Source And Hydrochemical Process Of High Arsenic Groundwater Based On Strontium And Calcium Isotopes

Arsenic chronic poisoning from drinking water caused by natural high arsenic(As)groundwater in human being has become one of most serious environmental geological problems in the world.There is a need to investigate the source of high As groundwater and the concomitant hydrogeochemical effects on As mobilization.Based on data of hydrogeochemistry,isotopes of strontium and calcium,this thesis explores the provenance of high As groundwater and reveals the hydro-geochemical effect on mobilization and enrichment mechanism of As in the Hetao basin of Inner Mongolia.This study is significant in both theory and application by providing theoretical supports on management of water resource in high As groundwater-affected areas.The main conclusions have been drawn as follows.Deep groundwater in the recharge-oxic zone(Zone I)was mainly of HCO₃-Ca,with As concentrations mostly lower than 50?g/L.⁸⁷Sr/⁸⁶Sr of groundwater in this zone were the highest in the study area between 0.71345 and 0.71522,and?^44/40Ca were the lowest(from-0.09‰to 0.24‰).The high ⁸⁷Sr/⁸⁶Sr indicated that groundwater in this zone was recharge by fissure water,and the low?^44/40Ca indicated that dissolved Ca²⁺were mainly sourced from the dissolution of silicates.Oxidizing environment caused by the recharge of fissure water was conducive to As immobilization by promoting As adsorption onto Fe oxides,and Ca²⁺could further enhance As adsorption by increasing positive charges on Fe oxides.Deep groundwater in flow-moderate reducing zone was mainly of SO₄-Cl–Na·Ca,with As concentrations mostly higher than 200?g/L.⁸⁷Sr/⁸⁶Sr of groundwater in this zone decreased(0.71192-0.71313),and?^44/40Ca increased(0.21‰to 0.45‰).The decreasing ⁸⁷Sr/⁸⁶Sr were caused by the incongruent dissolution of feldspar,and the increasing?^44/40Ca resulted from secondary calcium precipitation caused by continuous consumption of introduced CO₂ and organic degradation.Neutral to weakly alkaline p H and the decrease in Ca/Na induced by incongruent dissolution of feldspar contributes to As desorption from Fe/Mn oxyhydroxides/oxides.Secondary calcium precipitation could decrease As adsorption by directly isolating As from sediments.HCO₃^-could compete with As for adsorption sites on the surface of Fe/Mn oxyhydroxides/oxides.Organic degradation can enhance As mobilization by facilitating reductive dissolution of Fe oxides.Deep groundwater in flow-reducing zone(Zone III)was mainly of Cl-SO₄-Na,with As concentrations higher than 300?g/L.⁸⁷Sr/⁸⁶Sr of groundwater in this zone were the lowest in the study area(from 0.71167 to 0.71187),while?^44/40Ca were the highest(from 0.45‰to 0.49‰).The lowest ⁸⁷Sr/⁸⁶Sr values resulted from vertically mixing with shallow groundwater,and the mixing percentage of shallow groundwater was 69.2-88.4%.The highest?^44/40Ca were related to secondary calcium precipitation triggered by degradation of labile organic matter,which was introduced by vertical mixing of shallow groundwater.The organic degadation could trigger reductive dissolution of Fe oxides,and thus promote As release into groundwater.Secondary calcium precipitation,as a byproduct of reductive dissolution of Fe,had little effect on As mobilization since reductive dissolution of Fe oxides reached high level.