The oil spilled from the tanker Prestige when it foundered and sank Tuesday off the coast of Spain is less toxic to fish and wildlife than the crude oil spilled in Alaskan waters in 1989 by the Exxon Valdez, according to U.S. officials.
But the thick, gooey substance nevertheless threatens to coat and smother seabirds, marine mammals and shellfish on a long stretch of Spanish and Portuguese coastline, American experts say. And it will defy all available methods for recovering or dispersing spilled petroleum from the open water.
"Does it rank with the top spills in the world? No, I wouldn't say so. But then, if you live in Spain or Portugal right now, that isn't going to sell very well," said David M. Kennedy, director of the Office of Response and Restoration at the National Oceanic and Atmospheric Administration.
Kennedy's office has offered to assist Spanish and Portuguese authorities in predicting where and how the spilled oil will move and where it might come ashore.
Authorities fear that some or all of the Prestige's remaining 17 million or 18 million gallons on board might have escaped from the ship's tanks when it plunged to the bottom of the ocean.
"We still have a shoe to drop," Kennedy said. "That could change my evaluation of how bad it is."
The Prestige's cargo was intermediate fuel oil 380 (IFO 380), a product used as boiler fuel in large ships.
It is a "residual oil," left over after lighter products such as gasoline and heating oil have been extracted from the original crude. Refining also removes many of the most toxic substances in crude oil, including toluene and benzene, Kennedy said:
"You end up with something more like coal tar and asphalt."
As thick as molasses, IFO 380 must be heated or cut with diesel oil before it can be pumped. Not quite light enough to bob to the surface, it won't readily sink either. Kennedy compared it to the goop in a lava lamp.
"The material is floating, but it doesn't take much energy to knock it down in the water column," he said. So, much of the spilled oil might lurk 10 to 15 feet below the surface, well out of reach of any booming or skimming efforts.
Chemical dispersants are sometimes used to break up an oil slick, so wind, waves and biochemical activity can degrade it. But the chemicals don't work well on a "gelatinous" mass like IFO 380, Kennedy said.
"I see very little they're going to be able to do in the open ocean. The real question is what they'll do when it hits the shore," he said.
With relatively low toxicity, the fuel oil's biggest environmental threat is its potential for smothering and coating sea birds and marine mammals.
"There are plenty of experienced rehabilitators who are pretty good at mobilizing this stuff off oiled wildlife," said Gary Shigenaka, a marine biologist with NOAA's Hazardous Materials Response Division. "It's really labor intensive and it requires a lot of clean, hot water. But they're getting better at doing it."
Oysters, mussels and clams might also be at risk, he said. But with lower toxicity than crude, "I don't think we'd see a lot of shellfish [dying] or fish washing up dead."
Consumer perceptions, however, would likely hurt sales anyway and force authorities to close fisheries in the region, causing severe losses to fishermen and their communities.
The same could be true for tourism, as travelers read about the spill and decide to cancel their coastal vacations.
The cleanup over such a wide territory will require chemicals and hot water under pressure, carrying huge costs.
"And the longer it stays on the shoreline, the more tenacious it's going to be," Shigenaka said. "In some portions of the rocky Spanish coast, they may have to leave it in place and let natural processes take care of it."