Assessment of groundwater-driven dissolved nutrient inputs to coastal wetlands associated with marsh-coastal lagoons systems of the littoral of the outer Río de la Plata estuary

Abstract

In coastal wetlands the hydrological dynamics and in particular the groundwater flows play a critical role in the establishment of wetlands and in the transport of salts and nutrients. The aim of the work is to analyze the role that groundwater discharge has in the dynamics of the dissolved nutrients of the wetland associated with the coastal lagoon and marshes of the Punta Rasa Natural Reserve, which is located on the coastal sector of the southern end of the Río de la Plata estuary. A monitoring network in the form of transects was generated in order to define groundwater flows and take samples of dissolved species of N and P. The presence of sandy sediments with similar granulometric profiles in all geomorphological environments determines that the underground flow occurs in a homogeneous aquifer. From the dunes and beach ridges the fresh to brackish groundwater flows with a very low hydraulic gradient towards the marsh and coastal lagoon. The contributions of N and P would derive from the degradation of the organic matter of the environment, in the case of the marsh and coastal lagoon also from the tidal flow and discharge of groundwater, and possibly from atmospheric sources in the case of N. Since in all environments oxidizing conditions dominate, nitrification is the main process which is why the most abundant species of N is the NO3−. Under oxidizing conditions, P has a greater affinity for the sediments in which it is mostly retained, registering it in low concentrations in water. The discharge of groundwater from the dunes and beach ridges provides dissolved nutrients to the marsh and coastal lagoon. However, the low hydraulic gradient and the dominant oxidizing conditions determine that the flow is scarce and that it only acquires relevance in the contribution of NO3−.