Sources of groundwater salinity and potential impact on arsenic mobility in the western Hetao Basin, Inner Mongolia

•The build-up of groundwater salinity was related to natural processes.•Non-direct evaporation and mineral/evaporite dissolution contribute to groundwater salinity.•Co-occurrence of high As and high salinity groundwater were found.•High salinity groundwater (high SO4) may enhance As hazard in reducing environment.

The quality of groundwater used for human consumption and irrigation in the Hetao Basin of Inner Mongolia, China is affected by elevated salinity as well as high arsenic (As) concentrations. However, the origin of high salinity and its potential impact on As mobility in the Basin remain unclear. This study explores both issues using stable isotopic compositions and Cl/Br ratios of groundwater as well as the major ions of both groundwater and leachable salts in aquifer sediments. Limited variations in δ18O and δ2H (− 11.13 to − 8.10, − 82.23 to − 65.67) with the wide range of Total Dissolved Solid (TDS, 351-6734 mg/L) suggest less contribution of direct evaporation to major salinity in groundwater. Deuterium excess shows that non-direct evaporation (capillary evaporation, transpiration) and mineral/evaporite dissolution contribute to > 60% salinity in groundwater with TDS > 1000 mg/L. Non-direct evaporation, like capillary evaporation and transpiration, is proposed as important processes contributing to groundwater salinity based on Cl/Br ratio and halite dissolution line. The chemical weathering of Ca, Mg minerals and evaporites (Na2SO4 and CaSO4) input salts into groundwater as well. This is evidenced by the fact that lacustrine environment and the arid climate prevails in Pleistocene period. Dissolution of sulfate salts not only promotes groundwater salinity but affects As mobilization. Due to the dissolution of sulfate salts and non-direct evaporation, groundwater SO42 − prevails and its reduction may enhance As enrichment. The higher As concentrations (300-553 μg/L) are found at the stronger SO42 − reduction stage, indicating that reduction of Fe oxide minerals possibly results from HS− produced by SO42 − reduction. This would have a profound impact on As mobilization since sulfate is abundant in groundwater and sediments. The evolution of groundwater As and salinity in the future should be further studied in order to ensure sustainable utilization of water resource in this water scarce area.

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