Washouts as a source of dissolved elements to the coastal ocean in southern Brazil and its hydrogeological characteristics

Abstract

The coastal zone of Rio Grande do Sul state, Brazil, presents several surficial water bodies (washouts), which act as a drainage system of the coastal plain. Considering the entire extension of the coast, the number of washouts is very significant, thus, this study aims to assess in an unprecedented way the availability of nutrients, carbon and iron, dissolved in washout waters and its contribution in terms of concentration to the coastal ocean. Furthermore, due to groundwater in the beach region being one of the major sources for the formation and maintenance of these water bodies and influencing their hydrochemistry, we used the Ground Penetrating Radar as a tool to understand the interaction between the surficial and subterranean compartments, the influence of freshwater from the washouts on the aquifers and the extent of this interaction. The influence of the freshwater flux through the washout may reach 1000 m in length and a trough of up to 3.2 m. The medium outflow of freshwater by the washouts was heavily influenced by the rainfall, ranging between 0.12 m3 s−1 and 0.95 m3 s−1, in the dry and wet seasons, respectively. Throughout the Rio Grande do Sul coastal plain, washouts may represent 28% of the medium freshwater outflow from the Patos Lagoon Estuary and can top the outflow from Tramandaí Lagoon by 25%. The high outflow in the wet season impacted on the dissolved elements flux, with estimates of 274 ton y−1 for SiO44−, 0.42 ton y−1 for PO43−, 7.07 ton y−1 for total inorganic N, 1.7 ton y−1 for Fe and 262 ton y−1 for total dissolved C. Reckoning the values for the inorganic nitrogen flux, the washouts can support a potential primary production in the surf zone of 18 gC m−2 y−1, considering the entire Rio Grande do Sul coastal plain.