In our country, the intrusion of sea water to groundwater in coastal aquifers is becoming an increasingly important problem. In this study, the hydrochemical and isotopic properties of groundwater in the coastal aquifer of Bafra Basin, Kızılırmak Delta were investigated, and the effects of seawater on groundwater were revealed. For this purpose, 48 groundwater samplings were made from the coastal aquifer in June- 2016, October-2016, June-2017 and October-2017 periods, and major anion and cation, trace element and isotope analyzes were performed on the samples taken. According to the results of hydrochemical analysis, the Electrical Conductivity (EC) value of the groundwater was found to be higher than 2.500 μS/cm in all sampling periods. In addition, Na+, Cl-, SO4-2 ion contents were determined to be quite high according to the standards of the World Health Organization (WHO, 2011) and TS-266 Regulation on Water Intended for Human Consumption (2006). According to the isotope analysis results performed on the groundwater samples, it was observed that they were close to the Global Meteoric Water Line, were recharge by precipitation falling at low elevations, and were under the influence of seawater intrusion. However, it has been determined that this initiative has decreased with the introduction of the irrigation system of the Bafra Plain Irrigation Project.