Saltwater intrusion is one of the most significant groundwater challenges in the southern Laizhou Bay. Previous studies have predominantly focused on regional scales, leaving the vertical saltwater intrusion pattern relatively underexplored. This knowledge gap hinders the effective prevention and control of saltwater intrusion. This study utilized hydrochemical and stable isotopic methods combined with hydrochemical facies evolution diagrams to investigate the groundwater evolution and the processes of saltwater intrusion in a typical profile and saline–fresh groundwater transition zones. The results showed that the groundwater types in the study area were complex and diverse, with fresh groundwater, saline groundwater, and brine. Stable isotope and hydrochemical analyses indicated that mixing and evaporation of seawater were the predominant processes governing the evolution and salinity of groundwater. In the south of the typical profile, carbonate dissolution played a significant role, and the silicate dissolution may represent the primary water–rock interaction in the saline–fresh groundwater transition zones. Groundwater samples from various locations within the study area exhibited different stages of hydrochemical facies evolution, and the majority of the typical profile samples were in the salinization phase during the mixing process. The saltwater intrusion in the saline–fresh groundwater transition zone primarily occurred between −20 and −30 m, exhibiting a wedge-shaped saltwater intrusion pattern. This study enhanced the understanding of vertical saltwater intrusion.

The Chinese government hopes to implement groundwater table control to realize the sustainable utilization of groundwater resources based on controlling the current groundwater exploitation amount. In this study, a method to determine the control index of the groundwater table is proposed. In the method, the reasonable relationship between the groundwater table and groundwater exploitation amount is ensured using the groundwater numerical simulation model. The operability of the index determination is improved using a surrogate numerical model, and the annual hydrological dynamic is simplified to three scenarios of dry, flat, and wet. To verify this method, the Minqin Basin in Northwest China was chosen as a typical study area. It is assumed that the control index of groundwater exploitation in 2020 is 85,000 × 10³m³. Then, the preset annual water table index is calculated as [−0.70, 0.62, 1.13, −1.25, 1.36, 3.09] m [−0.77, 0.53, 1.05, −1.33, 1.27, 2.96] m, and [−0.83, 0.46, 0.99, −1.40, 1.20, 2.85] m for the chosen six monitoring wells, varying over the years with wet, flat, and dry scenarios. This method can ensure high precision, operability, and dynamic management when determining the control index of the groundwater table and satisfy the demand of managers.

Surface water quality, serving as a key link between domestic water use and agricultural production, impacts both the drinking water safety of local residents and the sustainable use of irrigated soil. To better protect water resources and enhance their sustainable value, this study collected 50 water samples from the areas surrounding Nansi Lake. Using the Piper trilinear diagram, Gibbs model, and ion ratio analysis, the main hydrochemical types were identified. Based on this, the entropy-weighted water quality index (EWQI) was used to evaluate the water’s suitability for drinking, while irrigation water quality indicators were applied to assess its suitability for irrigation. The results indicate that during both dry and rainy seasons, Na⁺ and SO₄²⁻ dominate the water, with average total dissolved solids (TDS) of 1279 mg/L and 1163 mg/L, respectively, indicating moderately elevated salinity. The ion concentrations follow the order: SO₄²⁻ > HCO₃⁻ > Cl⁻ > NO₃⁻ > F⁻ and Na⁺ > Ca²⁺ > Mg²⁺ > K⁺. From a hydrochemical perspective, mixed-type and Cl-Na-type waters prevailed in both seasons. The chemical composition of surface water in the study area is largely governed by rock weathering, with ions primarily originating from the dissolution of silicate and evaporite minerals. Furthermore, cation exchange processes play a significant role in shaping the evolution of the water chemistry. The water quality evaluation indicates that surface water in the study area is generally Class II, representing good water quality. However, Class IV and Class V water exist in some areas, where the primary exceedance parameter is SO₄²⁻, which is a key factor influencing water quality. Irrigation suitability is generally good. Systematic investigation of surface water hydrochemistry and quality is of great practical significance for ensuring safe drinking and irrigation water and promoting sustainable socio-economic development.