Source apportionment of groundwater contamination and spatial variability in the Bohai Sea region (China)

Understanding large-sample groundwater chemical dynamics is critical for sustainable coastal aquifer management. Using geostatistics, Self-Organizing Maps (SOM), K-means clustering, and Positive Matrix Factorization (PMF) on 1415 high-dimensional datasets from China's Bohai region (subregions: Bohai Bay, Liaodong Bay, Laizhou Bay), we investigate spatial dynamics, evolutionary classification, and source apportionment of groundwater chemistry. Results reveal pronounced spatial heterogeneity in salinity and contaminant ions, with a distinct hydrochemical gradient (Laizhou Bay area > Bohai Bay area > Liaodong Bay area) reflecting salinity-driven quality degradation. SOM indicates halite dissolution/seawater intrusion via Na⁺-Cl^- covariation (ranking: Laizhou Bay area≈ Bohai Bay area > Liaodong Bay area), while decoupled Ca²⁺-SO₄^2- and distinct F^--HCO₃^-/Ca²⁺-F^- gradients highlight the contributions of anthropogenic (primary) and natural contributions (secondary). K-means identifies quintuple groundwater groups across subregions, demonstrating consistent tripartite zonation: high-salinity coastal (SO₄·Cl-Na/Ca·Mg), transitional mixing, and inland freshwater (Ca·Mg-HCO₃) zones-characterized by declining seawater mixing and increasing anthropogenic pollution. PMF quantifies five anthropogenic-geogenic sources with subregional dominance: Bohai Bay features seawater intrusion (Factor 1: Na⁺-Cl^-) and industrial SO₄^2- (Factor 4); Liaodong Bay exhibits agricultural dominance (Factors 2-3: Mg²⁺, NO₃^--NO₂^--F^-) linked to intensive fertilization; Laizhou Bay shows paleo-brine impacts (Factor 1: Na⁺-Cl^--Br^-) with evaporated seawater signatures. Management priorities thus diverge: saline groundwater utilization and urban/industrial controls in Bohai/Laizhou Bay area versus agricultural efficiency in Liaodong Bay area. These findings demonstrate both the effectiveness of combining SOM and PMF for processing high-dimensional hydrochemical data and underscore the necessity for tailored groundwater protection strategies targeting critical subregional pollution sources.