ABSTRACTThe aims of this research are to determine hydrogeological and hydrogeochemical investigations of Sandıklı (Afyonkarahisar) basin and to plan of optimum groundwater management of the basin. The Sandıklı basin is situated in the west of the Aegean region and discharge to the Büyük Menderes Basin. The research area is occurred from Sandıklı Kuruçay basin and Küçük Sincanlı basin. It has 1556 km2 recharge area. The basin has a semi-closed basin property due to discharge to Kestel River. The river is appear from the Sandıklı basin and discharge to the Agean Sea via the Büyük Menderes River. Furthermore, Hüdai geothermal waters (70 0C) located in Sandıklı which emerge as result of the tectonics are one of the most important geothermal areas of the country. In the research area, Quaternary alluvium, Pliocene sediments and Cretaceous limestones are important aquifer units. According to groundwater level measurements of September-2008 and May-2009 terms, general groundwater flow direction of the basin is towards to the Büyük Menderes River. 1072 pumping wells are located in the research area. These wells are utilized to drinking and irrigation purposes densely. Decreasing of groundwater (average 10 m/year) has observed due to overexploitation of groundwater in the research area. Therefore, a new well dugging has not given permission by governmental associations nowadays. In addition, Hüdai geothermal water is also influenced negatively by overexploitation of the waters. It is shown that, detail hydrogeological investigations must be made in the Sandıklı basin.Hydrogeological properties which are controlled to using conditions of water sources are the most important factors. Representative locations were selected to general hydrogeochemical evaluating of groundwater in the basin and, physical and chemical analyses of groundwater was made. According to the classification of the waters, cold groundwater has Ca-HCO3 facies and Hüdai thermal and mineral water has Na-HCO3 facies.Key Words : Sandıklı (Afyonkarahisar) Basin, Hydrogeology, Hydrogeochemistry