Monday, January 18, 2010

Slow Biodegradation of Exxon Valdez Oil

As I have posted before, about 20,000 gallons of oil lingers in the beaches of Prince William Sound in Alaska. It is left over from the Exxon Valdez disaster in 1989 even though it was predicted to dissipate quickly. Apparently it has persisted due to a very slow rate of biodegradation brought on by the sound's physical conditions.

Boufadel said the beaches they studied consisted of two layers: an upper layer that is highly permeable and a lower layer that has very low permeability. He said 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, said Boufadel, have created a sort of sheltering effect on the oil, which often lies just 1-4 inches below the interface of the two layers.

Boufadel said that 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....

He said 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 his team 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.
The physical conditions described make finishing the clean-up much more difficult. At least now the cause is known, it may be easier to find a solution.