Seasonal Comparison of Groundwater Irrigation Suitability in the Coastal Zone of Northeastern Laizhou Bay Under the Influence of Seawater Intrusion

Coastal zones are sensitive areas where marine and terrestrial systems interact. Seawater intrusion, a typical coastal geological hazard, poses a serious threat to groundwater resources. This study takes the northeastern coastal zone of Laizhou Bay, a representative area affected by seawater intrusion in China and relying on groundwater for agricultural irrigation, as the research area and integrates hydrochemical analysis, irrigation hazards assessment, and a hybrid-weighted Irrigation Water Quality Index (IRWQI) to reveal seasonal changes in groundwater irrigation suitability. Results show that (1) groundwater hydrochemical facies exhibits a shift from HCO3-Ca type in the rainy season to Cl-Ca·Mg type in the dry season, with TDS and Cl− increasing coastward. The Huangshui River estuary displays a striking seasonal reversal: minimally affected during the rainy season, it becomes moderately severely intruded in the dry season, owing to the contrast between the perennial Huangshui River and adjacent ephemeral streams. (2) Salinity hazard (EC, PS) is the most immediate seawater intrusion consequence, with dry-season PS expanding inland and rendering estuarine groundwater unsuitable for irrigation. Although sodium and magnesium hazards remain below critical thresholds, strong Cl−–Na+ and Cl−–Mg2+ correlations in the dry season signal emerging risks. Bicarbonate hazard declines via conservative mixing with Ca·Mg-rich seawater, masking other hazards. Permeability hazard exhibits moderate seasonal deterioration. (3) Spatially, the IRWQI values are systematically lower during the dry season, with contiguous severe-restriction zones emerging along the Huangshui, Yongwen, and Jiehe River estuaries. These findings indicate that under reduced recharge, seawater intrusion dominates groundwater irrigation quality, triggering a seasonal tipping point. The study provides a scientific basis for adaptive coastal groundwater management.