For nearly a decade, scientists have puzzled over the persistence of oil from the Exxon Valdez oil spill.
A pair of Lower 48 researchers on Sunday published results from the first study to attempt an explanation for why that oil isn't degrading as much as expected.
Their findings have implications for any future attempt to remove residual oil from the beaches -- a proposal launched by state and federal prosecutors in 2006 that is still being negotiated with Exxon Mobil Corp.
An estimated 21,000 gallons of the 11 million gallons of crude the Exxon tanker spilled in Prince William Sound in 1989 remain on Alaska beaches.
For years, federal and state officials expressed optimism that the oil from the massive spill would dissipate completely. In fact, in the first five years after the spill, scientific testing did show oil was degrading at a fast clip. Prince William Sound's beaches started to look idyllic again, even though people -- and animals -- digging below the surface still could find oil.
Since then, federal scientists have gathered evidence that disputes spill cleanup officials' prediction that all of the toxic oil would disappear in a few years. Further, the scientists say, the rate at which the oil is disappearing from the beaches has slowed down.
That's because the rest of the oil is trapped in a layer of compacted beach sediment where it cannot easily degrade, said Michel Boufadel, the lead researcher of the $1.2 million study, funded by the Exxon Valdez Oil Spill Trustee Council. The study is being published in Nature Geoscience, a monthly scientific journal. Boufadel runs the Center for Natural Resources Development and Protection at Temple University's College of Engineering, and specializes in studying the movement of oil in water.
His testing over the past three summers in Prince William Sound showed that its gravel beaches consist of two layers: an upper layer that is highly permeable to water, nutrients and oxygen, and a lower layer that isn't, he said.
Exxon oil that nearby freshwater sources didn't flush out to sea slowly trickled into the lower layer and got stuck, he said.
The Temple University tests occurred on Smith, Eleanor and Knight Islands, southwest of Bligh Reef where the Exxon tanker grounded, causing the nation's worst tanker spill.
The researchers dug 12 holes on each beach to a depth of 4 to 5 feet. The first summer, they used shovels. That didn't turn out well. Once they hit the lower, compacted layer of sediment, "we kind of broke our backs," Boufadel said. Sometimes, it took 16 hours to dig one hole, he recalled.
The second year, they switched to augers and jackhammers, he said.
The researchers shipped the sediment, water and other samples back to Temple for analysis.
The study is part of a recent spate of studies funded by the spill council on the lingering effects of oil in Prince William Sound. The spill council is the agency in charge of spending the $900 million in Exxon Valdez spill restoration funds.
Based on its own studies, Exxon contends that the oil that remains in beach sediment isn't hurting the environment.
Boufadel's findings aren't surprising, and they don't demonstrate a need for further cleanup, says Exxon contractor Paul Boehm.
"Scientists who have studied spills for years know that after crude oil spills you can and will find buried oil many years later, but that it does no harm and does not need to be (removed)," said Boehm, a chemist and vice president for Exponent, an international consulting firm that works for Exxon on spill-related issues.
The spill council disagrees. Its own research concludes that the sound's otters and sea birds are still ingesting harmful amounts of residual oil when they forage for food.
"Further monitoring will be needed to determine whether the environment is truly on a trend to complete recovery," according to a recent spill council report.
The spill council has been getting regular updates on Boufadel's work, said Peter Hagen, a National Marine Fisheries Service scientist who is managing some of the council's lingering-oil studies, including Boufadel's.
Hagen said the council has discussed the possibility of funding pilot projects to determine the best ways to remove residual oil from the beaches.
For example, it might be possible to degrade the oil by injecting oxygen or nutrients into wells or trenches dug into the beaches, according to a recent council report.
Those methods might be preferable to digging the oil out, scientists say.
"I don't think anyone wants to rip up the beaches again," said Jacqueline Michel, a South Carolina scientist who is finalizing a council-funded study that will map where the residual oil lies and identify which places have the greatest amount.
In its recent report, the council said it can identify 50 beach segments -- a total length of about 1.6 miles -- where a significant amount of oil remains buried.
When state and federal prosecutors presented their plan for additional oil cleanup on shorelines to Exxon in 2006, they estimated it would cost $92 million over several years.
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