For nearly a decade, scientists have puzzled over the persistence of oil from the Exxon Valdez oil spill.

A pair of Lower 48 researchers on Sunday published the first study to attempt an explanation for why that oil isn't degrading as much as expected.

Their findings have implications for any future attempt to remove residual oil from the beaches -- a proposal launched by state and federal prosecutors in 2006 that is still being negotiated with Exxon Mobil Corp.

An estimated 21,000 gallons of the 11 million gallons of crude the Exxon tanker spilled in Prince William Sound in 1989 remain on Alaska beaches.

For years, federal and state officials expressed optimism that the oil from the massive spill would dissipate completely. In fact, in the first five years after the spill, scientific testing did show oil was degrading at a fast clip. Prince William Sound's beaches started to look idyllic again, even though people -- and animals -- digging below the surface still could find oil.

Since then, federal scientists have gathered evidence that disputes spill cleanup officials' prediction that all of the toxic oil would disappear in a few years. Further, the scientists say, the rate at which the oil is disappearing from the beaches has slowed down.

That's because the rest of the oil is trapped in a layer of compacted beach sediment where it cannot easily degrade, said Michel Boufadel, the lead researcher of the $1.2 million study, funded by the Exxon Valdez Oil Spill Trustee Council. The study is being published in Nature Geoscience, a monthly scientific journal. Boufadel runs the Center for Natural Resources Development and Protection at Temple University's College of Engineering, and specializes in studying the movement of oil in water.

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