NEWS FEATURE

The perplexing physics of oil NEWS FEATURE Massive amounts of oil, gas, and 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 of Louisiana and triggered what was fast hose into the erupting plume and started pumping in becoming the worst 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 . “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 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. In August 2012, and carcinogens from the oil and are so small that CARTHE members sought to map those flows in they can penetrate deep into the lungs? “Go to unprecedented detail with the Grand LAgrang- the literature, and you find we don’t know much,” ian Deployment (GLAD)—an experiment that seeded says Katz. the blowout region with 317 custom-made floats Adding still more complexity are the currents that designed to drift with the currents the way oil would, and stir the Gulf on every size scale, from local riptides at then tracked them via GPS for 10 days (5). GLAD was the the beach to the giant Loop Current: a powerful flow largest experiment of its kind ever conducted until that rises between the Yucatan Peninsula and Cuba, early 2016, when the consortium followed up with wanders around the Gulf in an erratic and hard-to- more than 1,000 drifters in the Lagrangian sub- predict path, and finally exits between Cuba and mesoscale experiment (LASER) (6). Florida to become the Gulf Stream. One nightmare In both cases, says D’Asaro, the drifter paths showed scenario during the Deepwater Horizon incident was the currents very clearly. But strikingly, he says, “we that the Loop Current would capture the spreading found that sometimes there were places where the oil slick and end up fouling a good chunk of Florida drifters gathered together”—typically at a junction or conceivably even the East Coast. That this didn’t between waters of different density. happen was purely a stroke of luck: the Loop Current One recurring example is at the mouth of the was flowing south of the Deepwater Horizon site at Mississippi River, he says. “There is a fan of fresh water the time of the accident and was in the process of coming out, making a rather sharp boundary with the

Waldrop PNAS | May 28, 2019 | vol. 116 | no. 22 | 10605 Downloaded by guest on September 27, 2021 saltwater in the ocean.” Salt water is heavier, so it di- The model also suggests that the dispersants in- ves underneath and creates a “front” that can collect jected at the wellhead enhanced this effect by floating things. shrinking the droplet and bubble sizes by about a During an oil spill, says D’Asaro, that can be good third, which increased the surface-to-volume ratio and news or bad news: “If there is oil on the salty side, it made it easier for volatiles such as benzene to dissolve will be prevented from going on shore. But if the front on the way up. That didn’t appreciably cut down intersects the shore, it will become a conduit for the the total amount of oil reaching the surface, says oil.” Either way, he adds, modelers need to learn how Socolofsky, but it definitely improved the air quality for to predict these fronts so that clean-up crews in future the cleanup crews. “The workers’ respirator alarms quit oil spills will know the best places to pick the stuff up. going off,” he says. In short, says Socolofsky, researchers now have a In the Plume good general understanding of what the plume Meanwhile, another group of CARTHE investigators looked like. Unfortunately, he adds, “that doesn’t was finding a whole new set of complexities in the answer the question of where the oil went.” For that, plume of oil and gas rising from the Gulf seafloor. he says, you’d need to calibrate the models with the “Think of it like a volcanic eruption, where the heat actual size distribution of the oil droplets coming out and force of explosion [send] the rock and hot gases of the wellhead, with and without dispersants. “That’s high into the atmosphere,” says Scott Socolofsky, a a challenging measurement,” he says, “and it was not civil engineer at Texas A&M University in College done on Deepwater Horizon.” Station. The heat and force were there in plenty, says Frustratingly, it’s also a measurement that’s almost Socolofsky. From a combination of observation and impossible to make in the laboratory. Droplet forma- experiment, as well as a detailed computer model of tion depends on the surface tension between the oil and water, which doesn’t scale. So to reproduce the full range of droplets coming out of the 50-centimeter “Think of it like a volcanic eruption, where the heat and Deepwater Horizon pipe, an experimenter would force of explosion [send] the rock and hot gases high into need a model pipe at least twice the size of the largest ” stable oil droplets, which are about 12 millimeters the atmosphere. across. (Anything bigger will quickly break up from —Scott Socolofsky unstable oscillations.) That works out to a minimum pipe size of roughly 25 millimeters. But a nozzle that big would fill up any lab-sized tank in minutes, turning the plume (7) that incorporated factors such as fluid it an impenetrable black. Most laboratory experiments dynamics, the buoyancy of oil and gas, and their sol- use nozzles with diameters of 1 or 2 millimeters. ubility in seawater, Socolofsky and other researchers This uncertainty in the droplet size leaves plenty of know that what came roaring up the broken drill pipe room for interpretation. For example, University of was a 100 °C, high-pressure mix of oil and natural gas Miami oceanographer Claire Paris and her collabora- that abruptly decompressed as it slammed into the tors have created their own model of the plume (8). It frigid, 4 °C bottom waters of the Gulf. The gas reacted incorporates much of the same chemistry and physics like the fizz from a shaken soda can, flashing into a as the model developed by Socolofsky and his co- mass of bubbles that helped break the oil into a cloud workers, including factors such as solubility and of fine droplets. And from there, says Socolofsky, “as buoyancy. But it uses different experimental data, the gas bubbles and oil droplets started to rise, their suggesting that the violence of the eruption from the lightness created a plume that entrained the ambient wellhead smashed the oil into droplets so small that ’ seawater and carried it along with them.” the dispersants couldn t have made them much smaller. “ But bubbles and droplets have only a limited ca- And if that was the case, says Paris, the injection of dispersants did not significantly change the amount of pacity to lift the dense bottom water, says Socolofsky. oil that reaches the surface. Maybe 3%.” So at a certain point, he says, “they started getting off Complicating things still further is the presence of this upward rising train: ‘This is as high as I can go.’” In all that gas in the outflow, mainly methane, ethane, Deepwater Horizon, this happened at a depth of and propane. CARTHE member Michel Boufadel, an roughly 1,100 meters, or about 400 meters above the environmental engineer at the New Jersey Institute of seafloor. The smaller drops and dissolved compounds Technology in Newark, recently worked with Özgökmen — spread out into an intrusion a kind of underwater and several other colleagues to reanalyze the Deep- mushroom cloud that was very dilute and hard to see water Horizon data (9) and concluded that there directly but that was detected from chemical traces. had been a lot more gas in the jet than people had Meanwhile, says Socolofsky, the larger droplets assumed. “It was not just big blobs of gas, but a very ’ and bubbles kept rising. But they didn t make it to the violent tumbling and churning,” says Boufadel, who surface, either, because the gas inside them steadily was a member of the panel that prepared the National dissolved into the surrounding seawater as they rose. Academy of Sciences’ recent consensus report on So did everything soluble in the oil droplets: his plume dispersants. So who knows what really happened model estimates that 27% of the original mass of the when dispersants were injected into this maelstrom. oil disappeared in this way. “Dispersants like to stay at the interfaces,” he says. So

10606 | www.pnas.org/cgi/doi/10.1073/pnas.1907155116 Waldrop Downloaded by guest on September 27, 2021 maybe they were reacting with the gas all along and and that could image even very small droplets over a not the oil. “There are not many experiments, or even wide range of distances. models for this kind of churn flow,” he says. The results are still being prepared for publication, To sort all this out, says Boufadel, “we need a full- says Boufadel. But the data taken so far cover the full scale experiment.” gamut of conditions, including churn flow with 50%/ 50% oil and gas, bubbly flow with 5–10% gas, and What’s Left in the Tank smooth flow with no gas. In the experiment’s second That’s where the huge tank in New Jersey comes in. run in July 2019, the group will measure how droplet By US law, says Özgökmen, you can’t put oil in the formation is affected under each condition by differ- ocean even for an experiment. So the researchers ent levels of dispersants. have turned to the Ohmsett facility, an above-ground Hopefully, the results will clear up some of the Deep- saltwater tank operated by the US Interior Department uncertainties about where the oil went after water Horizon in Leonardo, NJ. Roughly the size of four Olympic- . But it will definitely be a culmination of CARTHE’s work on the accident, says Boufadel. Like length swimming pools placed end to end, the facil- all the other Gulf of Mexico Research Initiative con- ity is designed for testing oil cleanup methods. But a sortia, the group is now moving into a data consoli- CARTHE team led by Boufadel took over the tank dation phase that’s geared toward integrating the June 18–29, 2018 in the first phase of their effort to immense amount of science that’s been done on the recreate the Deepwater Horizon disaster at something Gulf of Mexico—and improving the computer models approaching full scale. that response teams will use in the next oil spill. The focus in this initial phase was to nail down the That’s when, not if, says Boufadel. Future blowouts dynamics of droplet formation without dispersants. may or may not be as inaccessible as Deepwater Ho- For each run, Boufadel and his colleagues injected rizon, he says—although with oil companies drilling in some 10 tons of oil through a pipe that was being deeper and deeper waters around the world, that’s towed along the bottom of the tank to simulate cur- always a possibility. “But there are a lot of pipes un- rent flow. The pipe was 25 millimeters across, big derwater,” he says. “And if you have an oil release, it enough to generate the full range of droplet sizes, doesn’t have to be a mile below the surface.” which the researchers measured with a camera that Since Deepwater Horizon, says Boufadel, “it’s was developed for the task at the University of Miami good to be ready.”

1 National Academies of Sciences, Engineering, and Medicine, The Use of Dispersants in Marine Oil Spill Response (National Academies Press, Washington, DC, 2019). 2 C. P. Ward, C. J. Armstrong, R. N. Conmy, D. P. French-McCay, C. M. Reddy, Photochemical oxidation of oil reduced the effectiveness of aerial dispersants applied in response to the Deepwater Horizon spill. Environ. Sci. Technol. Lett. 5, 226–231 (2018). 3 C. Li, J. Miller, J. Wang, S. S. Koley, J. Katz, Size distribution and dispersion of droplets generated by impingement of breaking waves on oil slicks: Oil droplets generated by breaking waves. J. Geophys. Res. Oceans 122, 7938–7957 (2017). 4 N. Afshar-Mohajer, C. Li, A. M. Rule, J. Katz, K. Koehler, A laboratory study of particulate and gaseous emissions from crude oil and crude oil-dispersant contaminated seawater due to breaking waves. Atmos. Environ. 179,177–186 (2018). 5 A. C. Poje et al., Submesoscale dispersion in the vicinity of the Deepwater Horizon spill. Proc. Natl. Acad. Sci. U.S.A. 111, 12693– 12698 (2014). 6 E. A. D’Asaro et al., Ocean convergence and the dispersion of flotsam. Proc. Natl. Acad. Sci. U.S.A. 115, 1162–1167 (2018). 7 J. Gros et al., Petroleum dynamics in the sea and influence of subsea dispersant injection during Deepwater Horizon. Proc. Natl. Acad. Sci. U.S.A. 114, 10065–10070 (2017). 8 C. B. Paris et al., Evolution of the Macondo well blowout: Simulating the effects of the circulation and synthetic dispersants on the subsea oil transport. Environ. Sci. Technol. 46, 13293–13302 (2012). 9 M. C. Boufadel et al., Was the Deepwater Horizon well discharge churn flow? Implications on the estimation of the oil discharge and droplet size distribution. Geophys. Res. Lett. 45, 2396–2403 (2018). 10 S. Kleindienst, J.H. Paul, S. B. Joye, Using dispersants after oil spills: Impacts on the composition and activity of microbial communities. Nat. Rev. Microbiol. 13, 388–396 (2015).

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