Long-term persistence of oil from the Exxon Valdez spill in two-layer beaches

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Oil spilt from the tanker Exxon Valdez more than 20 years ago still persists in the gravel beaches of Prince William Sound, Alaska. Field data and numerical modelling indicate that some of the oil was trapped in the anoxic environment of the lower layers of the beaches when the water table was low. Oil spilled from the tanker Exxon Valdez in 1989 (refs 1, 2) persists in the subsurface of gravel beaches in Prince William Sound, Alaska. The contamination includes considerable amounts of chemicals that are harmful to the local fauna3. However, remediation of the beaches was stopped in 1992, because it was assumed that the disappearance rate of oil was large enough to ensure a complete removal of oil within a few years. Here we present field data and numerical simulations of a two-layered beach with a small freshwater recharge in the contaminated area, where a high-permeability upper layer is underlain by a low-permeability lower layer. We find that the upper layer temporarily stored the oil, while it slowly and continuously filled the lower layer wherever the water table dropped below the interface of the two layers, as a result of low freshwater recharge from the land. Once the oil entered the lower layer, it became entrapped by capillary forces and persisted there in nearly anoxic conditions that are a result of the tidal hydraulics in the two-layered beaches. We suggest that similar dynamics could operate on tidal gravel beaches around the world, which are particularly common in mid- and high-latitude regions4,5, with implications for locating spilled oil and for its biological remediation.