Spatiotemporal characteristics and source apportionment of heavy-metal pollution in the Yichang River basin

• Water quality assessed at 34 sites across wet, normal, and dry seasons. • Integrated WQI, PCA, and PMF/NMF to link water status with metal sources. • Identified 4 sources: industrial waste, runoff, baseflow, and sewage-sediment. • Drivers shifted seasonally: wet runoff, normal industry, dry baseflow/sediment. • Risks were low, but hotspots emerged in normal/dry urban-agricultural reaches. To reveal the spatiotemporal variations and key drivers of water quality in subtropical monsoon regions, this study investigates the Yichang River Basin in Guangxi. Thirty-four surface water sites were monitored across wet, normal, and dry seasons, covering 16 water quality parameters. Using Water Quality Index (WQI), Pearson correlation analysis, Principal Component Analysis (PCA), and combined Positive Matrix Factorization/Non-negative (PMF/NMF) models, the study found marked seasonal variation: good water quality during the wet season due to dilution and self-purification; degradation in the normal season from pollutant accumulation; and partial recovery in the dry season linked to reduced agricultural activity. Spatially, midstream and downstream urban-agricultural zones formed pollution hotspots, while upstream forested areas remained relatively clean. Multivariate analyses revealed that water quality is controlled by four main pollution sources: high-temperature industrial wastewater that drives temperature-regulated oxygen depletion, storm-induced flushing of urban and agricultural metal-laden runoff, oxygen-rich baseflow and geological background inputs, and coupled sediment–sewage release of NH₃–N, As and Mn under low-flow, low-oxygen conditions. These sources dominate in different seasons, reflecting a multi-source, seasonally shifting and process-driven pattern of water-quality change in the Yichang River Basin. These insights support improved watershed management and pollution source control in subtropical hilly regions.