Oil from the Exxon Valdez — not gone or forgotten. Credit: EXXON VALDEZ Oil Spill Trustee Council

Oil dumped in the Gulf of Alaska by the Exxon Valdez remains beneath the beaches of the area more than 20 years on because a porous layer of rock helps to lock the oil away from the air, researchers think.

Experts have puzzled over how such pockets of oil remained buried over the years. Shortly after the Exxon Valdez spill in Prince William Sound in March 1989, researchers calculated that the oil would dissipate within years or even months. Some suggested that natural processes such as microbial degradation would take care of most of the spill; others that the oil would get washed away from shorelines by waves or by high-pressure hoses. But pockets of oil have remained, buried half a metre below the surface of some beaches, including Eleanor Island, which received a substantial slug of the contaminant.

Eleanor Island has a 40-metre stretch of gravelly beach that is dotted with small boulders and rocks, left behind by waves that winnowed away finer sediments. Gaps between these pebbles and rocks are large enough to allow water and oil to seep through. Below them, slightly finer sediments and rocks got compacted together over time. These geological processes eventually resulted in two layers, with the top layer hundreds of times more permeable than the one below, according to research by Michel Boufadel and his postdoctoral fellow Hailong Li at Temple University in Philadelphia, Pennsylvania.

Going with the flow

Boufadel and Li measured how water flowed through the rocky sediments using several wells sunk in two sections perpendicular to the shore and tracked the salinity of the water table as it changed with incoming tides and as freshwater flowed down to the sea through the beach. They also looked at 'stagnant' areas of the water table that were untouched by tides.

Oil filtered through rocks until it became trapped underground. Credit: Center for Natural Resources Development and Protection

The researchers determined that in sections where freshwater flowed, the oil-containing groundwater was more likely to be carried out to sea through the pore-filled sediment layer. But where the water table was stagnant, oil would percolate into the less-permeable rock layer below. That layer also contains less oxygen, according to their measurements. Once trapped in the low-permeability layer by capillary action, the lack of oxygen may prevent the oil from breaking down chemically, or stop microbes from munching on the hydrocarbons that remained.

The beach's underground plumbing "is very important", says Boufadel. "It is the major mechanism for predicting how the oil would move over time." He suggests surveying other beaches with coarse glacial sediments in places such as Norway, to see whether they behave the same way to help to plan for potential oil spills.

Boufadel and Li, who is now at China University of Geosciences in Wuhan, suggest that such Arctic glacial areas, where shipping and oil drilling is likely to become more common as the climate changes, could end up as deep-sediment traps for oil spills. The study is one of several projects funded by the Exxon Valdez Oil Spill Trustee Council, and could support the council's efforts to acquire more money from Exxon for remediation. The work is published online today by Nature Geosciences1.

Rare bird?

Jeff Short, Pacific Science Director for the non-profit environment group Oceana in Juneau, Alsaka, says that the research clearly illustrates how pockets of oil could be preserved for decades in buried sediment. Researchers had assumed that beaches with such highly permeable surface rocks would be washed clean, but in 2001, Short and his colleagues reported that pockets of oil were more common than previously thought2. Their findings raised concerns over oil's lingering effects on animals such as sea otters, which dig deep to find clams and other food3.

But other researchers say that the aged oil is not a threat to animals. They are unlikely to be exposed to it because it is buried so deep, says Paul Boehm, principal scientist for consulting firm Exponent, which was hired by Exxon to examine many of the beaches in the affected area. Boehm also says that the Eleanor Island beach is "a rare bird", and that very few beaches have the same geological conditions, even in Prince William Sound.

"The beaches are so heterogeneous [and] the features where oil is sequestered are very patchy," Boehm says. He adds that if the question is remediation, it might be better to leave the oil where it is, rather than exhuming it and exposing the ecosystem to even more residue.