of the more stable hydrocarbons are still floating today, Ryerson said, although the report did not address that issue.

For all the light Ryerson’s report sheds, the fate of 25 percent of the released oil and gas is still a mystery.

Finding where it went is unlikely after two years. The oil and gas moved to the surface, dissolved, stayed suspended in the water or evaporated within two days. After two days, tracking the oil and gas became too complicated.

For the study, scientists measured oil and gas around the well and in the atmosphere for about six weeks during May and July 2010. The oil and gas mixture did not change over time, making it easier for scientists to extrapolate the results over the course of the entire three-month gusher.

“The consistency we see throughout the subsurface and airborne measurements is really striking,” Ryerson said.

The analysis closely matches the official flow rate estimate NOAA developed after the disaster. Scientists did not use any of the early NOAA data or videos of the underwater gusher to calculate the flow rate.

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