Identification of priority management tributaries to protect drinking water sources in the Yangtze River mainstream:...

Direct pollutant discharges caused by pollution incidents from tributaries into the Yangtze River mainstream pose a severe threat to drinking water safety. Risk assessments are crucial tools to support differentiated tributary pollution risk management. However, comprehensive and efficient assessment methods that are specifically aimed at protecting mainstream drinking water sources remain scarce. In this study, we propose a novel framework for the identification of priority management tributaries based on the simulation and assessment of risks posed by tributary arsenic (As) pollution incidents. The risk assessment accounted for the number of affected mainstream drinking water sources, the As exceedance magnitude, and the contamination duration. Tributaries were classified into four priority management levels based on the risk assessment results to support differentiated environmental management strategies. The framework was first applied to the Jiangsu section of the Yangtze River, where a total of 672 simulation scenarios were designed to account for the large number of tributaries and different hydrological years. The results indicated that 16 of the 224 tributaries were classified at the highest priority management level (Level IV). The As inputs from these tributaries generally affected more drinking water sources and resulted in higher exceedance magnitudes. In addition, the contamination persisted for longer periods. The remaining tributaries were classified as Level I, Level II, and Level III, with numbers of 125, 67, and 16, respectively. Differentiated management measures were proposed for the tributaries at the different priority levels. These measures included source-oriented control, water quality monitoring, engineering measures, and on-site inspections. Furthermore, some tributaries showed the sensitivity of priority levels to the As emission intensity, suggesting that those with upward shifts in priority warranted attention comparable to high-priority tributaries. A case study demonstrated that the proposed framework offered a scientific and effective decision-support basis for differentiated tributary management, thereby enhancing the protection of mainstream drinking water sources.