Role of freshwater in the persistence of the Exxon Valdez oil spill in a wave-exposed beach

More than 20 years after 1989 Exxon Valdez oil spill in Prince William Sound, Alaska, the oil still persists on some of the previously polluted beaches. As part of the investigation for the persistence of oil, we conducted a field study for water flow and solute transport along a clean transect of a tidal beach in Smith Island, Prince William Sound (PWS), Alaska that was heavily polluted by the spill. Field measurements of water table and pore water salinity were obtained, and the results were modeled numerically by the model MARUN (MARUN is a finite element model capable of simulating water flow and solute transport in variably saturated media while taking into account the effect of salt concentration on water density and viscosity). A random-walk particle tracking code was also used to investigate the flow path in the sediment. The simulations reproduced the observed water table and salinity accurately. The results suggested that the beach has two layers of very different hydraulic properties: A high permeability upper layer underlain by a low permeability layer. Particle tracking results indicated that the average pore water velocity in the upper layer and lower layer was 8 m/hour and 5.7×10-3 m/hour, respectively. We believe that due to the high freshwater recharge into this beach, the water table always remained above the interface of the two layers. Therefore, oil could not penetrate into the lower layer and persist there for a long period of time.