But the state of Alaska and the US government could ask for up to US$100 million more if they can show there is substantial, continuing environmental damage caused by the spill, and that the damage could not have been anticipated when a settlement with Exxon was signed in 1991.

A study by researchers at the National Marine Fisheries Service in Juneau, Alaska, indicates about six miles (9.6 km) of shoreline around Prince William Sound is still affected by the spill, with 100 tons (101.6 tonnes) of oil remaining in the area.

Mark Boudreaux, a spokesman for ExxonMobil, questioned the study's findings, and noted the oil company had previously responded to this research, which was based on field work done in 2003. Exxon and Mobil merged in 1999.

"We disagree with their conclusions," Boudreaux said by telephone from Irving, Texas. "We've done 350 peer-reviewed studies of Prince William Sound, and those studies conclude that Prince William Sound has recovered, it's healthy and it's thriving."

The study, which is to appear in the June 15 print edition of the American Chemical Society's journal Environmental Science & Technology, said oil from the Exxon Valdez remains on shorelines of Prince William Sound.

SEA OTTERS AND OIL

Some is on the surface and has weathered to a hardened, "asphalt pavement" state, while some is hidden under the surface in the inter-tidal area of local beaches, research chemist Jeffrey Short said in a telephone interview.

"The subsurface oil is typically liquid, smelly oil," Short said. "It looks like crude oil."

Sea otters and sea ducks -- both species that forage for food along the tide line -- are most likely to be affected by this, he said: "There's a clear link for ongoing exposure for animals that disturb sediments while they forage for prey."

Sea otters dig pits at high tide when the inter-tidal zone is covered with water, then they dive down and disturb the sediments as they look for clams.

If they encountered spilled oil in the process, they would probably get it on their fur and likely ingest some of the oil as they groomed themselves -- an essential habit for otters, which rely on their fur for warmth.

"Our study suggests that they would eat some of this stuff several times a year," said Short, one of five authors of the study.

He said little is known about any toxic effects to mammals which ingest oil, but circumstantial evidence implicates oil exposure as a possible cause of the lack of recovery of sea otter populations in the most heavily oiled parts of Prince William Sound.

ExxonMobil's Boudreaux noted the research was being released about two weeks before a June 2 deadline for Alaska and the federal government to seek additional payments from ExxonMobil in a provision that allows this part of the 1991 settlement to be reopened.