LAUSR

Polder areas are typical geographic units in the middle and lower reaches of the Yangtze River and subject to intensive anthropogenic influence, resulting in complex hydrochemical processes. Current understanding of these processes remains incomplete, limiting effective water resource management. This study employs an integrated approach combining hydrochemical diagrams, isotope tracing, and multivariate statistics to elucidate the spatiotemporal patterns and controlling mechanisms of ion composition in polder waters, and to quantitatively attribute ion sources. Results show that cations are dominated by Na+ and Ca2+, with elevated concentrations during the dry season compared to the wet season, while anions are primarily HCO₃⁻, exhibiting minimal seasonal variation. Significantly higher ion concentrations were observed in ditches than in ponds, indicative of distinct hydrological connectivity and anthropogenic effects. Hydrochemical facies transitioned seasonally: HCO₃–Ca dominated the wet season, whereas Cl/SO4–(Ca·Mg), HCO3–Ca, and Cl/SO4–Na types emerged during normal and dry periods, reflecting a shift toward mixed hydrochemistry under dual natural and human influences. Although rock weathering was the predominant control, anthropogenic activities amplified spatiotemporal heterogeneity in ionic composition. Quantitative source apportionment revealed that natural processes (e.g., rock weathering) contributed 80.95% of total ions, compared to 19.05% from human activities, highlighting their considerable role. This study provides the first systematic clarification of the formation mechanisms and driving factors of hydrochemistry in polder areas, filling a critical knowledge gap in this field and offering a scientific basis for water environmental protection and sustainable development in these regions.