Water quality varies significantly depending on degree of leaching from aquifer rocks and pathway rocks. Waters' chemical compositions are determined by degree of water-rock interaction, mineralogical and geochemical properties of lithological structures, and environmental factors. Golcuk Lake is used partly as the source of Isparta City's drinking water. Volcanic rocks in this region are lava, volcanic breccia, agglomerate, and tuff and range from andesite to trachyandesite. Main hydrogeochemical facies determined in the area are (Na+-HCO3-). Concentrations of Mg2+, Na+, SO42-, and Cl- in the lake water samples increased in some years resulting from ion exchange of rock forming minerals by rainwater. The purpose of this research is to provide a better understanding of long-term water-rock interaction and its effect on water quality. Water quality indices were calculated using the Turkish Standard (TS) 266 of the World Health Organization (WHO) and the use of Golcuk Lake water as drinking water was assessed by Isparta. The fluoride content in Golcuk Lake was determined to be higher than the average.