ANCHORAGE, Alaska -- Lingering crude from the nation's largest oil spill has weathered only slightly almost 18 years after the tanker Exxon Valdez ran aground and fouled hundreds of miles of Alaska shoreline, a new federal study concludes.

The estimated 85 tons of oil remaining in Prince William Sound is declining about 4 percent a year and even slower in the Gulf of Alaska, according to research chemist Jeffrey Short with the National Oceanic and Atmospheric Administration.

At that rate of decline, oil could persist for decades below the surface of some beaches along the gulf, Short and colleagues said in their report. The study is to be published in the Feb. 15 edition of Environmental Science & Technology, the journal of the American Chemical Society.

"Such persistence can pose a contact hazard to intertidally foraging sea otters, sea ducks and shorebirds, create a chronic source of low-level contamination, discourage subsistence in a region where use is heavy, and degrade the wilderness character of protected lands," researchers wrote in their conclusion.

The study was partially funded by the Prince William Sound Regional Citizens' Advisory Council, which was formed by federal mandate after the Exxon Valdez spill to monitor industry operations. Researchers, however, said their findings were not influenced by that sponsorship.

Exxon Mobil Corp. spokesman Mark Boudreaux said the company's Valdez team was reviewing the report.

The Exxon Valdez ran aground March 24, 1989, emptying 11 million gallons of crude oil into Prince William Sound.