Twenty years after the Exxon Valdez supertanker spilled nearly 11 million gallons of crude oil into Prince William Sound in southern Alaska, everything is pristine and natural again, right?

Not exactly.

A study in 2004 estimated that perhaps 25,000 gallons of oil remained along the sound’s gravel beaches and was degrading very slowly. So that raised a question for researchers: Why, despite one of the largest environmental cleanups in history, has some oil persisted?

Michel C. Boufadel, an environmental engineer at Temple University, and a colleague, Hailong Li, have provided an answer. In a paper in Nature Geoscience, they report that the oil has become trapped in a zone of low permeability below the beach surface.

“We could only answer this question by understanding the movement of water within these beaches,” Dr. Boufadel said.