Analysis Of Hydrochemical Characteristics And Formation Process Of Dongjiao Groundwater Source In Yinchuan City

Yinchuan City is located in the central of the Yinchuan Plain.Groundwater is one of the main resources of water supply in Yinchuan City,it plays a vital role in maintaining industrial,agricultural,and domestic water.So,a comprehensive analysis of hydrochemical characteristics and formation process of groungwater sources of Yinchuan City is particularly important.In this paper,on the basis of field investigation and data collected,confined aquifer I and confined aquifer II of Dongjiao Groundwater Source of Yinchuan City are taken as research objects.This paper analyzed the chemical characteristics and isotope characteristics of the groundwater source comprehensively and systematically,evaluated the water quality of confined aquifer I,which is the main water supply aquifer of Dongjiao Groundwater Source,defined the formation process of chemical composition of confined water,and analyzed the influences of human activities on the formation process of chemical composition of confined water,and provided a certain scientific basis for the rational development and utilization of Dongjiao Groundwater Source of Yinchuan City.The main conclusions are as follows:1.Using descriptive statistical analysis to analyze the chemical composition of confined water I and confined water II in the study area.It was obtained that among the two aquifers,the lowest coefficient of variation in the conventional components is HCO₃^-,indicating its content in groundwater is the most stable.For the coefficients of variation of the remaining conventional ionic components,confined water II is greater than confined water I,indicating the ion concentration of confined aquifer II is locally enriched to a high degree,and the content fluctuates greatly.2.According to Piper diagram analysis,the mainly cations of confined aquifer I are Mg²⁺and Ca²⁺,the mainly anion is HCO₃^-,the mainly cations of confined aquifer II are Na⁺and K⁺,and the mainly anion is HCO₃^-,sometimes HCO₃^-and Cl^–.Schoeller diagram was used to compare the change curves of water chemical composition of water samples in the same place but at different strata?confined aquifer I and confined aquifer II?.It is found that the change curves of most water samples which in the same place but at different strat are similar,and it could be seen that the hydraulic connection of the two confined aquifers is closely.3.Based on the data of hydrogen and oxygen stable isotopes in the study area,it can be concluded that the phreatic water is controlled by strong evaporation and is closely related to the surface water,and its renewability is strong.At the same time,the hydrogen and oxygen stable isotopes in the phreatic aquifer are more enriched than the confined water I and confined water II,it means that the phreatic aquifer is more closely related to the atmospheric precipitation,and also has a certain hydraulic connection with the confined aquifer.According to the qualitative analysis of the tritium isotope data in the study area,it is found that the tritium abundance gradually decreases with the increase of the depth of groundwater,indicating that the deeper the groundwater is,the weaker the renewability is.The low tritium content of the confined water II indicates that the sources of recharge may be precipitation or Yellow River water in historical period.4.The analysis results of single-index evaluation method showed that the NH₄⁺in confined aquifer I exceeds the standard seriously,the excess of NH₄⁺may be due to the decomposition of nitrogen-containing organic matter in the domestic sewage of the residents in the study area,and some area exists the pollution of industrial wastewater and agricultural fertilizers and pesticides.Fe and Mn exceeded the standard in some areas,it is because the runoff conditions of the study area are poor,Fe and Mn are easy to enrich.The high content of SO₄^2-,total hardness and TDS in one or two sampling points may be affected by human activities.The traditional Nemerow index method shows that the proportion of Class IV water in the total sampling points is the largest,which is 82.35%;the results of the improved Nemerow index method are more reasonable than the traditional Nemerow index method.The improved Nemerow index method shows that Class II water accounts for 47.06%of the total,and Class III accounts for 29.41%of the total.The results of fuzzy comprehensive evaluation which determines the weight by AHP-entropy weight method shows that 82.35%of the water quality in the study area are Class III.5.Confined aquifer I is mainly controlled by rock weathering,and partial area is also supplied by phreatic water which controlled by evaporation.Confined aquifer II is mainly supplied by precipitation or Yellow River water during the historical period affected by evaporation,and has also experienced rock weathering since the historical period.According to the Pearson correlation coefficient among water chemical components,it is concluded that the correlation coefficient between Cl^-and SO₄^2-is high.This may be caused by discharge of domestic sewage,excessive use of fertilizers,and the infiltration of irrigation return water in this area.On the basis of Pearson correlation coefficient and the anion-cation ratio coefficient analysis,it is concluded that the chemical constituents are mainly from the dissolution of halite,aluminosilicate,and sulfate.Its formation and evolution are mainly affected by mineral dissolution and ion exchange.In addition,human activities also affect the hydrochemical formation process of Dongjiao Groundwater Source.