Relative importance of hydrogeochemical and hydrogeological processes on arsenic enrichment in groundwater of the Yinchuan Basin, China

Arsenic (As) contamination in groundwater has been widely observed in the world and posed negative impacts on human health. Although factors controlling As concentration in groundwater have been intensively investigated, relative importance of hydrogeological settings and geochemical factors in As enrichment is less known. Groundwater samples from three hydrogeological transects and different aquifer sediment samples from the Yinchuan basin (China) were taken to systematically evaluate roles of hydrogeochemical and hydrogeological factors in As mobility. Dissolved As concentrations ranged between 0.04 and 263 mu g/L, and 46% of studied groundwater samples had As concentrations greater than the World Health Organization drinking water guideline (10 mu g/L). High As groundwater (As > 10 mu g/L) generally had ORP values between-175 and-75 mV, while low As groundwater mostly had ORP >-75 mV. Sedimentary Fe and Mn oxides were the major source of groundwater As in aquifers. In each transect studied, As concentrations were positively correlated with concentrations of dissolved Fe and/or Mn or negatively correlated with ORP values. This suggests that reductive dissolution of Fe(III) oxides released the adsorbed As into groundwater. In addition, desorption of As contributed to As enrichment in groundwater, which occurred at high pH and/or high HCO3- conditions. The hydrogeochemical characteristics played a leading role in the control of As concentrations in each transect. However, for all dataset, groundwater obtained in aquifers with higher hydraulic gradients generally had lower As concentrations. It indicates that, at the basin scale, hydraulic gradients and groundwater flow conditions were the dominant factors controlling As distribution. This study emphasizes the relative importance of hydraulic conditions in controlling As behavior at the basin scale yet neglected.