Characterization of spatial and temporal changes in groundwater chemistry and analysis of pollution sources in the middle and lower reaches of the Yangtze River.

The accelerating industrialization and urbanization in the Yangtze River Delta have imposed significant pressures on its groundwater quality. To elucidate the spatiotemporal evolution and source apportionment of groundwater chemistry, this study integrated hydrochemical analysis with multivariate statistical techniques, including correlation analysis (CA), absolute principal component scores-multiple linear regression (APCS-MLR), self-organizing map (SOM), and land-use analysis. Our findings reveal a clear temporal trajectory: from the 1980s to 2018, groundwater chemistry shifted from homogeneous HCO3-Ca·Na types towards more complex HCO3·Cl-Ca·Na and HCO3-Ca·Mg types, indicating an increasing anthropogenic imprint. Spatially, phreatic water exhibited greater ion concentration variability and higher susceptibility to pollution compared to confined water. The APCS-MLR model quantified three primary pollution sources: mineral leaching and enrichment (50.15%), mixed industrial-agricultural activities (16.92%), agricultural practices (12.88%). Notably, SOM analysis visualized the clustering of high TDS and ion concentrations in specific regions, while declining nitrate levels in recent years were attributed to increased precipitation dilution. This study underscores the dynamic interplay between human activities and natural processes in shaping groundwater chemistry, providing a scientific basis for targeted groundwater resource management.