This study was carried out in the La Pletera salt marsh area (NE of Catalunya), situated in the north of the Ter River mouth. This area presents a coastal lagoon system associated with the shallowest aquifer levels of the Ter River fluviodeltaic system (10-20 m thick). Which are composed of prograding alluvial deposits, replaced by coastal deposits (marshes, dunes, and beaches) in the areas closest to the shoreline. The salinity regimes of these lagoons fluctuate significantly according to the volume of freshwater inputs from surface and groundwater sources, the presence of underlying sediment with low permeability and the climate conditions. The aim of this study is to determine the variation in aquifer salinity, comparing the dry and wet periods of the year, as well as considering the aquifer-lagoon interactions. Fifteen Electrical Resistivity Tomography (ERT) campaigns were conducted periodically from June 2022 to November 2024, combined with continuous measurements of electrical conductivity (EC), temperature, and water levels of the main lagoons and superficial piezometers, located next to the lagoons. Additionally, during each campaign, potentiometric measurements were taken from nearby wells, to determine the water table evolution of the Ter River fluviodeltaic aquifer. The study period coincides with a severe drought episode, during which precipitation was 30% below the average, leading to reduced freshwater recharge to the aquifers. Potentiometric data show a constant decline in the water table levels of the aquifers, decreasing by 0.3 to 1.5 m during the study period. Meanwhile, the continuous EC measurements and the ERT profiles reveal a progressive inland advance of seawater and the leaching of hypersaline water from the lagoons to the aquifer. This study has been realized within the framework of the collaborative international Project consortium TREASURE (PCI2024-153436) and the AquiPondSys grant (PID2023-147186OB-I00), funded by MCIN/AEI/10.13039/501100011033.