Water flow and solute transport due to Macrotide in a gravel beach

A combined field and numerical study of water flow and solute transport was conducted along a clean transect of a tidal gravel beach in Smith Island, Prince William Sound (PWS) that was heavily polluted after the Exxon Valdez oil spill. Water table and pore water salinity were directly measured in the field. The numerical model MARUN, validated with the field observations, was used to simulate subsurface flow and dissolved salt transport processes. Our study identified a two-layer hydraulic structure in the beach, a high-permeability upper layer and a low-permeability layer at the bottom. Numerical simulations indicated that the density gradient between the saltwater and freshwater strongly affects solute transport in the intertidal zone of the beach. A sensitivity study indicated that depth to and slope of the bedrock had major effects on solute transport in the lower layer, but minor effects in the upper layer. A deep bedrock caused the water particles travelling in the lower layer to penetrate deeper in the beach. Our study revealed high terrestrial freshwater recharge to the transect which promotes the removal of oil in two-layer beaches by maintaining the water table at or above the interface of the two layers. The study presented in this paper has strong implications on the oil persistence and associated removal strategies along the Alaskan coastline polluted with oil. Findings from this work in relation to oiled beaches include: 1) oil tends to persist at locations of small freshwater recharge and 2) Prior to oil arriving to the shoreline, one could minimize oil penetration into the beach by releasing water onto the beach at the high tide line during low tides.