Two years ago this week, the wellhead that ruptured on the sea floor of the Gulf of Mexico, sending 4.9 million barrels of oil into the water, was capped. After three months, the end of the largest oil spill in the industry's history was in sight.
With the help of nearly 2 million gallons of toxic chemicals known as dispersants, BP and government agencies involved in the response managed to prevent most of the oil from reaching the shoreline. But while the Gulf Coast's seafood and tourism industries are slowly recovering, the effects of those dispersants - used underwater for the first time ever - remain murky.
"My stomach churns when I hear people say, 'We dodged a bullet,' because I've heard it so many times, but we shouldn't be so quick to wave this off," said James Cowan, a professor at Louisiana State University's School of the Coast and Environment. "This notion of 'Come back to the Gulf, eat seafood, it's fine' is a problem."
The health risks of dispersants used in the response to the Deepwater Horizon spill are not yet fully known, said Cheryl Murphy, an environmental toxicologist at Michigan State University, in part because the contamination that began in phytoplankton may take years to rise up the food chain to the seafood eaten by humans.
Scientists are already spotting red flags. Cowan said the rates of dolphin and sea turtle deaths have risen to highly unusual levels in the Gulf of Mexico.
There is also clear evidence, he said, that contamination has been making strides up the food chain, with 2 to 5 percent of the gulf fish population affected. His greatest worry now, he said, is for fishermen who handle fish with lesions containing highly concentrated pathogens linked to the dispersants, which accelerate the breakup of oil.
Significant gaps remain in researchers' understanding of the dispersants' environmental consequences, according to a report released by the Government Accountability Office earlier this month. Despite $15.5 million in federal funding for studies on dispersants - including $8 million since the gulf spill - scientists have yet to answer basic questions about how the chemicals affect underwater environments and what their potential risks to human health may be as the chemicals and their combinations with oil droplets move up the food chain.
The main dispersant used in the spill response was Corexit, a cocktail of 57 chemicals manufactured by Illinois-based Nalco. It was neither the most effective nor the least toxic oil dispersants, and it is banned from use in oil spills in the United Kingdom and Canada. But BP claimed that only Nalco could provide the quantities of dispersant needed the week of the disaster. By the time the well was capped, about 1.1 million gallons of dispersants were sprayed at the wellhead in addition to the nearly 1 million gallons poured onto the surface.
On Aug. 2, 2010, the Environmental Protection Agency defended the use of dispersants, arguing that the threat they posed to the environment was no greater than that of the oil. But the picture is much more complicated than that, many scientists say.
When oil combines with the dispersant chemicals, the result is more toxic than either substance individually, and the depth at which dispersants were released raises concerns that currents are carrying their dangers far beyond the wellhead, Cowan said.
After several members of cleanup crews aboard fishing vessels off the coast of Louisiana fell ill in late May 2010, fears about the health consequences of the dispersants in the gulf spiked. The National Institutes of Health is currently conducting a study of the more than 54,000 members of response crews deployed to the Gulf to determine whether their health suffered from exposure to crude oil and dispersants.
Some scientists also worry that the dangers will increase exponentially when the gulf is hit with its next major tropical storm, which could release massive amounts of oil and dispersant chemicals currently covered by sediment.
Nalco denied that its product has contributed to any human health problems on the gulf and believes the lawsuits against it lack merit, spokesman Roman Blahoski said, adding that the company had no part in decisions affecting where and how Corexit was used following the spill.
Corexit 9527, used more heavily in the beginning of the response, contains 2-butoxyethanol, a chemical believed to have caused health problems following the 1989 Exxon Valdez spill. The EPA, which considers it a chronic and acute health hazard, asked BP to find a less toxic alternative to Corexit, but BP agreed only to reduce the volume it was spraying.
Despite Nalco's effort to keep the ingredients a trade secret, the EPA released them after it was prompted to do so by Sen. Kirsten Gillibrand (D-NY) under the Toxic Substances Control Act.
It may be years before scientists understand how the dispersants used in the spill response will affect the marine environment and the food chain, in part because of the difficulty of studying dispersants at extreme depths. According to Ralph Portier, a professor of environmental science at Louisiana State University, the pressure levels at the sea floor renders methodologies used so far to study dispersants inapplicable.
"This is a case where the science hasn't kept up with the engineering, the deep ocean exploration and oil fuel production," he said. "The GAO report is correct. There's a whole block of data missing."
Another challenge to scientists, Portier said, is distinguishing the effects of the dispersant chemicals from those of the oil and those of the oil-dispersant mixtures.
Scientists will not be able to judge the recovery of the gulf ecosystem until the spill reaches its third anniversary, Portier said, but the consensus among scientists appears to be a "quiet hope."
At least as far as seafood is concerned, that hope appears to be warranted. So far, the seafood testing program coordinated by state and federal governments and BP has succeeded in preventing contaminated seafood from going on the market, said Kyle Graham, a spokesman for the Louisiana Coastal Protection and Restoration Authority.
Still, according to Cowan, some researchers believe the public has been misled about the true damage still unfolding in the gulf by states eager to protect domestic industries. Because the seafood crucial to the Gulf Coast economy has been deemed safe, the pollution's net impact on the aquatic ecosystem has had a lower profile in the press.
"The states are doing what they need to do, and I understand where they're coming from, but there is a potential for long-term toxicological effects in the gulf," Cowan said. "We are going to be dealing with this for quite a while."