Hydrogeomorphological control over groundwater composition in littoral environments in the outer estuary of the Rio de la Plata

Abstract

The aim of this work is to determine the hydrochemical characteristics of groundwater in littoral ridges, sand sheets, and lunettes environments to evaluate the control that hydro-geomorphological features exert over salinity and water chemistry. To achieve this, geomorphological units were mapped and a groundwater monitoring network was established comprising shallow exploration wells and pre-existing deeper wells. Hydrolithological characteristics of sediments were described during field surveys, and also analyzed by XRD technique and binocular magnifying glass observation. Physicochemical parameters of water were measured in situ and samples were taken for major and trace elements (As, Sr, Fe, and Mn) analysis. Both littoral ridges and sand sheets are constituted by shell fragments and sands which are highly permeable sediments where thicker and freshwater lenses are stored. Conversely, lunettes develop smaller lenses due to the presence of silty sediments, which are shallow salinized. Additionally, water—sediment interaction determines geochemical processes like precipitation—dissolution of minerals (mainly carbonates), cation exchange, amorphous and biogenic silica dissolution, and silicate weathering which also regulate the presence of elements, such as As, Fe, and Mn. Given the relevance of littoral ridges, sand sheets, and lunettes in the development of freshwater lenses for water supply for the population and ecological sustenance, it is important to understand the factors and processes which regulate groundwater quality, primarily in sectors where freshwater sources are scarce.