Spatial distribution and formation of high-fluoride groundwater in the Bashang region in Zhangjiakou

Groundwater in the Bashang region of Zhangjiakou has high-fluoride (F) levels, posing potential hazard to local residents．Here a total of 299 samples of shallow groundwater (well depth less than 50 m) were taken and analyzed．Spatial distribution of high F groundwater was subject to Arcgis analysis with inverse distance weight interpolation．Statistical analysis, hydrochemical analysis and hydrogeochemical simulation were applied to study the hydrochemical characteristics and to elucidate formation mechanism of high-F groundwater．Hydrochemical compositions were found to be mainly affected by rock-weathering, with HCO₃-Ca, HCO₃-Mg and Cl·HCO₃-Na being the dominant groundwater types in the study area．Fluoride level in groundwater was found to increase gradually from recharge area to discharge area in low-lying areas．High-F groundwater was present in low-lying discharge areas in the southern part of Kangbao County and northern part of Shangyi County, in lake-morass discharge areas in northwestern of Guyuan County, and in concealed basalt areas in northern part of Zhangbei County．Interactions between F-bearing minerals and groundwater, and ion exchanges were identified to be the primary mechanisms for the formation of high-F groundwater．Moreover, evaporation-crystallization process, dissolution of evaporites (e.g., halite and gypsum), precipitation of carbonates (e.g., calcite and dolomite), absorption-desorption process which occurred on the surface of clay minerals under alkaline environment were found to be the main controlling factors for F-enrichment in the aquifers in the Bashang region．