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ANCHORAGE, Alaska — Nearly 13 years after the tanker Exxon Valdez ran aground, spilling 11 million gallons of crude oil into Prince William Sound, federal scientists estimate about 10,000 gallons of oil remain buried under the shoreline. 

Jeff Short, a scientist with the National Marine Fisheries Service, released the findings of a survey of beaches conducted last summer. Short presented his data Tuesday on the first day of a four-day conference on the lingering effects of the spill. 

The survey of beaches in the spill area found that much of the oil lies beneath sediments in areas that are home to mussels, clams and other creatures. 

“We did indeed find quite a lot more oil than we expected to see,” Short said. “Most of the subsurface oil was in the fresh oil category, and by fresh oil I mean chemically, compositionally — it hasn’t really changed very much since late in the summer of 1989.” 

The beach survey was conducted to gather enough data to make a meaningful estimate of how much oil remained. 

Over 90 days last summer, a field crew visited 91 sites along about five miles of beaches, covering about 20 percent of the area classified as heavily or moderately oiled between 1989 and 1993, Short said. 

The workers dug 6,775 pits at random locations, then dug dozens of additional pits every time they found oil to calculate how far it had spread. 

The field crews discovered oil at 53 of 91 sites, in 568 different pits — about eight times more often than the scientists had expected. 

Although most of the pits were “lightly oiled,” about 20 contained oil that looked as fresh as that just a few weeks after the 1989 spill — “highly odiferous, lightly weathered, and very fluid,” they wrote in a preliminary report. 

In the end, Short and his team estimated that about 10,000 gallons of Exxon Valdez crude remains buried under about 27 acres spread along about 4.3 miles of shoreline scattered throughout the area. 

Other studies presented at the conference documented problems among certain species that forage on the nearby sea floor. They suggest that lingering oil is leaching into the food chain, where it hurts local populations of sea otters and harlequin ducks. 

Exposure to this oil may no longer threaten overall animal populations. But sea otters and harlequin ducks near Knight Island and Green Island have been ingesting hydrocarbons and apparently suffering liver and tissue damage, according to reports by biologists Brenda Ballachey of the U.S. Geological Survey and Dan Esler of Simon Fraser University in British Columbia. Otter and duck numbers in oiled areas have continued to decline, while populations in nonoiled bays fare much better. 

The fact that Exxon Valdez oil still has the power to harm wildlife, even if on a limited scale, is one of the most startling findings to come from a decade of research and monitoring, several scientists said. 

“The oil was quite a bit more persistent and quite a bit more toxic than we thought in 1989,” Short said. 

An Exxon Mobil official and a Maine chemist dismissed the idea that the spill still causes significant damage to life in the Sound. 

“What science has learned in Alaska and elsewhere is that while oil spills can have acute short-term effects, the environment has remarkable powers of recovery,” company vice president Frank Sprow said in a statement released from Exxon Mobil’s headquarters in Irving, Texas. 

Bowdoin College biochemist David Page, who has conducted studies for Exxon, said he was skeptical of Short’s findings. Page says that, for at least the last seven years, natural factors in Prince William Sound have been the major factor in governing ecological change.