The provenance of deep groundwater and its relation to arsenic distribution in the northwestern Hetao Basin, Inner Mongolia.

High-arsenic (As) groundwater has been widely found throughout the world. The source of groundwater would determine spatial distribution of groundwater As. In order to trace the source of high-As deep groundwater (DGW, depths > 50 m), groundwater, sediments, and local bedrock samples were taken to investigate chemical and isotopic compositions in the Hetao Basin, China. Results showed that 87Sr/86Sr in DGW gradually decreased with the increase in As concentrations along the approximate flow path. In recharge-oxic zone (Zone I), DGW was mainly recharged by fissure water, influenced mostly by weathering of phyllite bedrock and meta-basalt. In groundwater flow-moderate reducing zone (Zone II), DGW was mainly related to incongruent dissolution of feldspar. However, in groundwater flow-reducing zone (Zone III), DGW was partly recharged from shallow groundwater (SGW) with depths < 50 m. The mixing contributions of SGW to DGW in Zone III mostly exceeded 80% during groundwater irrigation season. In Zone I, DGW As concentrations were mostly lower than 50 μg/L due to oxic conditions. In Zone II, the weakly alkaline pH and the decreasing Ca/Na resulting from incongruent dissolution of feldspar caused As desorption, which was the major contribution to As mobilization (As mostly > 200 μg/L). In Zone III, the recharge of SGW introduced labile organic matter to support reduction of Fe(III) oxyhydroxides/oxides and predominantly led to As release into groundwater (As > 300 μg/L). This study has provided insights into the source of high-As DGW and the effect of SGW mixing on As mobilization.