NEWS FEATURE The perplexing physics of oil dispersants NEWS FEATURE Massive amounts of oil, gas, and dispersant streamed into the Gulf of Mexico during the Deepwater Horizon disaster. Understanding the chemistry and physics of this mix as it churned through the salt water turns out to be an exceedingly complex problem with plenty of unknowns. M. Mitchell Waldrop, Science Writer On April 30, 2010, 10 days after a blowout destroyed wellhead, which was still spewing more than 6,000 liters the offshore drilling platform Deepwater Horizon off of oil per minute, it stuck the end of a kilometers-long the coast of Louisiana and triggered what was fast hose into the erupting plume and started pumping in becoming the worst oil spill in US history, the well’s dispersants: detergent-like chemicals designed to frag- owner, British Petroleum, sent a remotely piloted sub- ment the hydrocarbons into tiny droplets. It was the start marine 1,500 meters down to the floor of the Gulf of of a campaign that would ultimately inject the plume Mexico. Once the vehicle arrived at the broken with almost 3 million liters of the chemicals. No one knew the ecosystem impact of using huge amounts of dispersants in deep water to break up the massive oil slick caused by the 2010 British Petroleum disaster, seen here via satellite one month after the blowout. Image credit: Science Source/NASA. Published under the PNAS license. www.pnas.org/cgi/doi/10.1073/pnas.1907155116 PNAS | May 28, 2019 | vol. 116 | no. 22 | 10603–10607 Downloaded by guest on September 27, 2021 debate among scientists. And the risks are even murkier. Dispersants by themselves don’t pose much of a near-term risk. They produce little more than burning eyes and coughing in humans, and except in the immediate vicinity of the Deepwater Horizon plume, the National Academy of Sciences report concluded, they never got close to acute toxicity thresholds for sea life living at the water’ssurface. But biologists are still trying to figure out the long- term threat to human and ecosystem health posed by millions of liters of the stuff combined with un- known quantities of crude oil laced with its own brew of toxins and carcinogens. Underlying it all is a mystery: where did the oil actually go, and how did the dispersants affect its movements? Before the wellhead was finally capped on July 15, 2010, it had released an estimated 760 million liters of oil—and as much as 25% of it remains An airplane releases oil dispersant over oil from the Deepwater Horizon unaccounted for. disaster off the shores of Louisiana in May 2010. All told, about 3 million liters Although definitive answers are hard to come by, of dispersant was used on the spill. Image credit: Science Source/United major clues have emerged in the years since the States Coast Guard. accident as researchers have studied the real-world physics of oil, water, and dispersants. They have Using dispersant at that depth was a roll of the analyzed and reanalyzed the data recorded during dice; the chemicals had been used before on surface the disaster, studied oil-droplet formation in the oil slicks with varying degrees of success but never laboratory (with and without dispersants), tracked in such cold, deep waters. No one could be sure currents in the Gulf with fleets of high-tech buoys, what effect they would ultimately have on the ocean and constructed innumerable computer simula- ecosystem, on coastal fisheries, or even on the oil tions. Researchers know vastly more than they once itself. The responders could only hope that the in- did about what happened to the oil in the deep sea jection would work as intended and that the resulting plume as it rose from the wellhead; how the oil oil droplets would be consumed by the Gulf’s many interacted with sunlight, wind, and waves as it petroleum-eating bacteria without ever making it to spread across the surface; and exactly what role the surface. the dispersants played. So did it work? That depends on whom you ask. And in June 2018, researchers embarked on the The oil companies certainly think it did, says Tamay largest experimental simulation of the Deepwater Ho- Özgökmen, a mechanical engineer at the University of rizon spill to date at a huge saltwater tank in New Miami in Coral Gables, FL, who has spent much of Jersey. In the two-phase experiment, which will con- the past eight years studying the Deepwater Hori- clude with a second series of experiments in July zon incident and its aftermath. The companies 2019, the scientists will gather a trove of data in hopes point to plummeting concentrations of toxic vapors of pinning down some of the last remaining uncer- over the oil slick—cleanup crews could finally work tainties stemming from a disaster whose scale and without respirators—and to aerial photographs sug- speed took everyone by surprise. gesting that less oil was reaching the surface. So from the companies’ perspective, says Özgökmen, On the Surface deep-sea dispersants have gone from being a des- The real-world chemistry and physics of the air-sea peration move to being standard operating proce- interface are about as complicated as it gets. As soon dure. “They’re preparing for it in future oil spills,” as oil from any spill hits the surface, for example, it he says. starts baking in the sun, boiling off volatile compounds The National Academy of Sciences’ Ocean Science and losing almost half its volume as it turns into a tarry Board tends to agree: In a draft consensus report re- gunk that resists dispersant action (2). The fumes were leased on April 5, the Board’s panel on oil-spill- bad news for the Deepwater Horizon cleanup crews; dispersant use concluded that yes, the deep-sea in- not only were the gases a fire hazard but also they jection had generally been effective at dispersing included some 40 times the allowed exposure levels the oil, making the hydrocarbons easier for bacteria for benzene, a known carcinogen. As much as 25% of to digest, preventing surface oil from fouling nearby the oil in that incident seems to have evaporated in shores, and enhancing workersafetybymitigating this way. exposure to hazardous oil-related chemicals (1). In addition, explains Eric D’Asaro, an oceanogra- But the report—and the numerous researchers pher at the University of Washington in Seattle, the studying dispersants’ effects—also emphasized the surface of the ocean isn’t like a flat puddle of rain- many remaining uncertainties. The benefits of mas- water. It moves, surges, and heaves. Breaking waves sive deep-sea dispersants are still a matter of intense and ocean currents are constantly shattering the oil 10604 | www.pnas.org/cgi/doi/10.1073/pnas.1907155116 Waldrop Downloaded by guest on September 27, 2021 slicks back into droplets and dragging them under again, he says, “until there’s an equilibrium between things that are carried up and carried down.” The finest droplets go deepest, says D’Asaro, who’sa member of the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE). This means that the so-called oil slick is ac- tually a thick layer of oil droplets extending down as much as 10 meters. Dispersants add another level of complexity (see Fig. 1), says Joseph Katz, a mechanical engineer at Johns Hopkins University in Baltimore, MD, who studies the effects of these chemicals with funding from a consortium funded by the Gulf of Mexico Re- search Initiative, which separately funds CARTHE. He works with a laboratory wave tank that allows him to introduce oil slicks and then watch through a system of lasers and microscopes as the breakers smash the slicks into an underwater cloud of oil droplets. “Without dispersants,” says Katz, “I found the size distribution to be understandable.” That is, the droplets showed a range of sizes down to about 100 microme- ters, or about as small as a turbulent eddy can get be- fore it’s dissipated by fluid viscosity. “But with dispersants, I couldn’t predict the distribution,” he says. Instead of a cutoff at 100 micrometers, he saw droplets as small as 1 micrometer (3). A closer look showed what was happening, says Katz: in the presence of dispersants, which lower the surface tension between oil and water, the droplets were developing all sorts of threads and tails. “They Fig. 1. Dispersants consist of surfactant molecules composed of a hydrophilic head group and a lipophilic tail (A). In seawater and oil, the hydrophilic look like sperm cells,” he says. In fact, the dispersants component turns toward the seawater and the lipophilic side toward the oil were concentrating in the tails, which would grow phase, spurring the formation of small oil droplets (B). Dispersants break up oil longer and longer until they broke up to produce slicks, sending dispersant-stabilized oil droplets into the water column (C). the microdroplets. Reprinted by permission of ref. 10, Springer Nature: Nature Reviews Above the surface, Katz found that dispersants Microbiology, copyright 2015. cause a 100-fold increase in the concentration of ultra- fine oil droplets floating in the air (4). It’s less clear how spinning off a giant eddy that kept the oil relatively these floating droplets form—the popping of bub- close to shore. bles, maybe?—but their presence raises new health However, that simply meant that the fate of the concerns: what happens when people breathe in Deepwater Horizon oil was subject to a host of poorly infinitesimal droplets that are filled with toxins understood, smaller-scale currents.