By Mark Kinver
Science and environment reporter, BBC News
Oil from the Exxon Valdez spill was found just inches below the surface
Large quantities of oil spilled during the 1989 Exxon Valdez disaster can still be found beneath gravel beaches in Alaska, a study has discovered.
Writing in Nature Geoscience, a team of scientists found that oil just a few inches down was dissipating up to 1,000 times slower than oil on the surface.
They suggested that a lack of oxygen and nutrients in the gravel was slowing the dispersal of the remaining oil.
The results could have implications for cleaning up future spills, they added.
Considered to be one of the worst environmental disasters of its kind, the Exxon Valdez tanker spilled 38,000 tonnes of crude oil into Alaska's Prince William Sound after the vessel hit a reef.
As a result, more than 2,000km (1,250 miles) of coastline was affected, killing thousands of seabirds and having a serious impact on the region's fishing industry.
In the five years after the disaster, the oil was shown to be dispersing at a rate of about 70% each year.
Most clean-up operations in the area ended in 1992 because the remaining oil was expected to disperse within a few years.
A later study discovered that the oil was disappearing at a rate of just 4% each year, and that an estimated 20,000 gallons remained in the beaches.
The spill was one of the worst environmental disasters of its kind
Researchers led by Professor Michel Boufadel from Temple University in Philadelphia, US, carried out a three-year study on a number of beaches to find out the cause behind the lingering deposits.
Prof Boufadel, director of the university's Center for Natural Resources Development and Protection, said the gravel beaches they examined were made up of two layers: a top level that was highly permeable, and a lower level that had very low permeability.
While the two layers were made from the same material, he said the lower level had become compacted as a result of tidal movements, limiting the volume of seawater that was able to penetrate the gravel.
In their paper, the team observed that the upper layer temporarily stored the oil, while it slowly and continuously filled the lower layer.
"You have a high amount of oxygen in the seawater, so you would think that the oxygen would diffuse in the beach and get down 2-4 inches (5-10cm) into the lower layer and get to the oil," said Prof Boufadel.
"But the outward movement of [fresh groundwater] in the lower level is blocking the oxygen from spreading down into that lower level."
Scientists carried out the study to find out why the oil was "locked" in beaches
He explained that oxygen and nutrients were needed to sustain micro-organisms that "ate" the oil.
However, without the necessary supply of the key ingredients reaching the lower level, the biodegradation of the oil was occurring at a much slower rate.
"We suggest that similar dynamics could operate on tidal gravel beaches around the world, which are particularly common in mid- and high-latitude regions," the team wrote in their paper.
"Thus, our findings are of direct application for the susceptibility of beaches worldwide to long-term oil contamination and provide guidelines for remediating oil-polluted beaches."
They added that climate change was reducing ice cover, "exposing the Arctic to oil exploitation and shipping" and increasing the risk of oil spills in the future.
Professor Boufadel and his team are now exploring ways to deliver the necessary levels of oxygen and nutrients to affected areas to accelerate the dissipation of the remaining oil.