Assessing groundwater quality threat to agricultural ecosystem health induced by seawater intrusion on the northeastern coast of Laizhou Bay: An irrigation risk perspective.

Abstract

Coastal agricultural systems are intrinsically Groundwater-Dependent Ecosystems (GDEs) whose sustainability is threatened by seawater intrusion. However, most existing studies focus on hydrochemical characterization rather than providing a spatially explicit, integrated assessment that links hydrochemical processes to agricultural GDE health risks through objective weighting and machine learning methods. To fill this gap, this study focuses on the northeastern coast of Laizhou Bay and establishes a "hydrochemical analysis - irrigation quality evaluation - risk assessment" framework to systematically investigate the impacts of seawater intrusion and conduct a comprehensive risk assessment of the agricultural GDE. The entropy weight method is applied to construct a Composite Groundwater Irrigation Risk Index (CGIRI), and a Random Forest model is employed for regional risk prediction. Results demonstrate that the groundwater is primarily of the Cl-Ca·Mg type. Seawater intrusion not only increases Cl- concentrations and the Seawater Mixing Index but also synergistically elevates Ca2+, Na+, Mg2+, and SO42- concentrations through hydrogeochemical interactions such as reverse cation exchange. Irrigation quality assessment indicates widespread salinity hazard, with 86% of groundwater samples classified as unsuitable for irrigation based on Potential Salinity, and seawater intrusion shows a potential to induce sodium hazards. The entropy‑weighted Random Forest model (R² = 0.97) predicts that the study area is predominantly characterized by medium risk (covering 77.5%), with risk levels increasing toward the coastline and strongly correlating with Seawater-intrusion Zone. The high‑resolution risk map generated in this study provides direct scientific evidence for zonal groundwater management, cultivation of salt‑tolerant crops, and long‑term water quality monitoring. The integrated framework established herein can serve as a methodological reference for assessing irrigation risks to GDEs in other coastal regions affected by seawater intrusion worldwide.