Study on chemical characteristics and formation mechanism of shallow groundwater in Shijiazhuang area

In order to study the chemical characteristics and formation mechanism of shallow groundwater in Shijiazhuang area, this paper is based on the water quality monitoring data of shallow groundwater from 2019 to 2020 in Shijiazhuang area , and comprehensively uses geographic statistics, Piper trigram, Gibbs model and ion ratio methods to analyze the chemical characteristics,the main chemical indicators affecting water quality and their formation mechanism of shallow groundwater in different hydrogeological units in Shijiazhuang area.The results indicate that the pH value of shallow groundwater ranges from 7.13 to 8.48, belonging to neutral water and weakly alkaline water in the study area; The mean values of TH and TDS are 391.03mg/l and 590.60mg/l, respectively, showing a gradually decreasing trend along the direction of groundwater runoff; The major cations and anions are HCO₃^-, SO₄^2-, Ca²⁺and Na⁺, while the major cations are Ca²⁺and Mg²⁺ in plateau and valley plain areas; Moreover, the spatial variation coefficients of Na⁺, Cl^-, SO₄^2- and NO₃^- ions are relatively large, indicating a certain degree of human interference, particularly in the alluvial plain and Taiwan region.The chemical types of shallow groundwater are mainly HCO₃•SO₄ -Ca•Mg, followed by SO₄•HCO3-Ca•Mg. Along the groundwater runoff path, the HCO₃•SO₄-Ca•Mg type water gradually transitions to SO_4•HCO₃- Ca•Mg type water,and the hydrochemical types gradually become more complex, with the emergence of SO₄•HCO₃- Na• Ca •Mg type water.The formation of chemical components is mainly influenced by rock weathering, evaporation and concentration and human activities in shallow groundwater . Among them, groundwater is significantly affected by human activities in alluvial plains and Taiwan areas. The formation of chemical components in samples with TH, SO₄^2-and NO₃^- indicators exceeding the standard  is mainly affected by atmospheric precipitation, dissolution of rock salt minerals, weathering and dissolution of sodium containing minerals such as sodium silicate rock/saltpeter, as well as agricultural activities, municipal sewage, and industrial activities in the area.