Hydrochemical dynamics of a wetland and costal lagoon associated to the outer limit of the Rio de la Plata estuary

Abstract

Coastal lagoons and marshes are among the most fluctuating and productive ecosystems in the world, which provide a wide range of ecosystem. The objective of this work was to evaluate the factors and processes that condition the hydrogeochemical dynamics in salt marshes associated with the development of small coastal lagoons located at the southern end of the outer estuary of the Rio de la Plata, where it confluences with the Atlantic Ocean. Four samplings were carried out where major ions composition was determined in seawater, surface water of the lagoon and tidal channel, and in groundwater of the marsh, measuring in situ water pH and the electrical conductivity (EC). The chemical data were analyzed considering the conditions and processes associated with both water balance and tidal regime. The coastal lagoon develops within a sandy spit with dunes which is connected to the sea by a narrow tidal channel where the entry of the tidal flow is limited. The tide that enters has strong chemical variability, reflected in EC ranging from 19.1 to 37.5 mS/cm, depending on the dominant source (ocean or estuary). In periods with evapotranspiration domain, tidal water in the lagoon evaporates resulting in soluble salts precipitation such as gypsum and halite. The subsequent tidal flood dissolves these salts causing that in these periods both surface water of the lagoon and groundwater in the marsh present EC values between 8.4 and 57.1 mS/cm. Conversely, when evapotranspiration is low, surface water of the lagoon and tidal channel and groundwater of the marsh have EC between 3.5 and 18.1 mS/cm, and freshwater discharge from the dune sector where the rain preferentially infiltrate becomes relevant. The results obtained show the strong chemical variability that this type of coastal wetlands presents, providing the basis for its conservation andmanagement of natural reserve areas.