Behavior of Salt Transport and Submarine Groundwater Discharge between an Estuary and the Underlying Coastal Aquifer

Coastal aquifers usually act as a source of freshwater received from terrestrial sources and supplied to the overlying estuaries in the form of submarine groundwater discharge (SGD). Those aquifers act also as a sink of salt fluxes received from the estuary. Heavier saline estuarine water underlain by a less dense terrestrial-based fresh groundwater results in an unstable flow and salt transport condition. Exchange of freshwater and saline fluxes between the aquifer and the estuary may have significant impacts on the estuarine ecosystem and therefore, it is important to be understood. Two numerical models were developed using SEAWAT to study the behavior of SGD and salt exchange at two transects across the Indian River Lagoon. Field data revealed that the normalized IRL salinity can vary from freshwater salinity of 0.3 to that of ocean salinity of 1, based on ocean salinity of 35 g/L. Results obtained from model simulations showed that a transition mixing zone of brackish water exists below the estuary. Salt fluxes are found to migrate from the estuary down into the aquifer even though SGD is discharging upward from the aquifer into the lagoon. Higher IRL salinity was found to cause more salt transported into the aquifer and lower SGD into the lagoon. Regression analysis revealed a straight-line correlation (r² =1) between increasing IRL salinity and decreasing SGD. Spatial distribution of the SGD below the lagoon obtained from the numerical models showed that the SGD occurs across the entire transect with the majority being received from the mainland.