[Hydrochemical Characteristics and Origin of Groundwater in the Central Guohe River Basin]

Groundwater is an important water source in the central Guohe River basin but pollution and water quality deterioration present a significant challenge. Here, 80 groups of groundwater samples were collected between June and September 2019 including 61 groups of shallow groundwater samples, 9 groups of middle groundwater samples, and 10 groups of deep groundwater samples. The hydrochemical characteristics and formation mechanisms of groundwater at these different depths were analyzed using statiatical techniques, Piper triangular diagrams, Gibbs figures, and ion ratios. The following results were noted:① Groundwater is weakly alkaline overall, and the dominant anion and cation at different depths were HCO₃^- and Na⁺. The shallow and deep groundwater mainly consist of fresh water while the middle groundwater is mainly brackish water. The hydrochemical typology of the shallow groundwater was mainly HCO₃-Ca·Mg and HCO₃-Na·Mg. HCO₃·SO₄·Cl-Na was found to be the dominant hydrochemical typology in the middle and deep groundwater. ② The chemical composition of the groundwater shows notable vertical variations. With depth, the mean mass concentrations of TDS, Na⁺, Mg²⁺, Cl^-, SO₄^2-, and HCO₃^- first increase and then decreasing, while the mean mass concentration of Ca²⁺ gradually decreases. These vertical differences are closely related to the sedimentary environment of the aquifer and the intensity of water-rock interaction. ③ The formation of groundwater hydrochemical characteristics is influenced by water-rock interactions, cation exchange, and human activities, with water-rock interaction dominated by sodium silicate dissolution. Human activities generally have the greatest impact on the shallow groundwater. ④ The water quality of the deep groundwater is notably better than that of the shallow and middle groundwater. However, excessive exploitation of the deep groundwater resource has led to the formation of a regional groundwater funnel, enhancing the difference in middle and deep groundwater levels. To reduce the risk of ground subsidence and the contamination of the deep water with middle brackish water, deep groundwater exploitation wells should be carefully positioned and regulated.