Tracing the Origin of Groundwater Salinization in Multilayered Coastal Aquifers Using Geochemical Tracers

Salinization represents a significant threat to freshwater resources worldwide, compromising water quality and security. In the Vieira de Leiria–Marinha Grande aquifer, salinization mechanisms are a complex interaction between seawater intrusion and evaporite dissolution. Near the coast, groundwater is mainly influenced by seawater, evidenced by Na-Cl hydrochemical facies, high electrical conductivity, and Na+/Cl−, Cl−/Br− and SO42−/Cl− molar ratios consistent with marine signatures. In areas affected by diapiric dissolution, besides elevated electrical conductivity, groundwater is enriched in SO42− and Ca2+ and in minor elements like K+, Li+, B3+, Ba2+ and Sr2+, and high SO42−/Cl− and Ca2+/HCO3− molar ratios, indicative of gypsum/anhydrite dissolution. The relationship between δ18O and electrical conductivity further supports the identification of distinct salinity sources. This study integrates hydrogeochemical tracers to investigate hydrochemical evolution in the aquifer with increasing residence time and influence of water–rock interaction, as well as the accurate characterization of salinization mechanisms in multilayer aquifers. A comprehensive understanding of these processes is essential for identifying vulnerable zones and developing effective management strategies to ensure the protection and sustainable use of groundwater resources.