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Reckoning With Oil Spills

Experts learn from Exxon Valdez and Deepwater Horizon to plan for future accidents

by Jyllian Kemsley
July 13, 2015 | A version of this story appeared in Volume 93, Issue 28

Workers spray oil-covered rocks on the shore of an Alaskan island, after the Exxon Valdez oil spill in 1989.
Credit: Natalie Fobes/CORBIS
Workers try to clean oil-covered rocks on an Alaska shoreline by spraying them with water after the Exxon Valdez oil spill.

Five years ago this week, engineers capped the leaking Macondo well 1,500 meters below the surface of the Gulf of Mexico, effectively ending the largest marine oil spill in U.S. history. The Gulf spill started three months earlier, when the Deepwater Horizon (DWH) oil rig exploded into fire—an accident that killed 11 workers—and then sank into the sea. During the 86 days of the spill, 4.9 million barrels of oil and 200,000 metric tons of hydrocarbon gas spewed into the Gulf.

Earlier this month, BP, the principal developer of the oil field, announced a proposed $18.7 billion settlement with federal and state authorities for environmental and economic damage related to the spill. The deal would bring the total that the company will spend on the spill to more than $40 billion.

The DWH spill came two decades after the second-largest spill in the U.S., when the oil tanker Exxon Valdez ran aground in Prince William Sound, Alaska, and released 260,000 bbl of crude into the water there. The area’s cold, rocky beaches and larger ecosystem still show vestiges of the spill.

Although the DWH and Exxon Valdez oil spills are the largest U.S. spills, they are far from the only ones that have made headlines recently. Also in 2010, a pipeline ruptured in Michigan and leaked 20,000 bbl of oil into the Kalamazoo River. In 2013, a train collision in North Dakota spilled 9,500 bbl of oil, resulting in a fire. Right now, workers continue to clean up 2,400 bbl from a pipeline rupture in California that has affected nearly 100 miles of coastline.

As long as we depend on oil for fuel and feedstock, more spills will occur within the U.S. and worldwide. The question is how to prepare for spills of all sizes to minimize harm to aquatic ecosystems and to the livelihoods of people who depend on those waterways. In particular, environmental scientists are concerned about possible spills in the Arctic, where Shell plans to start exploratory drilling this summer in the Chukchi Sea after receiving conditional approval from the Obama Administration in May. So what, then, can experts learn from the Exxon Valdez and DWH spills, looking back at them with the benefit of hindsight and thinking ahead to future spills?

Both spills started because of human error. The oil carried by the Exxon Valdez originated at the North Slope of Alaska. It was piped south to the Valdez Marine Terminal, then loaded onto the tanker. When the tanker left the port on March 23, 1989, ice was breaking off from the Columbia Glacier and entering the shipping channels, said Edward Page, a retired U.S. Coast Guard captain, at an oil-spill-response forum hosted by the University of New Hampshire (UNH) in October 2014. Page was involved in the Valdez spill response in 1989. The captain of the Valdez took the tanker out of the normal channel to avoid the ice, then left the ship’s bridge. The crew left in charge failed to follow the correct course, and the tanker hit a reef.

At first, weather conditions were good, and the spill seemed manageable, said Larry Dietrick, who retired in 2013 as director of the Alaska Department of Environmental Conservation’s Division of Spill Prevention & Response. The oil slick remained largely intact and would have been amenable to containment and attack by burning, skimming, and application of dispersants—mixtures of solvents, surfactants, and other compounds intended to break up oil into small droplets that are more easily biodegraded.

But although oil-spill plans and treatment permits were in place, equipment wasn’t, Dietrick said. Neither containment booms nor burn igniters were nearby. The barge that arrived to hold skimmed oil couldn’t because it wasn’t certified to do so. No planes were immediately available to apply dispersants, and once they arrived, the existing supply was far short of that needed for the amount of oil. The lack of on-scene equipment was compounded by the fact that the area is only accessible by boat or seaplane.

Then, three days after the Valdez hit the reef, a storm came in. Winds and waves broke up the slick and emulsified the oil, reducing the effectiveness of cleanup technology—emulsified oil is difficult to burn and dissipate, and skimmers are less efficient at collecting it.

The oil wound up spreading throughout the water and over 1,300 miles of remote, rocky shoreline at a particularly vulnerable time of the year for wildlife, said Marilyn Heiman, director of the U.S. Arctic Program of the Pew Charitable Trusts. The spill affected spawning herring, juvenile salmon, and migrating birds, as well as sea otters, seals, and killer whales. Native populations who depend on the ocean for food also felt the spill’s effects.

What ensued was a contentious three-year cleanup program that cost $3.8 billion (adjusted for inflation) and involved some 12,000 responders at its height. Because of the location, cleanup could occur only in the summer, and workers had to be housed in trailers mounted on barges.

The teams approached shoreline treatment in a few different ways. Some cleaning was done manually—washing rocks with rags or picking up oil with shovels. In other cases, workers moved oil-contaminated sediments from high on a beach down to the tideline to be naturally washed by waves. In some areas, crews sprayed water to flush off oil. The released oil from these efforts was then skimmed or absorbed into booms.

To better mobilize the oil, responders flushed some shorelines with heated seawater—to disastrous effect. “They sterilized the beach and killed the natural bacteria that were there,” Edward B. Overton, an emeritus professor of environmental sciences at Louisiana State University (LSU), told C&EN. “That really, really slowed down the natural degradation of the oil.”

Shoreline tests of dispersants to try to break up the oil showed poor results. Tests of fertilizer to enhance bacterial activity, on the other hand, were successful. Responders subsequently treated beaches with a combination of a granular fertilizer containing calcium phosphate, ammonium phosphate, and ammonium nitrate plus an oleophilic fertilizer containing urea. “Bioremediation turned out to be a very effective tool, and it wasn’t that intrusive,” said Robert L. Mastracchio, who was Exxon’s technical manager for the oil-spill cleanup and retired from the company in 2000 as vice president for engineering.

TOLL The two largest U.S. oil spills to date caused enormous environmental damage and cost billions of dollars to clean up.

Deepwater Horizon Report Card

Three years after the Deepwater Horizon spill, the national commission that investigated the disaster evaluated progress on its recommendations to improve spill prevention, preparedness, and cleanup.


◾ Assessment of the spill’s impact on and restoration of Gulf of Mexico ecosystems C+
◾ Safety of offshore drilling and environmental protection B
◾ Ability to respond to and contain offshore oil spills B
◾ Allocation of resources for future spills D
◾ Understanding of the Arctic environment and managing prospective oil development C


Federal government
Administration—implementing safety rules, conducting environmental impact assessments, integrating planning for the Arctic B
Congress—lagging on most recommendations, such as adopting a fee program to support regulation and oversight D+
Oil industry—adopting improved standards and procedures, procuring equipment for spill response B-

NOTE: Cleanup and settlement costs adjusted for inflation to 2015. SOURCE: Oil Spill Commission Action, “Assessing Progress,” April 17, 2013 (

In all, only about 10% of the oil was ever collected. The remainder was biodegraded, evaporated, weathered, or buried in sediments, where some pockets of unweathered oil still linger. “When that oil got down into the anaerobic zone, degradation essentially stopped,” Overton said.

Controversy arose when it came to assessing damage and tracking recovery. “The baseline is always a challenge in these things in terms of what was the status of the ecosystem before the oil hit,” said Robert Spies, principal of the company Applied Marine Sciences and formerly chief scientist of the Exxon Valdez Oil Spill Trustee Council. Without good, quantitative prespill data, scientists were left to make indirect comparisons of oiled versus unoiled areas. Additionally, it has been difficult to tease out oil-specific effects from other long-term pressures on the ecosystem, including fishing, predation, multiyear climate oscillations, and global warming.

Arguments also arose because different scientific camps had different injury definitions, standards of proof, and geographic frames of reference, Spies said. The government linked oil to ecosystem damage using a “weight of evidence” approach, whereas Exxon maintained that simply having one event follow another is not proof of cause and effect, Spies said. One study of juvenile pink salmon, for example, showed that those collected from oiled areas and exhibiting biomarkers of oil exposure grew less than those from unoiled areas. Exxon, however, argued that actual exposure of the salmon to Valdez oil was uncertain.

The ongoing effects of the oil on Prince William Sound are also uncertain. Most species appear to have recovered, including harlequin ducks and sea otters, which long showed signs of exposure, according to a 2014 National Oceanic & Atmospheric Administration (NOAA) report. Of the two main killer whale populations that were affected, one is still below its prespill population but appears to be slowly recovering, while the other seems to be dying out. The herring population in Prince William Sound crashed four years after the spill and has not yet fully recovered.

The year after the spill, Congress passed the Oil Pollution Act of 1990 without opposition and President George H. W. Bush signed it into law. The act established new requirements for spill response, outlined liability for responsible parties, and broadened the authorities of the federal government to direct spill cleanup.

“There was a political will in this country to fix the problems associated with tanker carriage of oil and not have another large tanker accident in this country. And guess what? We did that,” said retired U.S. Coast Guard Admiral Thad W. Allen, who was the National Incident Commander for the DWH spill and now works for consulting firm Booz Allen Hamilton.

But between 1990 and 2010, “as we focused so much on tanker safety and preventing a tanker accident, oil drilling moved offshore,” Allen added. Technological advances allowed companies to set up deepwater rigs that are able to drill thousands of meters below.

And so in 2010, the DWH rig wound up 50 miles off the coast of Louisiana in the Gulf of Mexico, connected by riser pipe to the Macondo wellhead at the seafloor 1,500 meters below, drilling down into a reservoir another 4,000 meters below that.

Workers called Macondo a “well from hell,” because it was hard to control. On the day of the explosion, as they were attempting to switch the well from drilling to production, gas and oil rose to the rig and ignited. From April 22, the day that the rig sank, until July 15, when the well was capped and the flow stopped, approximately 60,000 bbl of oil leaked into the Gulf daily.

Determining that flow rate was a critical challenge in the response. In addition to determining the appropriate scale of response efforts, “a number of solutions for stopping the flow of the well depended on how much oil was coming up,” said Marcia K. McNutt, who was director of the U.S. Geological Survey at the time of the spill and is now editor-in-chief of Science. For example, had BP engineers known the correct flow, they would have realized that the so-called top kill, which involved pumping in heavy drilling fluids to restrict the flow out of the well, wasn’t going to work, McNutt said.

The only prior similar event was the Ixtoc I oil spill in 1979, when a blowout released 3.3 million bbl of oil into Mexico’s Bay of Campeche, also in the Gulf of Mexico. “But there had not been a single peer-reviewed paper ever published from Ixtoc,” McNutt said, so there was no guidance for scientists looking at the flow rate out of the Macondo well. Eventually, they combined several approaches, including video analysis, acoustic methods, and reservoir modeling to determine the flow rate (Proc. Natl. Acad. Sci. USA 2012, DOI: 10.1073/pnas.1214389109).

As with the Exxon Valdez spill, responders tried to deal with the oil on the water by skimming, burning, and deploying booms to try to shield sensitive areas along the coast. In contrast to Alaska, the Gulf of Mexico area was relatively accessible to bring in necessary supplies and workers. BP was also able to recover some oil directly from the wellhead. Overall, however, skimming, burning, and recovery addressed only about 25% of the oil—the remainder wound up dissolved in the water column, evaporated to the atmosphere, or deposited on the coastline or seafloor.


The most contentious issue in the spill response was the use of dispersants. In addition to spraying dispersants on surface slicks, responders also injected dispersants into the jet of gas and oil gushing from the wellhead. The hope was that by adding the dispersants at the source, more of the oil would stay in the water and the dispersed oil would be more accessible to microbes for biodegradation. Less oil reaching the surface meant reduced worker exposure and less oil on beaches and in marshes.

But dispersants and chemically dispersed oil can be toxic, depending on concentration of the material and the species exposed to it. The question is whether the dispersants did more good than harm.

The answer is that we don’t know. For all the studies that have been done to look at dispersant effects on oil and toxicity, ultimately there is no control experiment of an identical spill without dispersants, McNutt noted. However, she added, from a response logistics perspective, dispersants were helpful. During periods when responders didn’t inject dispersants at the wellhead, the concentration of volatile organic compounds at the surface above rose so high that surface operations had to be halted for worker safety, hampering progress on capping the well or building additional capacity to collect oil from the wellhead. “If dispersants hadn’t been used, we probably would’ve had to wait several more weeks for the spill to end,” McNutt said.

The effects of the DWH spill are yet to be fully determined. Microbes appear to have quickly and successfully cleared gas and oil dissolved and dispersed in the water column, including the “deep plume” that formed about 1 km below the surface. Crews largely cleared oil that reached beaches, but they could do little for affected marshes. “Coastal marshes are soft,” LSU’s Overton said. “It’s hard to put people on them, never mind equipment.” Also, many questions remain about how much oil wound up on the seafloor and what’s happening to species there.

As in Alaska, oil that got down into anaerobic zones is degrading slowly, John Pardue, a professor of civil and environmental engineering at LSU, told C&EN. He has a field study ongoing to bubble oxygen into zones that are still contaminated to see if that can promote biodegradation.

The overall effects on species affected by the spill are unknown—the Natural Resource Damage Assessment is still ongoing. Lack of quantitative baseline data makes assessment difficult and uncertain, similar to the case in Alaska. Of particular concern is unusual mortality for dolphins and whales in the Gulf of Mexico that started in February 2010 and continues today.

Looking ahead to future spills, oil-spill experts highlight prevention first. “It doesn’t matter how much oil gets into the water, you’re not going to clean it all up,” said Fran Ulmer, who served on the national commission studying the DWH spill and offshore drilling and who is now head of the U.S. Arctic Research Commission. “The best strategy is to keep it from getting in the water in the first place.”

Beyond prevention, preparedness is key, and it comes in a few different forms. One is, as demonstrated in the Valdez spill, not to have just the plans in place but also the equipment.

In 2013, members of the DWH oil-spill commission gave federal agencies and the oil industry “B” grades for their efforts to address commission recommendations relating to prevention and spill response, such as in improving blowout-preventer performance and developing containment systems for deep water.

For issues relating to the Arctic, however, commission members reduced the grades to a “C.” The bar for appropriate preparedness in the Arctic is higher because of its icy waters, limited infrastructure, and little daylight for much of the year.

Basic science also comes into play for preparedness. Scientists need long-term ecological information in areas with significant petroleum development or transport to serve as a baseline, or benchmark, for understanding spill effects. “You don’t necessarily need to know every single thing about every single species, but you need to understand which are the crucial pieces for your ecosystem”—things such as the rates of change and hydrocarbon sensitivity of key species, said Deborah Glickson, a senior program officer with the National Research Council’s Ocean Studies Board, which last year published a study titled “Responding to Oil Spills in the U.S. Arctic Marine Environment.” Benchmarking is particularly important in the Arctic, Glickson noted, because the area is changing rapidly in response to global warming. “You can’t just rely on data that came from 1972,” Glickson said.

Understanding different oil types and how they behave and degrade in certain environments is also key for future spill response. Scientific methods to use biomarkers such as hopane to track oil from a particular source and to monitor how it weathers will aid in this area, Christopher M. Reddy, a senior scientist at Woods Hole Oceanographic Institution, told C&EN. Scientists developed such methods in response to the Valdez spill and extended them to new analytical tools for the DWH spill.

One oil type of particular interest is bitumen, or asphalt—the sticky, highly viscous material extracted from the Canadian tar sands in Alberta. It is transported as dilbit or synbit, in which the bitumen is diluted with a lighter petroleum product. Dilbit was the material spilled into the Kalamazoo River, and the bitumen component sank to the bottom—a new scenario for responders to deal with. The National Research Council’s Board on Chemical Sciences & Technology is currently working on a study of the effects of dilbit on the environment.

Better ecosystem and oil information would also be important for understanding when and where to use dispersants. “Certainly the biggest outstanding issue that I see right now is the controversy surrounding dispersants and their use not only in places like the Gulf of Mexico but in any marine environment,” Nancy Kinner, a professor of civil and environmental engineering and codirector of the Coastal Response Research Center at UNH, told C&EN. “There’s a lot of controversy and mistrust on both sides of the fence on this issue.” Her center is currently working on a project to review many features of dispersants, such as efficacy, physical transport and chemical behavior, degradation and fate, toxicity and sublethal effects, and public health and food security.

Besides studying dispersants, Kinner noted that scientists can help responders improve spill response by thinking through and coming up with solutions to various scenarios before a spill happens. A new initiative, Scientific Partnerships Enabling Rapid Response, aims to do just that. The goal is to build relationships between nongovernmental scientists and agencies “not during a time when everybody is going crazy,” Kinner said, both to help with planning and to have networks in place for when expertise is urgently needed.

With luck, that effort, along with regulatory changes and scientific progress, will mean that we’re better prepared for the next spills.  


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