SEASONAL WATER TABLE EFFECTS ON TIDE-INDUCED SALTWATER-FRESHWATER MIXING IN SANDY BEACHES

Fresh groundwater in coastal aquifers is often high in nutrients relative to seawater and thus poses an ecological threat to near-shore waters when discharged at the shoreline. The fate of these nutrients and other chemicals may be influenced by physical flow processes and saltwater-freshwater mixing in the beach aquifer prior to discharge. Field measurements of inland water table elevation and beach subsurface salinity distribution were obtained at tidal, spring-neap, and seasonal (salinity profiles obtained on average 1-2 months for 8 months) frequencies. The measurements revealed an upper tidally-driven saltwater recirculation cell beneath the beach surface that remained relatively unchanged in the vertical and to lesser extent horizontal directions over tidal and spring-neap cycles. Seasonality had the largest effect on mixing in the beach aquifer. In spring, the saltwater-freshwater mixing zone was pushed closer to the surface as freshwater intruded into the deeper regions of the beach aquifer. The areal extent of the mixing zone was largest during summer with salinities similar to those of the estuarine surface water found at depths of greater than 3 m. A fresh groundwater discharge zone was found between 2 and 7 m offshore of the low tide mark, and was present throughout the seasonal monitoring period except during August when the inland water table was relatively low. A variable-density numerical model was constructed to simulate groundwater flow and solute transport in a sandy beach subjected to tidal oscillations and a seasonally variable inland water table. Simulations show that tidal oscillations lead to saltwater exchange across the aquifer-ocean interface, resulting in saltwater recirculation and mixing with underlying fresh groundwater along the perimeter of the recirculation cell. The extent and intensity of saltwater-freshwater mixing varies at seasonal time scales, indicating that the terrestrial freshwater hydraulic gradient effects the salt distribution in the beach subsurface. The results demonstrate the role of different forcing conditions and the time scales involved in beach groundwater flow and mixing processes, which may affect contaminant fluxes to the coastal ocean.