THE ROLE OF STRATIGRAPHY AND GEOMORPHOLOGY ON THE PERSISTENCE OF THE EXXON VALDEZ OIL SPILL IN ALASKA

The 1989 Exxon Valdez oil-spills (EVOS) polluted around 2,000 km of rocky intertidal shorelines within Prince William Sound (PWS), Alaska. After two decades, subsurface oil persists in many initially-polluted beaches along PWS. The goal of present investigation is to identify beach hydrogeological factors related to the EVOS persistence in a tidal gravel beach located on Smith Island, PWS at the coordinates 147º 23' 6.41" W, 60º 31' 39.10" N. The sediments on this beach contain a large percentage of coarse gravel and its bedrock seems to be 3 to 4 feet deep, especially in the oiled areas. We installed 12 wells in two transects: six in the oiled transect and six in the clean transect. Based on field measurements of water level and salinity, and numerical simulations calibrated to these measurements, we concluded that the beach can be identified to have a two-layered structure: a high-permeability surface layer and a low-permeability layer. The hydraulic conductivity was found to be 10^-2 m/s for the surface layer and 10^-5~10^-4 m/s for the lower layer. The beach surface had a slope of 7% in the oiled transect and 11% in the clean transect, respectively. Our analysis concludes that the two layer stratigraphy along with a mild beach slope were the major factors for oil persistence. This study has implications on locating and bioremediating spilled oil on tidal beaches around the world.