SURFACE WATER HYDRODYNAMIC CONTROLS ON FLUID EXCHANGE ACROSS THE SEDIMENT-WATER INTERFACE IN A SALT WEDGE ESTUARY

Pore water biogeochemistry in bays and estuaries can alter nitrogen (N) loading to coastal ocean ecosystems through a variety of N cycling pathways. Tidal pumping in these systems is capable of driving large quantities of fluid and solutes across the sediment-water interface, potentially affecting biogeochemical processing of bottom nutrients and other contaminants in transit to coastal ecosystems. The reactivity of estuarine sediments is likely controlled by the relative inputs of freshwater and saltwater from the water column, however the rates of freshwater and saltwater exchange across the sediment-water interface in estuaries is not well understood. We combined a variable-density groundwater flow model with a surface water hydrodynamic model to investigate along-channel variability in tidal pumping of fresh and saltwater across the bed of a salt wedge estuary. Model results show that saltwater flux across the sediment-water interface is highest near the river mouth and decreases up the channel. Total fluid flux transitions from saline to fresh within the estuary mixing zone and decreases with distance up the estuary as the tidal amplitude decreases. The width of the brackish exchange zone decreases with an increase in river discharge owing to sharper salinity gradients along the saltwater wedge in the estuary. High river discharge also displaces the saltwater wedge in the channel seaward, resulting in reduced saltwater exchange and an increase in freshwater flux across a larger proportion of the estuary bed. The results demonstrate that surface water hydrodynamics is an important control on fresh and saltwater fluxes across the sediment-water interface in salt wedge estuaries. These insights into coupled groundwater processes and surface water hydrodynamics have implications for chemical fluxes to coastal ecosystems.