Abstract: When assessing the origin of salinity and the mechanisms of salinization in coastal aquifers, hydrogeologists may consider the combined use of certain geochemical tools to assess critical aspects of the hydrogeological setting of the system. These tools are based in the integrated use of chemical (major ions, trace elements and ionic ratios) and isotope parameters (oxygen, hydrogen, sulphur, carbon, strontium and boron). The problem of groundwater salinization in coastal aquifers, besides active seawater intrusion, may be affected by several human activities that accelerate the progressive deterioration of water quality, such as concentrated pumping, intensive agricultural practices including return flows or reuse of waste waters from urban or industrial origin. The characterisation of the perating processes and mechanisms of salinization is a requisite for a proper management of groundwater resources and for adopting remediation strategies. In this contribution the potential role of several isotopic tools in these studies is briefly described.

Abstract: In order to identify the origin of saline groundwater in the eastern part of Jeju volcanic island, Korea, a hydrogeochemical and isotopic study has been carried out for 18 observation wells located in east and southeast coastal regions. The total dissolved solid contents of groundwaters are highly variable (77–21,782mg/l). Oxygen, hydrogen, sulfur, and strontium isotopic data clearly show that the saline water results from mixing of groundwater with seawater. Strontium isotopic compositions and Br/Cl and I/Cl ratios strongly suggest that the source of salinity is modern seawater intrusion. Hydrogeochemical characteristics based on bivariate diagrams of major and minor ions show that changes in the chemical composition of groundwater are mainly controlled by the salinization process followed by cation-exchange reactions. The highly permeable aquifers at the east coastal region are characterized by low hydraulic gradient and discharge rate and high hydraulic conductivity as compared with other regions. These properties enhance the salinization of groundwater observed in the study area. Based on the Cl, Br, and δ18O data, seawater was determined to have intruded inland some 2.5km from the coastline. Considering the poor correlation of sampling depth and Cl concentrations observed, the position of seawater-freshwater interface is not uniformly distributed in the study area, due to heterogeneities of the basaltic aquifers.