ANCHORAGE, Alaska — An engineering professor has figured out why oil remains trapped along miles of gravel beaches more than 20 years after the Exxon Valdez tanker disaster in Prince William Sound.
An estimated 20,000 gallons of crude remain in Prince William Sound, even though oil remaining after the nearly 11 million-gallon spill on March 24, 1989, had been expected to biodegrade and wash away within a few years.
The problem: The gravelly beaches of Prince William Sound are trapping the oil between two layers of rock, with larger rocks on top and finer gravel underneath, according to Michel C. Boufadel, chairman of the Department of Civil and Environmental Engineering at Temple University. His study appeared Sunday in Nature Geoscience's online publication.
He found that water, which could have broken up and dissipated the oil, moved through the lower level of gravel up to 1,000 times slower than the top level.
Once the oil entered the lower level, conditions were right to keep it there, he said. Tidal forces worked to compact the finer-grained gravel even more, creating a nearly oxygen-free environment with low nutrient levels that slowed the ability of the oil to biodegrade.
"The oil could be maybe one foot below the beach surface and in contact with sea water with a lot of oxygen, but the oxygen doesn't get to it," Boufadel said.
He found that the upper layer of beach is so permeable that the water table falls within it as fast as the tide. The permeability of the lower level is so low, however, that the water table does not drop much within it, he said.
He said the study points out the susceptibility of beaches worldwide to long-term oil contamination, especially at higher latitudes where beaches tend to be gravel or a mixture of sand and gravel.
"As global warming is melting the ice cover and exposing the Arctic to oil exploitation and shipping through sea routes such as the Northwest Passage, the risk of oil spills on gravel beaches in high-latitude regions will be increased," the study says.