Groundwater showed a trend from oxic-suboxic conditions in the alluvial fan to anoxic conditions in the flat plain in the Hetao basin, with increases in As, Fe2+, NH4+, alkalinity and pH, and decreases in Eh and NO3-. A 1D reactive transport model was constructed using a model flow column by PHREEQC to quantitatively interpret geochemical processes related to As mobilization in a 5000 m-long hydrogeological unit. Results indicated that groundwater As concentration was mostly controlled by

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... ) oxide reduction and As adsorption processes. Observed Fe2+ was the combination of Fe(III) oxide reduction and mackinawite/siderite precipitation. Increasing alkalinity was the result of oxidation of organic carbon, rather than the dissolution of carbonates.