Oil from the Exxon Valdez spill lingers, as do disputes about its current impact on wildlife. An ecologist asks what a quarter century of research can tell us
In March 1989, the supertanker Exxon Valdez ran aground on Bligh Reef in Prince William Sound, Alaska. More than 40 million litres of crude oil spilled into the frigid waters. Pushed by a late winter storm, the oil eventually affected 2100 kilometres of remote, rocky shoreline.
It is estimated that as many as 250,000 seabirds, 2000 sea otters, 300 harbour seals, 250 bald eagles and untold numbers of invertebrates living on nearby rocks and beaches died. Fisheries closed, people's lives were disrupted and so lawyers went to work. Exxon faced a $5 billion damages claim for the worst spill in US coastal waters at the time, only since surpassed by the Deepwater Horizon disaster in the Gulf of Mexico.
Practical measures quickly followed. Coast Guard and state oversight to help protect the sound was improved, containment and clean-up equipment was stockpiled in case of future disasters and local fishermen were trained to respond to a spill. Tugs now escort tankers through the area and many new ships have double hulls.
Scientists also set to work, documenting what happened to the oil, its effects and the recovery of wildlife. The spill became the most intensively studied in the history of the oil industry.
Lingering reminder
By and large, these studies showed that nearly all of the oil disappeared from shorelines within a few years and most wildlife recovered in a decade or less. Yet bones of contention remain.
Some oil from the spill is still there, buried under boulders. Although most of it is highly weathered and in small deposits scattered over a tiny fraction of the shoreline that was hit, it represents a lingering vestige of the disaster.
And naturally there are fears it could still be affecting wildlife. Concerns have been raised about harm to harlequin ducks and sea otters, which both feed in affected areas and could be exposed to this oil if deposits are disturbed when the otters dig for clams.
So is this actually happening? Tissue samples, some as recent as 2013, showed that levels of the protein cytochrome P450 1A, an indicator of exposure to hydrocarbons, were higher in the livers of ducks and otters from areas with oil deposits than from nearby areas with no oil. This finding reinforces the suggestion that such huge spills have long-lasting ecological effects. In isolation, it looks worrying.
Safely sequestered
Thanks to the combined efforts of petroleum reservoir engineers, microbiologists, chemists, geomorphologists and ecologists, we now have the whole picture.
They showed that the residual oil had, in general, buried itself beneath boulders or cobbles in a way that meant it was safely sequestered from the seawater, and the toxicity of any hydrocarbons released would be rapidly diluted in any event. Biochemists observed that the protein marker in the ducks and otters is triggered by exposure to a wide array of hydrocarbons, as well as other compounds, many of which you might expect to find in a busy shipping and fishing area with a long history of shoreline industry. So it is unreliable as an indicator of exposure to just Exxon Valdez oil.
Ecological-risk modellers also demonstrated that probabilities of exposure to lingering oil, much less any ill effects, were vanishingly small. Finally, the oil is in areas with few clams, which are unattractive to otters. We know that oil residues persist in a few scattered locations, yet equally we know they no longer pose a threat to wildlife.
Science has worked as it should – initial findings led to additional work, which helped to resolve uncertain or inconsistent results. But after the spill, disagreements over issues like this often festered. Shouldn't the effects of something as overwhelming as an oil spill be obvious?
Communication is key
Distinguishing between spill impacts and everything else was challenging. Comparisons with pre-spill conditions or between areas with oil deposits and unaffected areas were confounded by environmental variation. Some studies assumed that the spill must have caused damage, while others made no prior judgment, leading to different questions and statistical designs. Studies varied in the criteria used to assess recovery or the standards used to evaluate evidence.
Some disagreements might have been avoided had there been better communication. But scientists were often partitioned by the litigation that followed the spill, stifling communication. Otherwise, unstated assumptions and methodological pitfalls could have been exposed and study designs shared.
Conducting rigorous science after a spill is not easy, but it can be done. Preconceptions must be minimised. Lessons about data and study design are obvious in hindsight, but they can easily be forgotten in the emotional atmosphere and urgency that follow a major spill. Co-operation and openness are essential. The Exxon Valdez has taught us many things that may help avoid such protracted disputes in the future.
The lessons of the disaster are timely. As the Arctic warms and the ice melts, oil development in remote, cold-water areas like Prince William Sound will accelerate. The likelihood of spills will increase. The people who have lived in these places for generations, and the biologically rich ecosystems on which they depend, will be especially vulnerable. And if the worst happens, they will want answers too.
John Wiens is chief scientist at ecology research group Point Blue in California, US. He is editor of Oil in the Environment: Legacies and lessons of the Exxon Valdez oil spill (Cambridge University Press)
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