In the heat of the 2010 Deepwater Horizon disaster, U.S. government and industry responders had to make a crucial decision. They were facing an enormous oil spill, gushing uncontrollably from a wellhead at the seafloor—at a depth where no oil spill had ever happened before. They were pitted in a high-stakes battle against big unknowns.
On Day 25 of the spill, the decision was made: They began to inject chemical dispersants, 10,000 gallons per day, at the severed wellhead 5,000 feet beneath the ocean surface in the Gulf of Mexico—an unprecedented and unproven experiment. The goal was to break up petroleum that surged uncontrollably from the wellhead into smaller droplets in the deep sea, with the hope of diminishing oil slicks and reducing the amount of harmful gases arriving at the ocean surface.
Was this the most opportune response? Seven years later, this decision continues to fuel contentious debate among a wide range of stakeholders, including Gulf beach residents, fishermen, the media, policymakers, NGOs, environmentalists, industry officials, and scientists.
Opponents say the dispersants themselves were toxic, may have exacerbated environmental damage, and were ineffective at breaking up the already dispersed oil erupting from the wellhead. Proponents say the dispersants helped diminish oil slicks on the surface, causing less oil to taint shoreline beaches and marshes. In a new study, we reveal a key benefit of using dispersants: The subsea dispersant injection likely allowed emergency responders literally to breathe easier.
Chemical dispersants have been applied to marine oil spills for at least a half century, but the debate recently has become more politicized and acrimonious. The National Academy of Sciences (NAS) warned about this in its 2005 study: Oil Spill Dispersants: Efficacy and Effects, predicting that “political issues” would become a factor influencing decisions about whether to use dispersants. In the aftermath of Deepwater Horizon, the NAS recently assembled a new committee to investigate questions about whether and when dispersants should be applied during oil spills, including deep-sea spills.
The Deepwater Horizon disaster can be viewed as two giant experiments in which Mother Nature responded to two different chemical inputs. The first experiment was the unintended and uncontrolled release of petroleum into the deep sea for 87 days, after the explosion on the Deepwater Horizon drilling rig caused the tragic deaths of 11 people. The second experiment was the unique decision by responders to inject chemical dispersants 5,000 feet below the ocean surface.
Unlike typical scientific experiments, these spontaneous experiments were unplanned and unreplicable. And in the midst of the crisis, with a priority and focus on controlling the spill and mitigating damages, scientists were not allowed the time to design and implement robust experiments to measure the impacts of the dispersant injection.
The dispersant, Corexit EC9500A, roughly resembles a mix of mineral oil, windshield-wiper fluid, and household dish detergent. During a typical oil tanker accident, this dispersant fluid might be applied to the sea surface to cause oil slicks to break up into smaller droplets that dissipate into waters of the open sea, so that less oil reaches ecologically sensitive coastlines. But what was the effect of injecting this dispersant into the bowels of the ocean at a depth of 5,000 feet?
Aerial photographs taken during the crisis suggest that the deep-sea dispersant injection may have helped dissipate the oil slicks at the ocean surface. Impromptu air quality measurements, taken to ensure the safety of emergency responders working at the disaster site, suggested that the air quality may have been improved on the boats. With limited people and little time to waste, responders accepted these data and continued to inject dispersant.
These air-quality measurements were neither comprehensive nor meticulous enough to reach the high bar needed for a scientific experiment. In the fray of crisis, the deliberate pace of scientific research often takes a back seat to urgent emergency needs. But this circumstantial evidence is not enough to inform high-stakes policymaking.
For the past seven years, we have worked toward better understanding what happened to oil and gas after it was released from the damaged wellhead on the seafloor. In a study published this week in Proceedings of the National Academy of Sciences, we finally have some answers to the central question: What did subsurface dispersant injection really achieve?
For our study, we built and tested a mathematical model that simulates the complex interactions among oil, gas, seawater, dispersant, ocean currents, and other factors that occurred during Deepwater Horizon. We focused on the situation from June 3, 2010, when engineers cut the riser pipe at the wellhead, until July 15, 2010. During this period, a large number of scientific observations were recorded nearby in the air and ocean—data that we needed to confirm that the model adequately simulates what actually occurred.
In the model, we carefully weave together several different dynamic physical and chemical effects that are “coupled” together. These included the changing chemical compositions, fluid dynamics, and interlinked movements of oil droplets, gas bubbles, and seawater in the deep sea. To understand what happened, these interdependent problems all have to be solved at the same time. The resulting model is very complex. The equations can be solved with a desktop computer—but only after three weeks of nonstop number-crunching!
To test the model’s representation of the real-world disaster, we compared the model predictions to several types of observations that were collected in both the sea and atmosphere near the disaster site. Nearly all of these comparisons revealed good agreement between the model and observations, which indicates that the model convincingly explains many aspects of what happened to oil and gas under the sea surface.
We then used the model to conduct a key test that was never made in 2010: We ran the model without adding dispersants to see what likely would have happened if dispersants had not been injected into the deep.
The results were stunning. The model showed that deep-sea dispersant injection had a profound effect on air quality at the sea surface. In the hidden darkness of the deep sea, the injection of the dispersant fluids caused the turbulent jet of petroleum fluids gushing from the wellhead to form oil droplets that were about 30 times smaller (by volume) than they would have been if dispersants were not present. This subtle change caused many volatile organic chemicals in the oil to dissolve more rapidly than if dispersant had not been injected. Instead of rising to the ocean surface, these rapidly dissolving chemicals became entrained in deep-sea currents, where they likely affected marine organisms on and near the seafloor.
These processes occurring in the deep sea were invisible to people working at the ocean surface. But as a consequence of dispersant injection, lower quantities of the most harmful volatile chemicals rose to the surface and outgassed into the atmosphere in the vicinity of people working to mitigate damages from the spill.
The model indicated that the dispersant injection decreased the overall concentration of all volatile organic chemicals in the atmosphere by only a modest amount (about 30 percent). But it substantially reduced the amounts of chemicals most harmful to humans, such as benzene and toluene. In the model simulations, the atmospheric concentration of benzene, for example, decreased by about 6,000 times above the area of sea surface where petroleum fluids were surfacing.
Without the dispersant injection, the model showed that benzene concentrations in the air 2 meters above the sea surface would have been 13 times higher than the levels considered acceptable to breathe during a 10-hour working day or a 40-hour work week, based on guidelines by the National Institute for Occupational Safety and Health (NIOSH). However, with dispersant injection, the model showed atmospheric benzene concentrations were 500 times lower than the levels considered acceptable to breathe by NIOSH.
These predictions depend on local weather conditions that can vary from day to day. However, we predict that cleanup delays would have been much more frequent if subsurface dispersant injection had not been applied.
Dispersant injection dramatically improved the air quality for the courageous emergency responders who were working around the clock to stop the flow of petroleum from the wellhead and mitigate damage. On days when the air quality was too poor, they had to don respirators or stop working, which delayed efforts to seal the wellhead. We predict that human health risks and work delays likely would have been much more frequent if subsurface dispersant injection had not been applied. That would have made a bad situation even worse.
This one study is not the final say on whether and when to use dispersants. It is another row on a ledger sheet called the “spill impact mitigation analysis,” which assesses various strategies and tools to reduce environmental and economic damage caused by oil spills. All potential positive and negative effects of dispersant injection need to be taken into account before decision-makers can reasonably judge their future use.
This study represents a collaborative effort of four far-flung research groups in Switzerland, Texas, New Jersey, and Massachusetts. It required a combination of knowledge in distinct fields: chemistry, physics, marine science, and computer programming. It comprised a seven-year commitment to incremental problem-solving and trans-Atlantic email debates. For us, the payoff is to answer finally an important question that has frustrated policymakers and the public for seven years. We hope this work supplies information that can help protect the health and safety of the people who will bravely accept the risks of future emergency responses during big oil spills on the open sea.
The study was funded by the Gulf of Mexico Research Initiative and the National Science Foundation.