Genesis of arsenic/fluoride-enriched soda water: A case study at Datong, northern China

The high As and F− groundwaters from Datong Basin are mostly soda waters with a Na/(Cl+SO4) (meq) ratio greater than unity, As and F− up to 1550 μg/L and 10.4 mg/L, respectively, and with pH between 7.6 and 9.1. Geochemical modeling indicates that the waters are oversaturated with respect to calcite and clay minerals such as kaolinite, and undersaturated with respect to primary rock-forming minerals such as anorthite and albite. The water chemistry also is affected by evapotranspiration. The degree of evaporative enrichment is up to 85 in terms of Cl−. Results of the hydrogeochemical studies indicate that the occurrence of soda water at Datong is the result of incongruent dissolution of aluminosilicates at one stage of their interaction with groundwater when the water is oversaturated with respect to calcite and evapotranspiration-related salt accumulation is not too strong. Studying the genesis of soda waters provides new insights into mechanism of As and F− enrichment in the aquifer system. Due to CaF2 solubility control and OH−–F− exchange reactions, F− can be enriched in soda water. And the high pH condition of soda water favors As desorption from oxyhydroxide surfaces, thereby increasing the concentration of As in the aqueous phase.