The purpose of this study is to investigate the hydrogeology of the Sarkikaraagac Basin in southwest Turkey and to assess its groundwater vulnerability. The groundwater of the basin, used for both drinking and irrigation, is discharged to the Beysehir Lake, the largest in the Lake District. Groundwaters have Ca-Mg-HCO3 and Mg-HCO3 facies. The most important pollutant factor is anthropogenic pollution, originating from dense and widespread agricultural activity, which moves through the groundwater to the Beysehir Lake. The groundwater vulnerability of the alluvial aquifer is determined with different parameters using the DRASTIC approach. The most vulnerable area, covering 30.23% of the basin, is strongly correlated with known pollution values.