Chemical Characteristics And Genetic Analysis Of Shallow Groundwater In Nanchang Area Of Ganfu Plain

In this paper,shallow groundwater in Nanchang District of Ganfu Plain is taken as the research object.Based on the understanding of hydrogeological conditions,the characteristics of hydrochemical components of shallow groundwater are analyzed.Gibbs graphic method,ion proportion coefficient,chlor-alkali index method,deuterium oxygen isotope method and other methods are used to study the origin and source of shallow groundwater.Reverse geochemical model was established through Phreeqc to analyze the specific formation process of groundwater chemistry along the reaction path,and the following conclusions were obtained:(1)Most of the shallow groundwater in Nanchang area of Ganfu Plain is slightly acidic,with HCO₃^-as the main anion and Ca²⁺and Na⁺as the main cation.Along the direction of groundwater flow,the concentration of TDS increases as a whole,and the concentration of all ionic components increases to varying degrees.The concentrations of HCO₃^-,Na⁺and Cl^-change relatively greatly.The chemical components in groundwater showed different spatial distribution characteristics,with Na⁺,K⁺and Ca²⁺being high in the middle and low in the east and west sides.The high-content areas of Cl^-and Mg²⁺are mainly located in the area from Wanshe Village to Xiawujia Village in Gangshang Town and near Wenjia Town in Machu Town.The overall concentration distribution of SO₄^2-is high in the west and low in the east.The overall concentration of HCO₃^-was high in the middle and low around the periphery.The spatial distribution characteristics of TDS concentration were generally high in the middle and low in the east and west sides.The areas with high concentration mainly existed in the densely populated central urban areas.(2)The main chemical types of groundwater are HCO₃-Na·Ca·Mg,HCO₃·Cl-Na·Ca·Mg,HCO₃-Na·Ca and HCO₃-Ca·Mg.The chemical components of rock minerals mainly enter the groundwater through leaching process.Na⁺and K⁺are from silicate rocks,Ca²⁺and Mg²⁺are from carbonate rocks,silicate rocks and evaporite rocks,and the anions such as SO₄^2-,Cl^-and HCO₃^-are from sulfate rocks and carbonate rocks.The main anion chemical types were HCO₃type and mixed type dominated by HCO₃,accounting for 82.71%.The main cationic chemical types were Na·Ca·Mg and Na·Ca,accounting for 74.34%.(3)According to the calculation results of chlor-alkali index and the ratio of(Ca²⁺+Mg²⁺-HCO₃^--SO₄^2-)/(Na⁺+K⁺-Cl^-),cation exchange between Ca-Na exists in shallow groundwater.The trend of D and ¹⁸O in groundwater is basically the same,and the fitting line of D and ¹⁸O isotopes in groundwater is close to the global precipitation line,which indicates the close relationship between shallow groundwater and precipitation.The saturation index of rock salt and potassium salt in groundwater is relatively small,ranging from-7.77 to-6.68,while the saturation index of gypsum,calcite and dolomite is relatively large,ranging from-4.82 to-0.86.The order of average saturation index is calcite>gypsum>dolomite>rock salt>potassium salt.The reverse geochemical simulation results show that along the flow path,the groundwater gradually evolved to the salinization and acid reduction state.Dissolution of gypsum,calcite,dolomite,rock salt,potassium salt and carbon dioxide gas is the main source of chemical components of shallow groundwater.