When the Deepwater Horizon oil well spewed at least 518 million liters of oil into the Gulf of Mexico in 2010, responders added an additional 7 million liters of chemicals, known as dispersants, to try to control the oil. That move prompted questions about whether the brew of dispersants and oil was more toxic to the environment than the oil itself.
It appears the answer is largely no, concludes a committee of top oil spill experts in a report examining dispersants issued today by the National Academy of Sciences in Washington, D.C.
Dispersants, which can break slicks and clumps of oil into smaller droplets that sink, have long been dogged by questions about safety and effectiveness. But today’s report concludes the chemicals can help cope with oil spills, depending on the circumstances. The panel cautioned, however, that questions remain about the health effects on people and the effectiveness of dispersants in some situations.
“The take home is that this is a tool that has a place in the toolkit,” says David Valentine, a geochemist and microbiologist at the University of California, Santa Barbara, who served on the 17-person committee that produced the report. “It has the potential to make a bad thing from getting worse.”
The report comes as oil wells probe deeper and deeper waters. Some so-called ultradeep oil projects in the Gulf of Mexico now stand in nearly 3 kilometers of water—twice the depth of the Deepwater Horizon well. That reduces the effectiveness of strategies to corral oil at the surface, and puts dispersants on center stage, says Steve Murawski, a fisheries biologist at the University of South Florida in St. Petersburg who served on the committee.
During the Deepwater Horizon spill, dispersant was sprayed across the ocean to break up surface slicks. In an act of desperation, responders also injected dispersant directly into the geyser of oil erupting from the broken wellhead just above the sea floor. The hope was to keep the oil from reaching the surface, where it could endanger workers, surface-dwelling animals such as turtles and birds, and nearby beaches and marshes.
Concerns were quickly raised that the dispersant alone, or a toxic mixture of oil and dispersant, imperiled people and wildlife. Some studies have found such a mixture can harm deep sea corals and slow oil-eating bacteria.
But the academy concluded that overall, research showed the combination was more toxic to marine creatures than oil alone only at levels greater than 100 parts per million. That concentration is far greater than what was found during the Deepwater spill, except right around the well, Murawski says. “Generally that plume was much less than 100 parts per million, and more like one or less.”
Chris Reddy, a chemical oceanographer and expert in oil chemistry at the Woods Hole Oceanographic Institution in Massachusetts, welcomed the findings for putting laboratory results into a real world context. Reddy, who reviewed the report but didn’t serve on the panel, said that when he worked on Gulf Coast beaches after the spill, he heard more talk of problems associated with the dispersant than the oil. “I still can't wrap my head around that,” he says.
The report was less clear cut about dispersant’s effects on people. Two recent epidemiological studies—one of Coast Guard workers and another of Deepwater spill responders—found evidence that people who reported being exposed to a dispersant suffered higher incidences of respiratory problems such as coughing and wheezing, as well as skin irritation. But the academy report warns the studies had shortcomings that made it hard to separate the effects of dispersant from the toxic fumes produced by the oil itself.
That same uncertainty emerged about a key question of how to tackle a future deep ocean oil spill: whether its effective to add dispersant deep underwater at the oil’s source. “That was a pretty vigorous debate within the group,” Murawski says. “I don’t think we’ve come to a final resolution on that.”
The debate revolves around the size of oil droplets as they surge from a wellhead under the extreme pressures of the deep ocean. Droplets smaller than 70 microns wouldn’t be buoyant enough to float to the surface even in the absence of dispersant, instead forming an underwater mist of oil. But if the droplets are larger, the chemical treatment could help break them up. Studies since the Deepwater spill have given conflicting pictures of droplet dynamics. The uncertainty stymied the academy panel, prompting it to call for further research, including the possibility of building a facility able to better simulate the pressures, temperatures, and oil flow in a deep ocean spill.
The continued questions underscore the need to have researchers ready to deploy instruments before a major oil spill, to complement efforts to reconstruct events after the fact, several scientists involved in the report said. For example, deploying special cameras at a broken wellhead could help decipher droplet behavior. And finding biochemical markers that provide a more objective gauge for tracking workers’ exposure to dispersant could enable better health studies. One goal of the report is to encourage government agencies and other research institutions to prepare ahead of time, says Gina Coelho, a panel member and principal scientist at the Sponson Group, a private oil spill science consulting company based in Mansfield, Texas. “The time to be building the design for a study of human health,” she says, “is not during an oil spill.”