The salinization process was investigated with hydrogeochemical tools to evaluate the origin of salinity in the Selçuk Plain. Na/Cl and Cl/Br molar ratios of coastal zone that covers an alluvial aquifer, karstic discharges, and a wetland are similar to the local seawater ratio. According to mixing ratios, seawater addition can reach 9-18% in both seasons at the coastal zone especially in karstic springs that are a kind of seawater-freshwater mixing points. A thematic map of the Salinization zone was constructed for the Selçuk Plain using geographical information system tools with different parameters such as major ion ratios, EC values, mixing ratios, and sodium adsorption ratio values. High correlation between Cl and Br, B, Li and Sr explains the salinization process that comes from marine intrusion. The δ 18 O and δD composition of water samples varies between −6.7‰ and −2.9‰; −37‰ and −20‰, respectively. The coastal zone waters are plotted on the mixing line due to the effects of the salinization process in the plain.