Saltwater intrusion in the Hel Peninsula (Poland): Local controls and geophysical insights

The Hel Peninsula, Poland - a narrow, low-lying coastal landform bordered by the Baltic Sea and Puck Bay. This study examines the spatial dynamics of saltwater intrusion (SWI) in a vulnerable peninsular aquifer. An integrated approach was applied, combining Electrical Resistivity Tomography (ERT) surveys with well logs, geological maps, and meteorological data. Six shallow ERT profiles, reaching depths of ∼30 m below sea level, were analyzed to capture the transition from freshwater lenses to saline zones. Analytical modeling using Fetter’s solution, calibrated to the Kuźnica profile, was adapted to represent the three-phase system of freshwater, bay water, and seawater. Results show that SWI is governed by peninsula morphology and subsurface lithology. Asymmetric intrusion patterns emerged when comparing Puck Bay and Baltic Sea profiles. Although Fetter’s model provided an ideal first approach, it mainly served to interpret geophysical anomalies that revealed a more complex SWI boundary than assumed in analytical solutions. However, the analytical solution facilitated the geophysical interpretation, indicating “geophysical SWI boundaries”. Across all profiles, lithology was generally uncomplicated, with fine-grained sands (mainly) or medium sands found beneath a shallow soil layer, and only rare interbeds of finer fractions were observed. Moreover, the study is the first integrated investigation of SWI conducted on the Hel Peninsula and, more broadly, in Poland. • patterns of saltwater intrusion resulting from geology in the peninsula were identified. • conductivity cross-sections imaged the distribution of fresh and saltwater in the peninsula subsurface. • Fetter’s solutions were applied to find the salt-fresh water contact zone.