As concentration and speciation were studied of 146 groundwater samples of the Datong basin,northern China. It was shown that Astot,As(Ⅲ) andAs(Ⅴ) are in the range of0.4-1932, 0.2-1060 and 0.2-872 μg/lrespectively and the ratio of As(Ⅲ)/As(Ⅴ) is 0.36-4.1, with an average of 1.4. High arsenic groundwaters from the Datong basin are mostly soda waters, with TDS between 0.4 and 10.5g/l and 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 are always undersaturated with respect to primary rock forming minerals such as anorthite and albite. The water chemistry is also affected by continental salinization. Results of our hydrogeochemical studies indicate that the occurrence of soda water is one natural stage of water-aluminosilicates interaction, which happens when the water is saturated with respect to calcite, and under the condition that salt accumulation of evaporative enrichment is not strong or even absent. The soda-forming process creates a favorable geochemical environment for As enrichment in groundwater at Datong.
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