11.2 The Datong Alluvial Aquifer, Shanxi, China

Northern China has several areas affected by high F concentrations in groundwater and one of the best studied is the Datong basin. Endemic arsenicosis and fluorosis in this area have been known for more than 22 years (Wang, 1998). The groundwater geochemistry for the shallow system (< 80 m) evolves from a Ca-HCO3 type recharge water around the margins of the basin through an intermediate mixed zone with increased Cl and HCO3 concentrations to a zone closer to discharge (near the center of the basin) with the highest concentrations of F averaging 7.2 mg/L for 27 wells and with a maximum reported value of 80.9 mg/L (Guo and Wang, 2005). This zone also has the highest pH values and the highest concentrations of Cl and HCO3. In the center of the basin the discharge water is also of a Na-HCO3 type with high F but lower concentrations than the intermediate zones. Further work by Wang and others (2009) reinforced the relation between Na-HCO3 type waters. High F concentrations showed that calcite and fluorite were saturated to supersaturated for many of the wells thus limited the Ca and F concentrations. The F concentrations varied from 0.14 to 39 mg/L and pH values ranged from 7.37 to 9.13. A study by Li and others (2012) found similar trends for 486 groundwater samples from the basin with a maximum F concentration of 22 mg/L, as well as a correlation with elevated pH (as high as 9), Na-HCO3 type waters and evaporation. Another 70 wells were sampled in a follow-up study (Su et al., 2013) and indicated that shallow to intermediate wells contained higher F concentrations than the deeper ones and that calcite and fluorite solubilities were reached and exceeded in groundwaters from several wells. Saturation indices for calcite and fluorite are shown in Figure 14.

Graphs showing calcite and fluorite saturation indices as a function of HCO3 concentration

Figure 14  a) Calcite saturation indices as a function of HCO3 concentrations for Datong Basin recalculated from data of Su and others (2013). b) Fluorite saturation indices as a function of HCO3 concentrations for Datong Basin recalculated from data of Su and others. (2013).

The study by Pi and others (2015) showed further evidence for the importance of evaporation using water isotopes and the correspondence of high F concentrations with waters that had reached calcite saturation and should precipitate calcite to keep the Ca concentrations low and F concentrations high. Several of these studies showed a positive correlation of F concentrations with HCO3 or alkalinity and sometimes with Cl concentration. In Figure 15a, a plot of F concentration against HCO3 shows the positive qualitative trend and Figure 15b shows that PCO2 also influences the F concentration. This plot points out the importance of PCO2 as an independent variable.

Graphs showing fluoride concentration increasing with increasing HCO3 and PCO2 concentrations

Figure 15  a) Fluoride concentration increasing with increasing HCO3 concentrations for Datong Basin using data of Su and others (2013). b) Fluoride concentrations also increasing with increasing PCO2.


Fluoride in Groundwater Copyright © 2022 by Pauline L. Smedley. All Rights Reserved.