Isotope and minor element geochemistry of high arsenic groundwater from Hangjinhouqi, the Hetao Plain, Inner Mongolia

High As groundwater is widely distributed in the northwestern Hetao Plain, an arid region with slow groundwater flow. Arsenic concentration in groundwater ranges from 1 to 1000 μg/L. Most water samples have elevated salinities, with Cl and/or HCO3 as the dominant anions and Na as the dominant cation. High concentrations of As in shallow aquifers are associated with strongly reducing conditions, as evidenced by high concentrations of dissolved organic C (DOC), NH4, dissolved sulfide, arsenite and dissolved CH4, and relatively low concentrations of NO3- and SO42-. Results of the hydrochemical, and H and O isotope geochemical studies indicate that evapotranspiration is an important process controlling the enrichment of Na and Cl as well as trace elements such as As, B, F and Br in groundwater. In Na-HCO3-dominated groundwaters, As, B and F were enriched. Decades of irrigation using Yellow River water has resulted in elevation of the groundwater level, which has accelerated salt accumulation in shallow groundwater and surface soil. In addition, irrigation is responsible for the release of some components from aquifer materials and mixing with saline groundwaters, as indicated by minor element and isotope geochemical data. Used to trace groundwater flow paths, Sr isotope composition also indicates that bedrock weathering is one of the primary sources of As in groundwater in the study area.