Exploring secondary contamination in river and spring waters: A novel phenomenon arising from seawater intrusion into deep coastal aquifers.

Seawater intrusion, characterized by the infiltration of a seawater wedge into deep coastal freshwater aquifers, poses a significant threat to water quality. Despite extensive research on this hydrogeological phenomenon, monitoring remains challenging, often requiring deep drilling that exacerbates aquifer contamination. This study proposes a novel approach for monitoring seawater intrusion based on hydrogeochemical indicators, specifically focusing on secondary contamination observed in river and spring waters along the Black Sea coast of Russia. Using mass spectrometry, we analyzed the abundance and concentrations of 71 elements, identifying key indicators of secondary contamination influenced by the low hypsometric altitudes of the region. In total, 30 river and spring water samples were analyzed. Notable findings include a positive correlation between excess trace elements and Cl and SO, characteristic of seawater. River waters exhibited lower mineralization compared to spring waters, with consistently higher Mn/Br, Cs/Br, and ∑ rare earth element/Br ratios, serving as reliable indicators of secondary contamination. This study allowed us to propose a geochemical indicator (GIM) GIM = ∑ Mn Cs REE/Br which reflects the secondary contamination intensity. The decrease in GMI dynamics indicates an increase in the impact of the secondary contamination process on surface and spring waters. This innovative approach minimizes the reliance on extensive drilling, providing valuable insights for the study and monitoring of seawater intrusion in coastal aquifers.