Hydrogeochemical and isotopic investigations on the origins of groundwater salinization in Çarşamba coastal aquifer (North Turkey)

The aim of this study is to determine the origins of salinization and the main hydrogeochemical process that controls the chemistry of Çarşamba coastal aquifer in Turkey. Therefore, a total of 33 groundwater samples and three seawater samples were analyzed in the coastal region of Çarşamba Plain in July 2019, and for these samples’ physical parameters, major ions and environmental isotopes (δ18O, δ2H, and 3H) values were determined. Piper, Chadha, Gibbs diagrams and Stuyfzand Classification Systems were used to determine the origins of salinization and the key hydrogeochemical process controlling the groundwater chemistry. According to Stuyfzand classification system, the study showed that the freshwater and fresh-brackish water main types are the most widespread in the study area. Six water subtypes were observed in the study area that include CaHCO3, CaMix, NaMix, NaCl, NaHCO3, and MgHCO3. In addition, the subtypes CaMix indicated the locations of the transition zone, where the groundwater rich in Ca and HCO3 and gradually enriched in Na changes from CaMix with HCO3 as dominant anion to eventually CaCl and NaCl subtypes. Furthermore, the subtypes NaMix, NaHCO3, and MgHCO3 showed the locations of the transition zone where the flushing of the saline aquifer by freshwater takes place. All groundwater samples from study area had a positive cation exchange code, and show that four hydrogeochemical facies composed of CaHCO3, Ca–Mg–Cl, NaCl, and NaHCO3. Besides, groundwater samples have been influenced by two main mechanisms: the water–rock interaction and evaporation–crystallization. According to δ18O, δ2H, and 3H analysis, the water samples have meteoric origin, shallow circulation, and a short residence time.