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Sea water intrusion into the groundwater is considered as important environmental issue for the coastal aquifers in Turkey. This research used an experimental analysis method and focused on the assessment of aquifer water quality due to sea water intrusion in the delta coastal area of Kızılırmak River, which is the third longest river, in Turkey, located in north-central region of the country. For this purpose, various analyses were conducted during four periods in June and October of 2016 and 2017 and in 48 wells during each period to identity hydro-chemical properties and isotopic composition of groundwater in delta coastal area. The physical properties, such as pH, electrical conductivity (EC), and total dissolved solids (TDS), of the groundwater samples were determined, and major anion cation, trace element, oxygen, deuterium, and tritium isotope analyses were performed on these samples. Since mean EC values were higher than 2500 µS/cm during each period, it was concluded that groundwater was not potable and unusable. Values of ions, such as sodium (Na+), sulfate (SO42−), and chloride (Cl−), among others exceeded the limits defined in Turkish Standards (TS266), World Health Organization (WHO), and Environmental Protection Agency (EPA). Groundwater was classified as inconsumable in terms of permeability index, sodium adsorption ratio, Na%, and Kelly Ratio (KR) values. With the Piper diagram, it was determined that the groundwater samples were mixed water and sea water intrusion characteristics. According to the TDS-ion graphs, it was concluded that the main mechanism controlling the groundwater chemistry in the Kızılırmak Delta is evaporation. The experimental analysis results showed that sea water intrusion seen in the north-western part of the delta aquifer is effective up to 3 km inland from the coast. For this reason, policies on groundwater management should be rearranged urgently for the coastal areas in the Kızılırmak delta with the condition that pumping of the wells are stopped. [ABSTRACT FROM AUTHOR]