The BP Deepwater Horizon offshore oil rig burning in 2010. (U.S. Coast Guard via AP)

The spread of oil from the Deepwater Horizon disaster in the Gulf of Mexico was far worse than previously believed, new research has found.

As the worst offshore oil spill in U.S. history approaches its 10th anniversary in April, a study by two University of Miami researchers shows that a significant amount of oil and its toxic footprint moved beyond fishery closures where it was thought to be contained and escaped detection by satellites as it flowed near the Texas shore, west Florida shore and within a loop current that carries Gulf water around Florida’s southern tip up toward Miami.

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In their study, published Wednesday in Science, the researchers dubbed it “invisible oil,” concentrated below the water’s surface and toxic enough to destroy 50 percent of the marine life it encountered. Current estimates show the 210 million gallons of oil released by the damaged BP Deepwater Horizon Macondo well spread out over the equivalent of 92,500 miles.

But the oil’s reach was 30 percent larger than that estimate, the new study says. “Oil in these concentrations for surface water extended beyond the satellite footprint and fishery closures, potentially exterminating a vast amount of planktonic marine organisms across the domain,” the researchers wrote. The findings show that the government’s understanding of how oil flowed from Deepwater Horizon was limited and that it underestimated the extent to which marine life was killed or poisoned by toxic crude.

The study comes as the Trump administration is preparing to finalize a sweeping proposal that would allow the oil and gas industry to buy leases in every part of the Atlantic, Pacific and Arctic oceans, in addition to a leasing expansion in the Gulf. The administration’s bid to issue permits to several companies to map the Atlantic sea floor to search for oil and gas deposits has been stalled for more than a year by a federal court challenge.

In addition to the largest expansion of lease permits in American history, the administration has rolled back oil platform safety regulations meant to protect workers and avoid another event such as the fatal Deepwater Horizon explosion.

The April 20, 2010 Deepwater catastrophe was triggered by a blast that killed 11 workers and sank the oil platform. Thick, toxic oil billowed from a damaged well for five months off the Louisiana coast before workers finally managed to seal it. Spreading with squid-like tentacles, the oil reached Texas, Mississippi, Alabama and Florida.

In a massive spill response, federal workers, contractors and volunteers sought to detect it, contain it and use chemicals to disperse it. Yet large amounts of oil reached beyond the containment effort and were never fully accounted for until now, the study says.

Claire B. Paris-Limouzy, a professor of ocean sciences at the University of Miami Rosenstiel School and the study’s senior author, said a model she started developing during the Deepwater Horizon blowout provided a fuller picture of the oil’s footprint than the two-dimensional satellite imagery provided by the National Environmental Satellite, Data and Information Service.

It allowed researchers to trace Deepwater Horizon’s oil from its source and show how it was manipulated by wave action, mixed with ocean plumes and sank and rose to and from the ocean floor. Between June 28 and July 1, 2010, slightly more than two months after the disaster started, Hurricane Alex swept through the Gulf with powerful southeasterly winds “enhancing and mixing and bleaching the Deepwater Horizon oil,” the study said. The model tracked it all.

© Charlie Riedel/AP An oiled Brown pelican was rescued from Barataria Bay off the coast of Louisiana after the Deepwater Horizon oil spill in the Gulf of Mexico in 2010. (Charlie Riedel/AP)

Paris-Limouzy’s co-author, Igal Berenshtein, a post doctoral associate at U-M Rosenstiel, said he immediately noticed a discrepancy between the model’s footprint and the satellite’s.

“That is what kicked off our study,” Berenshtein said. “The extent was larger than the satellite footprint and the fishery closures” — areas that federal marine scientists designated as contaminated zones. “One of them must be wrong, right? There was strong support that the footprint extended beyond the satellite data and the closures.”

Oil in smaller and lighter concentrations that the satellite could not detect reached larger areas of the West Florida Shelf around Tampa than previously known. It also extended beyond Naples and curled around Florida’s southern tip. Farther west, it reached an area known as the Texas Shores.

Although the oil was lighter in concentration than oil on the surface, it was extremely toxic, Berenshtein said. “Basically, when you have oil combined with ultraviolet light it becomes two times more toxic than oil alone. Oil becomes toxic at very low concentrations."

Berenshtein said he was startled by the model’s results. “I think it kind of changes the way you think about oil spills,” he said. “I didn’t think this way before I did this study. I assumed that the satellite image captures the oil spill and that’s it. People have to change the way they see this so that they know there’s this invisible and toxic component of oil that changes marine life.”

The Deepwater Horizon oil spill “was no regular oil spill,” Paris-Limouzy said, and cannot be examined simply with satellite images. “It happened in the deep ocean. Between the deep sea floor and the surface is a lot of water.” Oil in that water is tossed by hurricanes, tropical storms and natural wave action, among other things, far from the surface.

“If you want to respond to this kind of spill, you have to know where the entire mass is, the amount of oil that came out of the well, and know that the footprint is not only on the surface, but in three dimensions,” she said.