Abstract Drinking water in the Hetao basin relies significantly on deep groundwater. Little is known about the occurrence of As and its distribution in deep groundwaters of the Hetao Basin. This study observed that this deep groundwater is affected by the presence of As in a nearby mountainous area. Arsenic concentrations gradually increase from the alluvial fans to the flat plain of the basin. Four hydrogeochemical zones were distinguished based on hydrogeological conditions, together with the distribution of redox-sensitive elements and their relationship with As concentrations. Although As mobilization is fundamentally controlled by the reductive dissolution of Fe-bearing minerals, reduction of SO42-also affects groundwater As concentrations. If the supply of SO42- is sufficient, further reduction of SO42-may lead to a decrease of As levels through co-precipitation with pyrite. This information is helpful for choosing localities for safe drinking water wells in mountain-edge alluvial fans.