This paper investigates the hydrodynamics in a tidally influenced sandy beach polluted by the Deepwater Horizon (DWH) oil spill. The oil was brought to the beach during a storm, and subsequent wave activity buried it under about 0.15 m of clean sand. We obtained field measurements of water table, pore water salinity, and soil moisture at a transect, and we used the two-dimensional finite element model MARUN (Boufadel et al. 1999) to simulate water flow below and above the transient water table while taking into account the effect of salt concentration on water density. The model was calibrated by fitting to the observed data and numerical simulations demonstrated the following: As the water was approximately one meter below ground surface, the physical washout of the buried oil by groundwater flow seems impossible. High salinity was observed in both landward well (~25 g/l) and seaward well (~39 g/l). This was probably because the beach is bounded on the landward side by Pensacola Bay, which has a salinity comparable to that of the Gulf of Mexico. The soil moisture in the oil-contaminated layer of the beach was around 20%~30%, indicating the oiled sediment was unsaturated. This range appears to be sufficient for the biodegradation of oil based on the literature.