weathering of fluorine-bearing minerals. This leaching contaminates groundwater that feeds into surface waters, where evaporation leads to higher concentrations and rainfall to diluted concentrations. Thus, the overall concentrations are associated with geological features of these arid and semi-arid areas as well as the basin structure, hydrochemical facies of the groundwater, and local climate conditions. Due to less precipitation than evaporation, surface waters are being enriched in fluoride. High concentrations of fluorine in the Qingshui River are the results of water--rock interaction and evaporation. The latter increases TDS and fluoride concentrations; intensified ionic strength as a result of Qingshui River further enhances the solubility of fluoride.

The main source for drinking water in the investigated region is well water. Although well water was not directly investigated, the high fluorine concentrationsdetected in river water suggest that both groundwater and well water will also have a high fluorine content. The consumption of such contaminated drinking water is the most likely cause for the high incidence of fluorosis reported for this area. The data collected since the 1980s identified multiple regions with a high incidence of endemic fluorosis, especially in south-central Ningxia. Fluorosis cases concentrate in two villages located in the upstream region of the Qingshui River within the Yuanzhou District and in Xiji County along one of its tributary arms. The fluorosis incidence decreases further downstream of the Qingshui River. The spatial distribution of villages and population of endemic fluorosis present the following features: headwater \> river valley basin; mainstream \> tributary; upstream \> midstream \> downstream. These distribution