THE ROLE OF HYDROLOGY IN THE PERSISTENCE OF THE EXXON VALDEZ OIL SPILL

The 1989 Exxon Valdez oil spill contaminated around 800 kilometers of shoreline in Prince William Sound, Alaska. Despite extensive cleanup efforts and nearly 20 years of natural weathering, subsurface oil residues persist in patches in some beaches. The hydrogeological mechanism causing the oil persistence was not fully understood due to the complex surface and groundwater interactions in the intertidal zone including tides, inland freshwater recharge, sediment heterogeneity, seawater density-effect and beach landforms. Based on field data and numerical simulations, we show that the persistence of oil is due to the two-layered structure (a high-permeability surface layer underlain by a low-permeability layer) in conjunction with a small freshwater recharge. The surface layer probably provided the oil a temporary storage for its slow, continuous filling of the lower layer whenever the water table dropped below the interface of the two layers due to small freshwater recharge from inland. The oil did not seem to have penetrated the lower layer at locations where the freshwater recharge was large. The persistence seems to be due to the lack of oxygen resulting from the tidal hydraulics in the two-layered beaches. This was not considered in prior studies dealing with the spill. This study has implications on locating and bioremediating spilled oil on tidal gravel beaches widely distributed around the world, especially in mid- and high-latitude regions.