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.

The exploitation of groundwater in arid and semi-arid areas, especially in basins where groundwater is intensively exploited, is likely to have adverse effects on the originally fragile ecological environment, which also greatly alter the hydrogeochemical evolution process. Over-abstraction of groundwater in the Haolebaoji watershed of the Ordos Basin located in the semi-arid regions has led to a series of changes in the groundwater system, which has attracted considerable attention from environmental protection organizations. However, the origin and geochemical evolution of groundwater in the Haolebaoji watershed have not been revealed. In this study, the Haolebaoji watershed is selected as the typical study area to investigate hydrogeochemical evolution under the intensive groundwater exploitation. Groundwater samples were collected and tested for major ions and stable isotopes (δ¹⁸O, δD). Various approaches including the ion proportional relationship diagram, chlor-alkali index, saturation index, Gibbs diagram, and principal factor analysis were used to reveal the hydrogeochemical processes regulating the groundwater geochemistry. The groundwater in the study area is divided into five hydrochemical types according to the Piper diagram. It was found that the chemical composition of groundwater in the study area is mainly controlled by the dissolution of calcite, dolomite, gypsum, and halite. The cation exchange intensity gradually increased with the flow of groundwater from the recharge to the discharge area. Rock weathering plays a controlling role in the formation of groundwater geochemistry, but it is also controlled by evaporative crystallization in some runoff and discharge areas. Groundwater is predominantly recharged by modern local atmospheric precipitation, and deep wells receive water supply during cold periods. The study findings provide important information for the development of sustainable groundwater management strategies for the Ordos Energy Base.

Aquifer diffusivity is a basic physical parameter used in hydrogeological calculations and is important for the evaluation and rational utilization of water resources, pollution prevention, and wetland protection. In this study, with the assumptions of aquifer isotropy, i.e., no vertical flow and constant saturated aquifer thickness, by using the Fourier transform and convolution theorem, a 1D analytical solution of the phase spectrum for an unconfined aquifer system was derived, which subsequently led to a phase spectrum solution for aquifer diffusivity. To test the efficacy, the proposed method was applied to a study site in the middle reach of the Hutuo River in the North China Plain. The estimated aquifer diffusivity ranged from 1.9 × 10³ to 4.9 × 10⁴ m²/d, with a mean of 2.2 × 10⁴ m²/d, which was consistent with the results obtained using power spectral analysis, pumping tests, and inverse numerical models. The phase spectral approach proposed in this paper can estimate the aquifer properties on a larger scale. If long time series of hydraulic heads are available, it can estimate hydrogeological parameters accurately and quickly. Considering the similarity of the linearized governing equations, it can also be applied to the river–aquifer system and the confined aquifer system.