Cation exchange and leakage as dominant processes in controlling salinity and strontium in sandy and argillaceous coastal aquifer: Insights from hydrochemistry and multi-isotopes

[Display omitted] •Evaporite dissolution influences salinity, yet doesn’t clarify the Sr variety.•Silicate & carbonate weathering majorly contribute to Sr in shallow groundwater.•Cation exchange drives Sr increase in deep groundwater.•δD/δ18O and 87Sr/86Sr reveal deep brine leakage to shallow groundwater. Coastal groundwater is crucial for supporting ecosystems and meeting the water needs of coastal communities. However, over-exploitation of groundwater resources, including fresh water and deep brine, leads to groundwater depletion and exacerbates saline intrusion, posing significant challenges to sustainable development. This study focuses on the south bank of Laizhou Bay, a typical area threatened by saline intrusion due to the salt industry’s extraction of deep brine and over-exploitation of fresh groundwater. To assess the impact on salinization processes, we employed hydrochemistry and multi-isotope techniques to study strontium (Sr) distribution and salinization in coastal groundwater, unraveling their origins and explaining hydrochemical processes. The hydrochemical analysis revealed that evaporite dissolution primarily contributes to salinity, especially in deep brine and saline water (TDS > 30 g/L). The relationship between 87Sr/86Sr and chlor-alkali index (CAI) and saturation index (SI) indicated distinct sources of Sr. In fresh and brackish water (