In arid and semi-arid regions with limited water resources, groundwater-surface water interactions exert significant influence on the water quality and quantity of regional surface water. In this study, the variations of water quality of a reservoir located in high fluoride groundwater at the Datong Basin were monitored and evaluated based on the hydrogeochemical and isotopic analysis. Average fluoride (F-) concentration in reservoir water was elevated from 0.45 mg/L in wet season to 0.76 mg/L in dry season. Around the reservoir, the pore-water and groundwater were characterized by high fluoride concentration with average concentration of 2.21 mg/L and 2.54 mg/L, respectively. The features of hydrogen and oxygen isotopes of different type water samples indicated the strong evaporation and hydrologic connection between reservoir water, pore-water and groundwater. The 222Rn mass balance model was employed to quantify groundwater discharge rates and Ffluxes into the surface water. Groundwater discharge rates were quantified at 280.91 mm/d during the wet season and 116.61 mm/d during the dry season, corresponding to Ffluxes of 3.26 & times; 102 mg/m2 center dot d and 4.34 & times; 102 mg/m2 center dot d, respectively. The Fflux rates from high fluoride groundwater to reservoir exhibits significantly spatiotemporal heterogeneity that more pronounced rate was shown in the upstream area of groundwater flowing than the downstream area. The high fluoride groundwater resulted from the weathering of fluoride-containing sediments serves as the main source of reservoir fluoride via the periodic groundwater discharge. The results provide new insights for understanding the changing and its mechanisms of surface water quality in the arid and semi-arid regions.