Spatial distribution of arsenic along groundwater flow path in Chaobai River alluvial–proluvial fan, North China Plain

The North China Plain has been identified as a potential high-arsenic (As) groundwater area; however, little is known about the As distribution along the groundwater flow path. In this study, 154 groundwater samples were collected from the alluvial–proluvial fan of the Chaobai River, which lies in the north of the North China Plain. The chemical compositions of the samples were analyzed to investigate the spatial distribution of groundwater As and to understand the hydrogeochemical processes that controls As mobilization. Results revealed that groundwater As concentrations were < 1 μg/L in the piedmont recharge zone (zone I) where groundwater flushing was strong, while high (< 1–73 μg/L) in the middle runoff zone (zone II), which features a moderate permeability and hydraulic gradient. In the downward plain zone (zone III) under stagnant hydraulic conditions, groundwater As concentrations reached 111 μg/L. Vertically, high-As groundwater mainly occurred in shallow aquifers (depths < 100 m) in zones II and III. In addition, iron (Fe), manganese (Mn), and NH₄⁺ concentrations showed increasing trends similar to those of As along the groundwater flow path, whereas the NO₃⁻ concentrations and redox potential (Eh) showed decreasing trends, which implies a gradual change in the redox conditions. The systematic variation in the As concentrations along the groundwater flow path and the correlations between As and redox sensitive components suggest that groundwater As would occur via the reductive dissolution of Fe/Mn oxides and that slow groundwater flow is an important factor that promotes As enrichment in groundwater.