of skeletal fluorosis in Ningxia {#Sec9}
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Since 2009, Ningxia Disease Control and Prevention Center commenced to monitoring endemic fluorosis associated with drinking water in 27 natural villages of fluorosis areas in 9 counties/cities/districts (Lingwu City, Litong District, Tongxin County, Yanchi County, Qingtongxia City, Pengyang County, Xiji County, Haiyuan County, and Shapotou District). Skeletal fluorosis was monitored every 5 years. In 2009 and in 2014, 27 villages highly affected by fluorosis were monitored specifically and data on skeletal fluorosis are available. This condition was detected in 550 out of 1384 patients in 2009 (39.74%). A majority of these cases were detected in low-endemic areas (474 cases, 35.84%), while 44 (3.18%) were from medium-endemic and 32 (2.31%) from high-endemic areas. In 2014, 519 skeletal fluorosis cases out of 1398 were identified inthat of tributary, implying that the import of tributary into main stream waters no longer exerted an additive effect on fluorine concentration. During the dry season, rivers are mainly replenished with groundwater, which in this case is rich in fluorine due to leaching from fluoride-bearing rocks. During the wet season, the rivers are mainly replenished via rainfall, thereby diluting the overall fluoride concentration. Nevertheless, the tributary waters produced higher concentrations, most likely because the main river was fed by upstream alluvial sources low in fluorine. During the normal season, the latter effect seemed to be weaker, resulting in higher fluorine concentrations in the main river.Fig. 5Spatial distribution of surface water fluorine concentrations in Qingshui River basin in April (**a**), in July (**b**), and in November (**c**)

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