Geochemistry of river and spring waters during seawater intrusion into deep coastal aquifers

Seawater intrusion into coastal freshwater aquifers is a widespread hydrogeological phenomenon that poses a significant risk to coastal freshwater resources, often exacerbated by anthropogenic pressures and low hypsometric relief. This study investigates the hydrogeochemical effects of seawater intrusion into coastal aquifers on the chemistry of river and spring waters in the Abrau Peninsula on Russia’s Black Sea coast. Analysis of major ions and 71 chemical elements in water samples collected over three hydrological seasons (2022, 2024, 2025) revealed a persistent bicarbonate-chloride calcium-magnesium-sodium composition. This pattern is indicative of subsurface seawater intrusion into coastal aquifers, with secondary contamination of surface waters occurring through discharge of salinized groundwater via ascending springs and fractured zones exposed by deeply incised valleys. Consistently elevated concentrations of marine-associated elements (Cl, Br, S, Na, Ca, Mg, B, Sr) were recorded across all sampling years and sites. On the basis of these findings, a composite geochemical index – the Geochemical Indicator of Secondary Contamination (GISC) – was developed to diagnose and monitor coastal surface water salinization. GISC values show strong correlations with seasonal hydrological conditions and geomorphological features, exhibiting lower values during arid periods and higher values in areas where geological structures facilitate vertical discharge from confined aquifers. The indicator provides a practical, non-invasive tool for detecting secondary contamination of surface waters resulting from seawater intrusion into aquifers, thereby supporting sustainable management of freshwater resources in vulnerable coastal systems.