Oil spilt from the tanker Exxon Valdez in 1989 can still be found on the Alaskan coast because of the two-layered structure of local beaches, according a study online this week in Nature Geoscience. As oil exploitation and shipping in the Arctic region becomes more feasible owing to global warming, effective environmental protection and clean-up of spilt oil will become increasingly important.

Michel Boufadel and Hailong Li investigated the groundwater dynamics of a beach on Eleanor Island, Alaska, which was contaminated by the Exxon Valdez oil spill, using field measurements, tracers and numerical simulations. They found that the upper layer of the beach acted as a reservoir for the spilt oil, protecting it from weathering and loss of fluidity. From this reservoir, the oil entered the lower layer whenever the water level fell below the interface between the two layers. Because of the low oxygen content in the lower layer, the oil is not degraded and can persist in the long term.