The immediate effects of an oil spill are all too apparent: dead wildlife, oil-covered marshlands and contaminated water chief among them. Now the results of a study published online by the journal Environmental Science and Technology suggest that contaminating oil can persist in the marine environment--perhaps indefinitely--even if surface sediments appear healthy.

Christopher M. Reddy of Woods Hole Oceanographic Institution and his colleagues studied a sediment core collected from marshland in Buzzards Bay near West Falmouth, Mass. In 1969, the barge Florida ran aground there and released 700,000 liters of oil into the environment. The team found that the top and bottom sections of the core were clear of petroleum residue. The section between six and 28 centimeters, however, contained levels of petroleum chemicals in concentrations similar to those detected immediately after the accident. "The long-term biological effects of oil contamination at this site is unknown since animals burrowing into these sediments can be exposed to high levels of some of these compounds," Reddy says. "It is clear from this study that oil spills can have a long-term impact on a coastal environment."