THE EFFECTS OF TRANSIENT OCEANIC AND TERRESTRIAL FORCING AND BEACH TOPOGRAPHY ON FLOW AND SOLUTE TRANSPORT IN COASTAL AQUIFERS

Nutrient, metal, and carbon fluxes in submarine groundwater discharge can impact the chemistry of nearshore marine ecosystems. These solutes can undergo biogeochemical transformations in the saltwater-freshwater mixing zones of beach aquifers, so understanding the driving mechanisms and timescales of flow, transport and mixing are important for estimating chemical fluxes to the sea. A variable-density groundwater flow and solute transport model was used to investigate intertidal salinity dynamics over tidal, spring-neap, and seasonal cycles in a sandy beach under tidal influence. A continuous 1-year simulation with tidal forcing with 5 tidal constituents and a variable inland head boundary was used to examine the effects of tidal stage, spring-neap variability in tidal amplitude, and seasonal variability in freshwater flow on intertidal salinity dynamics and fresh and saline discharge. Salinity dynamics across the top layer of the model provide insight into the importance of the three time scales in controlling the width of the fresh discharge zone, the horizontal extent of the circulation cell, and the location of the top of the lower saltwater-freshwater interface. The size of the saltwater-freshwater mixing zone along the perimeter of the cell was examined at the three time scales. The largest change in size occurred in response to seasonal inland water table fluctuations, followed by spring-neap variability in tidal amplitude, and tidal stage. The dynamics of the circulation cell were consistent for simulations with and without a beach berm. Groundwater residence time and velocity of circulating seawater were estimated by injecting a bromide solution into a sandy beach and tracking the movement of the plume through the subsurface. The results were confirmed with a numerical tracer experiment and show the pathway of circulating saltwater between the high and low tide marks. Fresh and saline discharge varied over the three time scales with saline discharge inversely proportional to the elevation of the inland water table at the seasonal time scale. The size of the mixing zone and temporal and spatial patterns of intertidal salinity over the three time scales is likely to affect groundwater chemistry in these dynamic systems.