In the first five years after the accident, the oil was disappearing at a rate of about 70 percent and calculations showed the oil would be gone within the next few years. However, about seven or eight years ago it was discovered that the oil had in fact slipped to a disappearance rate of around four percent a year and it is estimated that nearly 20,000 gallons of oil remains in the beaches.
Researchers from Temple University say the beaches they studied consist of two layers: an upper layer that is highly permeable and a lower layer that has very low permeability. They say that, on average, water moved through the upper layer up to 1,000-times faster than the lower layer, and while both layers are made up of essentially the same materials, the lower layer has become more compacted through the movement of the tides over time. These conditions have created a sort of sheltering effect on the oil, which often lies just 1-4 inches below the interface of the two layers.
Oil from the Exxon Valdez spill continues to be found in the beaches along Alaska's Prince William Sound. Temple University researchers have found that the low concentrations of oxygen and nutrients in the beaches, along with the water flow in the beach's lower layer, have hindered the aerobic biodegradation of the remaining oil.
(Photo Credit: Michel Boufadel/Temple university)
Using groundwater hydraulic studies, the researchers found that the net movement of water through the lower layer of beach was outwards, so it is preventing oxygen from diffusing through the upper layer to where the oil is located.
"You have a high amount of oxygen in the seawater, so you would like to think that the oxygen would diffuse in the beach and get down 2-4 inches into the lower layer and get to the
oil," said co-author Michel C. Boufadel. "But the outward movement of the water in the
lower level is blocking the oxygen from spreading down into that lower layer."
The oxygen and nutrients are needed for the survival of micro-organisms that eat the oil and aid in aerobic biodegradation of the oil. But without the proper concentrations of the nutrients and oxygen along with the slow movement of water, anaerobic biodegradation is probably occurring, which is usually very slow.
The team says that because of Alaska's pristine environment, you would expect to find a low concentration of nutrients and this recent study confirmed the earlier findings. What Boufadel and Li found was, on average, that the nutrient concentration in the beaches was 10 times lower than what is required for optimal aerobic biodegradation of oil. They also found that the oxygen levels in the beaches are also insufficient to sustain aerobic biodegradation.
Boufadel and Li are now exploring ways to deliver the much needed oxygen and nutrients to the impacted areas in an effort to spur aerobic biodegradation of the remaining oil.
Citation: Hailong Li, Michel C. Boufadel, 'Long-term persistence of oil from the Exxon Valdez spill in two-layer beaches', Nature Geoscience Online, January 2010; doi: 10.1038/ngeo749
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