Replenishment Impacts on Hydrogeochemistry and Water Quality in the Hutuo River Plain.

To investigate the influence of the Hutuo River (North China) ecological water replenishment project on the hydrogeochemical processes of groundwater, 64 groundwater samples collected at different time intervals after four replenishment events, and four samples from the Middle Route of the South-to-North Water Diversion Project water, were analyzed for water chemistry. Hydrogeochemical methods such as the Piper diagram, chloride-alkalinity index, and ion correlation were employed to analyze the characteristics of groundwater chemical evolution through replenishment. The results demonstrated that the hydrochemical types of groundwater in the study area underwent significant changes during continuous replenishment in the Hutuo River region. During the initial replenishment period (October 2019), the dominant hydrochemical type of groundwater in the study area was Mg-Na-HCO₃-SO₄, whereas the dominant type in the Middle Route of the South-to-North Water Diversion Project water was Ca-Na-SO₄-HCO₃. As the replenishment continued, the hydrochemical types of groundwater in the study area evolved into Ca-Na-Mg-HCO₃-SO₄, Na-Ca-Mg-HCO₃-SO₄, and Ca-Na-Mg-SO₄-HCO₃. The groundwater experienced a dissolution of calcite, gypsum, nitratine, carbonate rocks, and gypsum, accompanied by dilution effects, resulting in reduced ion exchange as replenishment progressed. The input of the high quality Middle Route of South-to-North Water Diversion Project water effectively promoted groundwater quality improvement, leading to an overall decrease or stabilization of components other than Ca²⁺ in the groundwater. Water quality was assessed using the entropy water quality index, with indicators including Na⁺, SO₄²⁻, Cl⁻, pH, total dissolved solids, NO₃⁻-N, NO₂⁻-N, F⁻, Al, As, and Zn. The evaluation results showed that, except for one medium-quality water sample, the water quality of the other samples was suitable for drinking and domestic purposes during the early replenishment period. The Middle Route of the South-to-North Water Diversion Project exhibited excellent quality (Rank 1), and as replenishment progressed, all water samples demonstrated good quality by October 2020, with a gradual improvement. [ABSTRACT FROM AUTHOR]