On March 24, 1989, the Exxon Valdez ran aground in northern Prince William Sound, spilling 42 million liters of crude oil and contaminating 1,990 kilometers of shoreline. Some 2,000 sea otters, 302 harbor seals and about 250,000 seabirds died in the days immediately following the spill. Now researchers writing in the journal Science caution that more than a decade later, a significant amount of oil still persists and the long-term impacts of oil spills may be more devastating than previously thought.

Charles H. Peterson of the University of North Carolina at Chapel Hill and his colleagues compiled and analyzed the findings of dozens of previous studies. The results, Peterson says, "showed that oil has persisted in surprisingly large quantities for years after the Exxon Valdez spill in subsurface reservoirs under coarse intertidal sediments. This oil was sequestered in conditions where weathering by wave action, light and bacteria was inhibited, and toxicity remained for a decade or more." Exposure to this oil, in turn, caused additional animal deaths. Salmon, for example, had increased mortality for four years after the spill because incubating eggs had come into contact with it. Larger marine mammals and ducks, meanwhile, suffered ill effects because their prey was contaminated. The team estimates that shoreline habitats such as mussel beds affected by the spill will take up to 30 years to recover fully.

The findings should inform the development of better ways to assess the ecological risks of large-scale oil spills, the scientists say. In addition, the work applies to other areas of environmental remediation. "Recognition that chronic exposures of fish eggs to oil concentrations as low as a few parts per billion lead indirectly to higher mortality shows the critical need to better control stormwater runoff of petroleum hydrocarbons and other toxins," Peterson remarks. "In a developed country like the U.S., an amount of petroleum equal to the Exxon Valdez oil spill is spilled annually for every 50 million people."