Evaluation of seawater intrusion in a sandy unconfined coastal aquifer using hydrodynamic analysis and vertical electrical soundings (Pehuen Co, Argentina)

Abstract

This study evaluates seawater intrusion (SWI) in the sandy unconfined coastal aquifer of Pehuen Có (Buenos Aires Province, Argentina) through the integration of hydrodynamic analysis and vertical electrical soundings (VES). The objective was to characterize the geometry of the freshwater–saltwater interface and identify mechanisms controlling salinization in a low-exploitation coastal system.Hydraulic gradients indicate active groundwater discharge toward the sea under present conditions. The Ghyben–Herzberg relation predicts interface depths of about −80 m at the beach front relative to mean sea level. However, geoelectrical data suggest that the main saline wedge is located between −90 and −140 m and is preferentially hosted in lower-permeability units underlying the sandy aquifer, highlighting the influence of stratigraphic heterogeneity on intrusion geometry.Additionally, a shallow salinization process (0–11 m depth) was identified in the beach sector. The observed resistivity patterns are consistent with a tide-influenced salinization process linked to the flooding of semipermeable Holocene marine deposits exposed in the intertidal zone, which may temporarily retain saline water and promote localized vertical mixing. Similar processes have been described in the literature under different conceptual frameworks, and are here inferred for the first time in the study area.Under sea-level rise scenarios, increased tidal inundation may intensify shallow salinization and modify vertical salinity distribution. The proposed conceptual model provides a useful framework for supporting groundwater management and coastal conservation strategies in similar sandy coastal environments, while acknowledging the limitations associated with the absence of independent validation of salinity distribution.