Understanding salinity dynamics in coastal aquifers is critical for managing groundwater quality under increasing climate pressures. This study investigates seasonal salinity variations in the shallow aquifer of the La Pletera salt marsh (NE Iberian Peninsula), where hypersaline lagoons interact with an unconfined aquifer composed of fluvio-deltaic sands and clays. Using time-lapse electrical resistivity tomography (ERT) in combination with continuous monitoring of groundwater and lagoon water levels, temperature, and electrical conductivity (EC), we identified key patterns of aquifer-lagoon connectivity and salinity recharge. Temperature correction of bulk EC was applied to account for thermal effects in the active zone down to 5.5 m depth, where temperature varies significantly due to atmospheric temperature changes leading EC variations up to 30 %. In the 3-12 m depth range, where clean sands predominate, ERT data revealed salinity anomalies (up to 132 mS/cm) that were consistent with the salinity of overlying hypersaline lagoons (mean for FRA lagoon: 73.7 +/- 19.2 mS/cm). This observation supports the interpretation that lagoon water recharges the aquifer under specific hydraulic conditions. Time-lapse ERT also imaged a significant increase in aquifer salinity after rainfall events (with accumulated precipitation >30 mm over 30 days), particularly in areas adjacent to the FRA and M03 lagoons. Such salinization is driven by the hydraulic gradients triggered by the rapid recharge of the lagoons by rainfall, which causes their hydraulic head to rise above those of the connected aquifer. Additionally, ERT revealed a progressive landward migration of the seawater wedge, driven by a regional groundwater decline during extended drought periods. These findings highlight the importance of including lagoon-aquifer interactions in salinization assessments and provide a transferable framework for managing groundwater quality in other Mediterranean or semi-arid coastal systems, under climate change scenarios.