Aqueous geochemistry and its influence on the partitioning of arsenic between aquifer sediments and groundwater: a case study in the northwest of the Hetao Basin

Abstract

The consumption of groundwater containing high arsenic concentrations has caused serious public health problems in the Hetao Basin in Inner Mongolia. However, the relationship between As contents of aquifer sediments and in groundwater at the same depth under different redox conditions is currently poorly understood. In order to determine this relationship in the Hetao Basin, four multilevel wells were set up to monitor As and other physiochemical parameters in groundwater at different depths in the aquifer. Sediment samples were also collected at equivalent depths to groundwater sampling intervals during the drilling of boreholes. Results showed that groundwater As concentrations ranged between 2.26 and 583 μg L−1. In suboxic shallow groundwater (depths 14 m) with relatively lower Eh values. The As contents of sediments had a negative correlation with the grain size of sediments, showing the high affinity of fine grained sediments to As. Positive relationships were observed between dissolved As concentrations and contents of extractable As (including S1: weakly bound As, S2: strongly bound As, and F3: Fe/Mn oxide bound As) in aquifer sediments for a set of depth-matched samples in reducing and suboxic environments. The relationships showed 263 µg L−1 dissolved As per 1 mg kg−1 S2-extracted As in reducing environment, and 9.4 µg L−1 dissolved As per 1 mg kg−1 S2-extracted As in suboxic environment. In the reducing environment, As tended to be partitioned into the liquid phase, where S2-As showed a better correlation with dissolved As than S1-As. In the suboxic environment, As tended to be fixed in the solid phase, where S1-As of aquifer sediments was closely related to concentrations of dissolved As in groundwater.