Freshening processes in a prograding littoral system of the Río de la Plata–sea interaction area

Abstract

The aim of this study is to evaluate groundwater freshening processes and the development of freshwater lenses in a littoral environment associated with the coastal progradation of beach ridges in the coastal area associated to the interaction zone of the Río de la Plata and the sea. To achieve this, a geomorphological analysis was conducted through the interpretation of satellite imagery, digital elevation models, and field observations, along with sediment sampling and texture analysis. Based on the geomorphology, transects were defined for conducting electrical resistivity tomography (ERT), and a monitoring network for groundwater and surface water was established. In situ measurements of pH and electrical conductivity (EC) were made, and water samples were collected for major ion determination. The geomorphological, sedimentological, geophysical, and hydrochemical studies have identified areas where freshening processes generate freshwater lenses in a littoral environment, initially dominated by saline water. These freshening processes are directly associated with the infiltration of rainwater under humid climate conditions and water surpluses. Infiltration primarily occurs in beach ridge deposits, which are positive landforms with high permeability. Spatially, it was observed that the formation of freshwater lenses mainly occurs in the more continental environments (older within the progradation of the beach ridges), where beach ridges exhibit greater topographic expression and are typically amalgamated deposits. In contrast, in the more coastal zones near the estuary, the development of freshwater lenses is limited by the lower topography of the deposits, which restricts rainwater infiltration and, consequently, reduces freshening processes. A similar situation occurs in the inter-ridge depressions towards the continental zone, where rainwater infiltration is present but limited, resulting in areas of freshwater-saline water mixing. This analysis of the distribution of freshwater lenses demonstrates spatial variation in freshening processes that is controlled not only by geomorphology (presence of beach ridges) but also by the evolutionary sequence of these landforms (progradation). The findings from this study are crucial for the development of sustainable water management and supply strategies in coastal regions, like the one studied, where freshwater resources are scarce.