Hydrodynamic factors affecting the persistence of the Exxon Valdez oil in a shallow bedrock beach

We report a field study and numerical modeling of multicomponent flow in a tidal gravel beach in Knight Island, Prince William Sound, Alaska, where oil from the 1989 Exxon Valdez oil spill persisted. Field measurements of water table, salinity, and tracer (lithium) concentration were obtained for around a week during the summer of 2008. The numerical model MARUN was used to simulate the field observations. On the basis of field experiments and numerical simulations, the beach was identified to have a two-layered hydraulic structure: a high-permeability surface layer underlain by a low-permeability lower layer. The hydraulic conductivity was found to be 5 × 10^-2 m s ^-1 for the surface layer and 7 × 10^-6 m s ^-1 for the lower layer. The simulations reproduced the observed water table, salinity, and lithium concentrations accurately. The small flow entering the beach from the land side resulted in a beach water table dropping below the interface of the two layers. This seems to be the major reason for the presence of oil in the lower layer. The exchange flow between the beach and the sea due to tidal influence was ∼2.12 m³ d^-1 m^-1. The patterns of inflow and outflow rates showed that the maximum seawater-groundwater exchange occurred in the middle to high intertidal zone, which explains the persistence of oil in the lower intertidal zone. To explore bioremediation of the beach with nutrient amendment, a numerical simulation of nutrient application on the beach surface was conducted, where the applied nutrient concentration was 5,000 mg L^-1. The results showed that the nutrient concentration remaining in oiled areas after a week was larger than 50 mg L^-1, which is larger than that needed for maximum microbial growth (2-10 mg L^-1). This implies that the bioremediation via nutrient application on the beach surface could be adopted if nutrients were the only limiting factor.