Almost 15 years after the Exxon Valdez tanker spilled millions of litres of oil into the pristine waters of Alaska's Prince William Sound, contaminants released in the accident continue to befoul the area, according to a new study in the latest issue of the journal Science.
While the massive oil slick and thousands of dying seabirds that made headlines in 1989 have vanished, the study says wildlife continues to be poisoned by oil residues in subtle ways that are harming the young of many species and are undermining population recoveries.
In many of the worst-affected areas, the beaches and tidal flats appear clean from the surface, but researchers were able to strike petroleum by merely pushing spades into the ground and turning over cobblestones to find seemingly fresh oil not much different than the day it gushed from the Valdez.
"In the Alaskan coastal ecosystem, unexpected persistence of toxic subsurface oil and chronic exposures, even at sub-lethal levels, have continued to affect wildlife," the study concluded.
It said some areas may need up to 30 years to fully recover.
Although the Valdez accident was not the worst oil spill in history and doesn't even rank in the top 10 for the amount of petroleum dumped into the environment, it is considered the world's worst tanker mishap because it occurred in an area of almost unbelievable natural bounty, frequented by colourful puffins, killer whales, salmon and sea otters.
ExxonMobil spent almost $2.2-billion (U.S.) cleaning up the mess from the spill, but stopped in 1992 after the state of Alaska and the U.S. Coast Guard declared the cleanup had been completed.
The Valdez incident is the most extensively studied oil spill in history because it occurred in such an ecologically sensitive region, and the new study raises further questions over whether the damage has been largely mitigated.
The study's lead author, marine ecologist Charles Peterson from the University of North Carolina, said merely standing on the tidal areas affected by the spill causes oil residue to seep to the surface. "That oil sheens, stinks, stains everything you touch and obviously has a lot of characteristics that fresh oil did," he said.
Normally after a spill, oil quickly breaks down into carbon dioxide and water when it is exposed to air, sunlight and wave action. But in the case of the Valdez spill, hydrocarbons seeped down into rocky and sandy areas below the surface in many sheltered coves and inlets, slowing the breakdown process to a crawl. Oil residues aren't a problem in more open coastal areas, where natural processes have largely eliminated the oil.
ExxonMobil says pollution hot spots, such as the ones found by the researchers, exist in Prince William Sound, but it says they are small, isolated and have been found elsewhere in the world after large oil spills.
But Mr. Peterson said these sheltered areas tend to have the most fertile environments for many aquatic species, meaning the ecosystems with the most potential for wildlife diversity also carry the greatest threat to these animals because they tend to be places where oil doesn't break down quickly.
One of the most significant findings of the research is that the long-term effects of a spill harm wildlife in a profoundly different way than during the initial discharge of oil.
When birds or mammals first encounter an oil slick after an accident, their fur or feathers are coated, causing them to lose their ability to keep warm. They die of hypothermia, drowning or the ingesting of toxic hydrocarbons. A quarter of a million seabirds died in the days just after the Valdez spill, as well as up to 2,800 sea otters.
But the study said animals are now being affected by chronic, low-level exposure to toxic chemicals, such as polycyclic aromatic hydrocarbons (PAHs), that are leaching from the remaining oil. Laboratory research has found that PAHs are toxic to salmon eggs in concentrations as low as one part per billion, an extremely small reading.
Researchers have also found that sea otters and harlequin ducks have high levels of enzymes found in animals whose metabolism is trying to rid itself of toxic substances. The study said this finding could explain why sea otter populations have risen only 4 per cent annually since the spill, compared with rates of 10 per cent observed after the hunt for otter pelts was ended.