Partitioning and reactivity of iron oxide minerals in aquifer sediments hosting high arsenic groundwater from the Hetao basin, P. R. China

High arsenic (As) groundwater has been widely found in the Yellow River catchment, which has posed potential risks on health of local residents. However, it is still unknown how fractions and reactivity of Fe-bearing minerals control As mobility in groundwater system. As a typical basin in the catchment, the Hetao basin was selected as the study area to investigate roles of fractions and reactivity of Fe-bearing minerals in groundwater As. Seventy sediment samples and eight groundwater samples were collected in a representative borehole in the basin. Sequential extraction and kinetic experiment were carried out to characterize fractions and reactivity of Fe-bearing minerals, respectively. Results showed that groundwater in shallow zone (between 10 and 36 m depths) had lower ORP values and higher As concentrations (average = 279 μg/L) than deep zone (average = 158 μg/L). In shallow zone, the predominant reactive Fe phase was reducible oxides (Feox2), while in deep zone it was carbonate associated Fe (Fecarb). Feox2 had the highest amounts of As and As/Fe ratios, followed by Fecarb and easily reducible oxides (Feox1). Moreover, As/Fe ratios in Feox2 had the best correlation with As/Fe ratios in groundwater. The largest partition coefficients were obtained between As in Feox2 and groundwater As (Kd-ox2). Kinetic experiments showed that sediments having higher reactivity (k') and heterogeneity (γ) had higher As/Fe ratios. Reactivity of sediment samples fell into the range between synthetic lepidocrocite and poorly crystalline goethite. Arsenic partition coefficients between reactive minerals and groundwater (Kd-reac) showed a positive correlation with ORP values, indicating that reductive dissolution of Fe(III) oxides played the important role in As mobilization. Therefore, reducing conditions, high k', γ and Feox2 contents in sediments led to high As concentrations in groundwater.