Inorganic Component-organic Carbon Characteristics Of Aquifers And Its Controlling Effect On Arsenic Enrichment In Groundwater

The arsenic content in groundwater in Hetao Basin of Inner Mongolia is as high as 500μg/L,far exceeding the standard of drinking water arsenic concentration（10μg/L）stipulated by the World Health Organization（WHO）,which has caused great harm to the health and quality of life of local residents.In the past studies,a large number of samples and analyses have been made to the groundwater in this area,and the distribution characteristics and enrichment mechanism of arsenic in groundwater have been understood to some extent.However,the control effect of hydrogeochemical characteristics on arsenic release,migration and transformation in groundwater is lacking from the perspective of the sedimentary environment in which groundwater is located.Through field investigation and laboratory analysis,this study studied the hydrogeological conditions of aquifer,main and trace elements,different occurrence states of arsenic,spatial distribution characteristics of organic carbon and its isotopes in sediments,and discussed the main factors of arsenic release and migration in groundwater.The main conclusions of this paper are as follows:（1）The groundwater in the study area is mainly affected by precipitation and Yellow River irrigation,and the groundwater hydrochemical type is mainly SO₄-Na type.The redox sensitive components in groundwater present a certain distribution law along the flow direction,and the whole piedmont is in a partial oxidation environment,while the plain area is in a reductive environment.The arsenic content in groundwater increased gradually from piedmont to plain area,and the highest arsenic content reached 247μg/L.Redox conditions of groundwater play an important role in controlling the enrichment of arsenic,while competitive adsorption is not significant in this study.（2）Sediments are a direct source of arsenic in groundwater.Si O₂ and Al₂O₃ are the main components in the sediments,which are mainly concentrated in the piedmont coarse sediments.However,Fe,Mn and As are mainly enriched in the plain fine sediment,and there is a significant positive correlation between them,indicating that the arsenic in the study area mainly comes from Fe and Mn oxides/hydroxides.Through the step-by-step arsenic extraction experiment,it was found that the occurrence of arsenic in amorphous iron oxide or hydroxide accounted for the largest proportion in the plain area,and there was a good positive correlation between arsenic in different occurrence states in sediments and arsenic content in groundwater.（3）Organic carbon plays an important role in promoting the release of arsenic.From the piedmont to the plain area,organic carbon is utilized by microorganisms under reduction conditions,accompanied by the release of arsenic from iron and manganese oxides/hydroxides into groundwater.In this study,there is no significant positive correlation between groundwater arsenic and organic carbon isotopic values(δ¹³C_TOC),indicating that organic carbon isotope fractionation caused by microbial utilization has little effect on the overall organic carbon isotopic composition.The relationship betweenδ¹³C_TOC and C/N values indicates that the organic matter in the study area mainly comes from aquatic plants and algae,which is related to the formation environment of Hetao Basin to some extent.