Using H, O, Rn isotopes and hydrometric parameters to assess the surface water-groundwater interaction in coastal wetlands associated to the marginal forest of the Río de la Plata

The interaction between groundwater and surface water flows determines the ecohydrological characteristics of coastal environments which are periodically flooded by tidal flows. The Punta Lara Natural Reserve constitutes the southernmost vestige of the marginal forest of the Río de la Plata, which develops on the margins of Las Cañas stream. The aim of this work is to assess the interaction between groundwater and surface water in the reserve. For this purpose, isotopic data and hydrometric parameters for surface water and groundwater were obtained. δ18O, δ2H and 222Rn activity were measured from three sampling campaigns performed in winter of 2017 and 2018 and early summer of 2017. A monitoring network composed of shallow groundwater boreholes and surface water measuring points was designed to record groundwater and tidal levels, together with data of daily rainfall. The results obtained show that during high tides or extraordinary high tides, water spreads through the Las Cañas stream and floods the coastal levee infiltrating in the sandy sediments. Thus, the water of the Río de la Plata constitutes a source of periodic recharge to the groundwater within the levee, being of greater magnitude than the recharge registered during the events of intense rains. Conversely, groundwater discharge processes dominate during low tide. These results may contribute to the understanding of surface water - groundwater interaction processes as conditioning factors of the environmental characteristics.