Hydrochemical Characteristics Of Groundwater And Their Relation To High Fluoride Concentration In Zhangwei River Basin

Endemic fluorosis caused by high-fluoride groundwater is considered as a serious environmental problem.Characteristics,occurrence conditions and formation mechanism of high-fluoride groundwater are the research hotspots in the field of hydrogeology.High-fluoride groundwater is widely distributed in the plain area of Zhangwei River Basin,but its distribution characteristics and formation mechanism are unknown.Based on collecting groundwater,pore-water and sediment,this study investigated chemical characteristics,distribution,occurrence conditions and formation mechanism of high-fluoride groundwater,which is of theoretical and practical significance.Major conclusions are shown as follows.1.Groundwater in the study area is weakly alkaline.In shallow fresh groundwater and deep groundwater,the main cation is Na⁺,and HCO₃^-is the main anion.In shallow saline groundwater,the main cation is Na⁺and the main anion is SO₄^2-.The shallow fresh groundwater with high-fluoride concentrations is mainly of HCO₃·Cl·SO₄-Na and HCO₃-Na type,While the shallow saline groundwater with high-fluoride concentrations is generally of SO₄·Cl-Na type.The deep high-fluoride groundwater is of HCO₃·Cl-Na and HCO₃-Na type.2.Hydrogen and oxygen isotopes of groundwater and pore-water are located near the global and local precipitation lines,indicating that the source of high-fluoride groundwater in the study area is sourced from atmospheric precipitation.Hydrogen and oxygen isotopic characteristics of groundwater show that the evaporation of shallow groundwater is more intense,and the source of deep groundwater may have lower temperature or higher altitude.The hydrogen and oxygen isotope values of pore-water in deep aquifer are obviously more negative compared with those in the shallow aquifer.The linear relationship betweenδD andδ¹⁸O of pore-water in the aquitard sediments has a high slope because the pore-water occurs in a mild and humid environment.3.Correlation between total fluoride contents in sediments and fluorine content in pore-water extracted from corresponding sediments is significant（p≤0.05）,with a correlation coefficient of 0.318.It indicates that total fluoride content in sediments is an important factor controlling fluorine concentrations in pore-water.4.In shallow aquifer,high-fluoride groundwater is more strongly concentrated by evaporation,while in deep aquifer,it is mainly controlled by dissolution-precipitation,cation exchange,desorption and competitive adsorption.The palaeo-climate evolution since 2.5 Ma BP was reconstructed by analyzing the sediment palynology and pore-water isotopes,showing that in a warm and humid climate,the increasing degree of hydrolysis of clay minerals led to the enrichment of fluoride in pore-water.