Soluble components of sediments and their relation with dissolved arsenic in aquifers from the Hetao Basin, Inner Mongolia

High groundwater arsenic (As) and salinity have been detected in aquifers of the Hetao Basin in Mongolia which have caused serious public health concerns. The objective of this study was to characterize the distributions of the soluble components in sediment in different lithologies and depths and to assess the relationship between soluble As in sediments and dissolved As in groundwater. One hundred and one sediment samples and 13 groundwater samples were collected from four boreholes at varied depths. In addition to total chemicals and mineralogical phases of sediments, the soluble components (including major ions and As, Fe, and Mn) in sediments and dissolved chemicals in groundwater were analyzed. Clay or silty clay had relatively higher EC values (189-805 mu S cm(-1)) than aquifer sands (approximately 92-261 mu S cm(-1)). The major soluble components were Na+, Ca2+, HCO3 (-), and SO4 (2-), which were more variable in clay samples than fine sand samples. Soluble As concentrations ranged between 2 and 950 mu S cm(-1), and high contents generally occurred in clay sediments with high contents of soluble Fe and Mn. A comparison of chemicals between soluble components in sediments and dissolved species in groundwaters at matched depths showed that chemicals were preferentially partitioned into sediments at the mountain front and deep aquifers (> 60 m), while partitioned into groundwater in the shallow aquifers (< 60 m) of the flat plain. Arsenic was preferentially partitioned into groundwater in aquifers with relatively low dissolved SO4 (2-). Groundwater components were mostly sourced from corresponding sediments. In clay sediments, As was desorbed from the surface sites along with other soluble components. Under reducing conditions, reduction of Fe oxides with high surface sites for As adsorption led to a weak association of As with other phases (such as carbonates), and therefore resulted in high dissolved As concentrations and low As partition between sediments and groundwater in deep aquifers.