Arsenic Release Characteristics Of Aquifer Sediments And Their Significance In Groundwater Arsenic Enrichment In Typical Arid-semiarid Basins

High arsenic groundwater is widely distributed and seriously harmful,threatening the health of hundreds of millions of people.The arid-semiarid basin is a common groundwater distribution area with high arsenic content.Arsenic in groundwater mainly comes from aquifer sediments,and pH and Eh control the adsorption and release of arsenic in sediments to a certain extent.However,it is not clear which of the two factors has a greater impact on the release of arsenic.Therefore,the typical arid-semiarid basins in China,Hetao Basin in Inner Mongolia and Yinchuan Basin in Ningxia,were selected as the study areas.Based on the analysis of the distribution and hydrochemical characteristics of high arsenic groundwater,the characteristics and influencing factors of arsenic release from aquifer sediments under different pH and Eh environments were studied,which could provide theoretic support to the formation mechanism of high arsenic groundwater in arid-semiarid basins.The following main understandings were obtained:?1?The groundwater in the study area is weakly alkaline,and the pH is generally between 7.2 and 8.6.Aquifers in Hetao basin are reductive,shallow phreatic aquifers in Yinchuan basin are weak anoxic,and deep confined aquifers are weak oxic.More than half of the water samples collected were groundwater with high arsenic content?As>10 ug/L?.Arsenic content of groundwater in Hetao area is generally higher than that in Yinchuan basin.The contents of As,Fe and Mn in reducing/weak reducing aquifers are relatively high,and they are positively correlated.The pH of groundwater with high arsenic content is generally higher.?2?Compared with the sediments of Yinchuan Basin,the iron oxides in the sediments of Hetao Basin have higher reductive activity and heterogeneity.These iron oxide minerals have poor crystallinity and large specific surface area.They have strong ability to adsorb As in oxic environment and strong release potential of As in anoxic environment,which is one of the important factors affecting the content of As in groundwater.?3?Low pH contributes to the dissolution of Fe oxide minerals and promotes the reduction reaction of Fe³⁺while high pH weakens the electrostatic interaction between sediments and As,resulting in the desorption of As.Low Eh contributes to the reductive dissolution of iron and manganese oxides in sediments,thus releasing As.The reductive dissolution of iron and manganese oxides contributes more to As content in groundwater than acid dissolution and arsenic desorption.?4?Arsenic content in groundwater in Hetao Basin is affected by both pH and Eh.As content in groundwater mainly comes from reductive dissolution of iron and manganese oxide minerals and desorption of As.Arsenic content in groundwater in Yinchuan Basin is mainly affected by Eh and is insensitive to pH.Arsenic mainly comes from reductive dissolution of iron and manganese oxide minerals.