Mechanism of Groundwater Hydrochemical Evolution Under the Control of Land-Sea Gradient

Unraveling the integrated mechanisms governing groundwater hydrochemical evolution along a complete land-sea gradient is crucial for coastal water resource management and seawater intrusion prevention and control. This study focused on the Bailang River Basin (southern Laizhou Bay, Bohai Sea, China). Based on 45 systematically collected groundwater samples, we integrated hydrochemical statistics, Piper trilinear/Gibbs diagrams, and ion ratio analysis to clarify the hydrochemical evolution of the research area from mountains to coast. Results show: (1) The upstream reservoir area is dominated by low total dissolved solids (TDS) freshwater (360.1-1648.7 mg/L, HCO₃-Ca·Mg type), which is mainly controlled by carbonate and silicate weathering; (2) midstream plain: transitional brackish-saline water (702.1-32,322.3 mg/L, shifting to Na-Ca-Cl type), affected by cation exchange, agricultural return flow, and weak evaporation; (3) coastal area: high-TDS brine (15,398.9-86,169.6 mg/L, Na-Cl type), driven by paleoseawater residual, modern seawater intrusion, and intense evaporation. This study identifies a "source-sink" evolutionary pattern controlled by the coupling of rock leaching, cation exchange, seawater intrusion, and evaporative concentration. This pattern explicitly links geomorphic gradients to hydrochemical differentiation. This work clarifies how natural processes and anthropogenic activities synergistically shape the coastal groundwater hydrochemical spatial pattern, providing a scientific basis for sustainable groundwater management and seawater intrusion control in similar regions.