Soluble Components Of Sediments And Their Relation With Soluble Arsenic In Aquifers From The Hetao Basin,Inner Mongolia

Groundwater with high salinity and high arsenic concentrations has been widely found in aquifers from the Hetao Basin,which has posed serious public health concerns and environmental problems.An extraction method with a solid-to-liquid ratio of 1:5 was used to extract soluble components of sediments and saline alkali soils.Distribution of soluble salinity and soluble As in aquifer sediments was investigated,and the related hydrogeological processes controlling the relationship between dissolved components and soluble components.The main findings are shown as follows.(1)Lithology and sampling depth were the main factors controlling the sediment soluble salinity.Contents of soluble salinity generally exhibited a decreasing trend along depth.Due to the low permeability,high soluble salinity was observed in clay or silty clay samples.In contrast,there is relatively low soluble salinity in fine sand due to groundwater flushing.Soluble Ca2+ and Na+,SO42-and HCO3-were the major cations and major anions in soluble components,respectively.(2)Lithology and redox conditions exhibited remarkable effects on soluble As.Nearing land-surface(<15 m),contents of soluble As kept constant(<18 ?g/kg)or showed an increasing(3–133 ?g/kg)trend along the depth.Low contents of soluble As occurred in aquifer sands(generally <50 ?g/kg)due to groundwater flushing.High contents of soluble As were generally observed in clay sediments which had high contents of soluble Fe and Mn,indicating that soluble As was mainly originated from dissolution of Fe/Mn oxides.A good correlation between soluble HCO3-and soluble As was observed in aquifer sands and shallow sediments.Soluble SO42-showed greater influence on soluble As in sediments from the mountain front.(3)In areas with different irrigation water(Yellow river water and groundwater),high pH and contents of soluble components were observed in the saline alkali soils(313-660 mg/g and 9.07-10.16,respectively).They had similar major ions in soluble salinity,with Na+ and SO42-as the major cation and major anion,respectively,which may from dissolution of thenardite and halite.Low contents of soluble As and Fe were detected because most of these trace components were immobilized onto soil surfaces in oxic environment.(4)The enrichment of groundwater As was mainly caused by competitive adsorption and reducing conditions.Major components were preferentially partitioned in sediments at the mountain front,while into groundwater in the flat plain.Cation exchange showed a notable effect on the partitions of cations,especially in the flat plain.In weakly reducing conditions,As was preferentially partitioned in sediments due to adsorption.However,under reducing conditions with relatively low dissolved SO42-,HCO3-and As were easily dissolved into groundwater.