Seawater intrusion assessment in sandbar aquifer based on numerical modeling and electrical resistivity tomography, southern Baltic coast, Poland.

This study investigates the origin and development of seawater intrusion (SWI) in the Vistula Spit, southern Baltic Sea coast (Poland), using Electrical Resistivity Tomography (ERT) and SEAWAT numerical modeling. ERT surveys identified zones of reduced resistivity associated with saline water, while seasonal fluctuations in near-surface saturation were observed between spring and autumn. Hydrogeochemical data from the Krynica Morska water intake corroborated these findings, showing elevated chloride concentrations (121–228 mg/L) in wells located within a long-term cone of depression. The spatial coincidence of the intake center and the IG-1 Triassic brine borehole complicates the interpretation of salinity origins. However, SEAWAT simulations demonstrated that high abstraction rates alone could not reproduce the observed low-resistivity anomaly. Only scenarios involving a potential discharge from the IG-1 borehole yielded consistent salinization patterns. The study highlights a dual hazard: seawater intrusion and brine discharge. Consequently, ongoing hydrogeochemical monitoring and follow-up ERT surveys are planned to further assess these hydrogeological risks. The results highlight the importance of a multi-method approach, as integrating ERT observations with numerical modeling to reduce uncertainty in model calibration and conceptual model development.