A study in 2004 estimated that perhaps 25,000 gallons of oil remained along the sound’s gravel beaches and was degrading very slowly. So that raised a question for researchers: Why, despite one of the largest environmental cleanups in history, has some oil persisted?
Michel C. Boufadel, an environmental engineer at Temple University, and a colleague, Hailong Li, have provided an answer. In a paper in Nature Geoscience, they report that the oil has become trapped in a zone of low permeability below the beach surface.
“We could only answer this question by understanding the movement of water within these beaches,” Dr. Boufadel said.
Field measurements showed that the beaches have two layers — a top one, a few inches to a few feet thick, that is roughly a thousand times as permeable as the layer below. The composition of the two is not very different, Dr. Boufadel said, but it is quite likely that compaction due to tidal forces has made the lower one less permeable.
Dr. Boufadel said oil floating on the water remained in the upper layer until changes in the water table allowed it to drip slowly into the lower layer, where it remains.
“In the lower layer there’s not enough motion and not enough oxygen for the oil to degrade,” he said.
But the oil can be released when otters or other creatures dig into the beaches. Even in their field studies, Dr. Boufadel said, when they would dig into deeper sediments, “the whole place would smell of oil.”
One possibility for cleaning up the trapped oil, Dr. Boufadel said, would be to inject chemicals into the lower layer that would promote biodegradation.