Regional pollution source apportionment and emission thresholds determination in Zunyi's major watersheds:...

Zunyi City, in the Yangtze River Economic Belt, faces significant pollution challenges in the Wujiang and Chishui watersheds. This study analyzed water quality changes from 2021 to 2023. To support current and future water environment management, pollutant emissions in 2022 were estimated, and major pollutants were traced during flood and non-flood seasons. Emission thresholds were established for districts and counties firstly, identifying key areas for pollution control. This research innovatively integrates urban non-point sources and forest soil erosion into emissions calculations and introduces pollution emissions threshold. Results indicate that agricultural non-point sources, particularly livestock farming and crop cultivation, are the dominant contributors to watershed pollution, while urban non-point sources play a comparatively minor role, for both two studied watersheds. Natural factors including drought and forest soil erosion also significantly influence pollution degree, particularly during flood seasons when increased erosion intensifies pollutant transport, which was especially pronounced for Chishui watershed. Methodologically, the Positive Matrix Factorization (PMF) model demonstrated greater accuracy and reliability than the Absolute Principal Component Scores-Multiple Linear Regression (APCS-MLR) model in apportioning complex pollution sources, for both two studied watersheds. Based on pollution emissions and source apportionment, regional emission thresholds for COD, NH₃-N, TN, and TP were proposed. The results reveal that COD emissions in Xishui, Huichuan, Bozhou, and Chishui exceeded their respective thresholds, with TN and TP also surpassing limits in some areas. In summary, this study provides a scientific basis for targeted water quality management in Zunyi City and validates the PMF model as a reliable tool for source apportionment in complex watershed environments.