Analysis ofhydrochemical evolution characteristics of phreatic water in the Guanzhong Basin

Groundwater resources in Guanzhong Basin play an irreplaceable role in maintaining regional economic development and ecological environment balance. This study analyzed 420 of phreatic water samples from 2011, 2015, 2018 and 2021 in Guanzhong Basin. Multiple analytical methods were applied, including entropy weight-TOPSIS for comprehensive groundwater quality assessment, integrated pollution evaluation, Gibbs model, ion correlation, to study the hydrochemical evolution characteristics and influencing factors of groundwater. The results show that from 2011 to 2021, the ion concentrations of phreatic water display a decreasing trend with time. The phreatic water quality in Guanzhong Basin is mainly Class II, accounting for 31.5%. In 2011, 2015, 2018 and 2021, the average closeness coefficient C was 0.088, 0.083, 0.054 and 0.053, respectively, showing a decreasing trend year by year. The phreatic water in Guanzhong Basin was dominantly unpolluted and slightly polluted. The average value of comprehensive pollution index from 2011 to 2021 is 6.01, 4.25, 3.69 and 3.57, respectively. In the basin, Na⁺, Cl^-, HCO₃^-, Ca²⁺, Mg²⁺ and SO₄^2- are mainly derived from the dissolution of halite, carbonate, silicate and gypsum. High concentrations of Cl^-、SO₄^2-、F^-、Fe、Mn、NO₃-N, may also be related to human activities such as intensive urbanization, livestock farming expansion, and excessive agricultural fertilization. From 2011 to 2021, the phreatic water quality of Guanzhong Basin exhibited a continuous improvement trend. These results provide a scientific basis for rational allocation and optimization of groundwater resources and high-quality protection of ecological environment in the Guanzhong Basin.