Influence of beach hydrodynamics on saltwater transport and oil persistence in beaches

The exchange of water and solutes across the aquifer-ocean interface plays an important ecological role in both aquifer and the open water. Saltwater intrusion could prevent utilization of groundwater for either drinking water or irrigation. In addition, the flux of solutes, such as nutrients, from inland groundwater sources plays an important role in shoreline ecology, especially in embayments. This research relies on a three-pronged approach, involving a laboratory beach experiment, field scale experiments and numerical analyses. The laboratory beach experiment is designed to investigate the fundamental processes occurring in beach aquifers. It explores the effects of buoyancy (or lack thereof) on the flushing of freshwater to sea. The results indicate that the increase in water pressure when freshwater inundates saltwater systems exceeds the pressure obtained assuming the system is filled with freshwater. This has implications for the evaluation of stress on aquifers and the management of coastal aquifers.;Five hypotheses are designed and tested to explore factors affecting the beach hydrodynamics. One of the hypotheses establishes that the density gradient between saltwater and freshwater does not play a role in the intertidal zone of beaches. This hypothesis is tested through numerical modeling and laboratory experiments and is rejected. The results show that the density gradient has significant effect on solute transport in the intertidal zone. Another hypothesis states that depth (and slope) of bedrock greatly affects solute transport in beaches. Numerical investigations indicate that the depth of bedrock greatly affects solute transport in homogenous beaches while it has minor impact on that in the upper layer of two-layer beaches. A third hypothesis states that in locations of a large freshwater recharge, it is less likely to find oil. Numerical simulations reveal that freshwater recharge promotes the removal of oil in two-layer beaches by maintaining the water table at or above the interface of the two layers.;Findings from this work in relation to oiled beaches include: 1) oil tends to persist at locations of small freshwater recharge, 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, and 3) bioremediation of oil polluted beaches should be conducted using deep injection as amendments applied on the beach surface would washout rapidly to sea.;The field study is conducted along two transects of a tidally influenced gravel beach in Prince William Sound (PWS), Alaska, which was heavily polluted by the Exxon Valdez oil. The observed data of water table, pore water salinity and tracer (lithium) concentration indicates high freshwater recharge to the clean transect and low freshwater recharge to the oiled transect. The numerical model MARUN is used to reproduce the observations of water pressure and pore water salinity at both transects. Based on the field experiments and numerical simulations, the beach can be viewed as a two-layer system from a hydraulic point of view with a high permeable upper layer underlain by a layer with low permeability.