Influence of groundwater discharge on the hydrochemistry of coastal lagoons and marsh environments in Punta Rasa Natural Reserve area, Argentina

The Punta Rasa Natural Reserve encompasses a system of small coastal lagoons, which are developed in a sand spit that limits with the coast of the southern end of the outer estuary of the Río de la Plata. The aim of this work is to estimate the discharge flow of groundwater from the environments of beach ridges and dunes adjacent to the coastal lagoons and evaluate how it influences the hydrochemistry of the associated lagoons and marshes. For this, a monitoring network was defined covering sampling points in the sea/estuary, coastal lagoons, and groundwater located in transects that perpendicularly cross the main body of the coastal lagoon. In each transect, water levels, pH, electrical conductivity, and 222Rn were measured in situ and samples were collected to determine the major ions. The analysis of groundwater flow profiles and 222Rn values allowed the identification of the existence of groundwater discharge flows in the three coastal lagoons. It was also possible to estimate the different components of the groundwater discharge flow and the velocity and discharge flows. Regarding chemistry, although coastal lagoons and marsh water are saline and show Na-Cl hydrochemical facies that reflect tidal flow contribution, in the areas of groundwater discharge from the dunes and beach ridges there is a decrease in the electrical conductivity of the marsh water associated with an increase in the concentration HCO−3+CO−23. This is because the water from the dunes and beach ridges is fresh to saline and has higher concentrations of HCO−3+CO−23 than sea water. Hydrodynamic differences in the three lagoons are associated with the relationship between tidal flow contributions—groundwater discharge responds to morphological changes owing to the evolution of the sand spit that encompasses the lagoons.