The findings of the study, conducted by research chemist Jeffrey Short and his colleagues at the National Marine Fisheries Service in Alaska, suggested that despite efforts to clean up the spill, up to 10km of coastline was still affected, and up to 100 tonnes of oil may still be contaminating the waters.

“This study shows that it is very plausible that exposure to Exxon Valdez oil is having a material impact on many shore-dwelling animals and is contributing to their slow recovery in some parts of Prince William Sound," Short said.

“Sea otters, for instance, have yet to re-inhabit Herring Bay, the most oiled bay we studied, and the population of otters elsewhere around northern Knight Island continues to decline. Unfortunately, because much of this oil is buried in beach sediments and not exposed to weathering and other elements that might degrade it, it could remain hazardous to wildlife for decades."

The field research was conducted in 2003 at over 600 pits dug at random across 32 stretches of shoreline on northern Knight Island, one of the earliest and worst affected areas during the spill.

Oil from the Exxon Valdez was discovered at 14 of the sites.

Up to half of the oil was found in low-tide regions, with the potential to impact predatory wildlife in the region.

Of all the oil discovered by Short’s research team, more than 90% of surface oil and all of the subsurface oil was determined to originate from the Exxon Valdez.

Short claimed that sea otters, which dig pits in low-tide areas to discover clams and other prey, could come into direct contact with Exxon Valdez oil at least once every two months.

Reuters has reported that if the research is verified, it could cost ExxonMobil Corporation an additional $US100 million to further remediate the site, in addition to the $900 million already spent.

Exxon and Mobil merged in 1999, some 10 years after the Exxon Valdez ran aground in March 1989, spilling over 11 million tonnes of oil into the Prince William Sound.

An ExxonMobil spokesman told Reuters the company disagreed with Short’s conclusions, based on 350 peer-reviewed studies indicating that the area had recovered from the incident.