The Impact of Water Diversion Projects on Total Phosphorus Content in Large Shallow Lake Bays: A Case Study of the Xinmeng River Project in the Taihu Lake Basin, China

Introducing low-phosphorus water to eutrophic lakes is a straightforward and common treatment measure. The objective of the Xinmeng River water diversion project is to improve the water quality of Zhushan Bay in Taihu Lake by introducing water from the Yangtze River. However, because the water diversion route passes through a plain river network area with high phosphorus content, it leads to two problems that still need to be solved: first, whether the concentration of exogenous total phosphorus entering Zhushan Bay can be controlled; second, whether the total phosphorus content will increase due to water diversion. By establishing a numerical model for hydrodynamics and water quality along the main river of the Xinmeng River Project, as well as in the surrounding river and lake areas, we simulated and analyzed various scenarios regarding hydrodynamics and water quality within the diversion area. The results show that when the proportion of Yangtze River water exceeds 80%, the total phosphorus (TP) content of Zhushan Bay can be reduced to the lowest state. The comparison between the main stream and circulation scale in the bay can reflect the change process of TP content. Water diversion reduced the total phosphorus content in Zhushan Bay by 24%–34%. The increased diversion flow results in greater water loss along the diversion route, reducing the reduction efficiency of TP. Replacement of water within the original river network (through the diversion and drainage of several hubs) before water diversion resulted in a reduction in TP content in Zhushan Bay while simultaneously improving the overall efficiency for reducing total phosphorus levels within this bay area. This research provides valuable insights into the distribution of diverted waters within plain river network environments while also considering their impact on total phosphorus contents in large shallow lake bays.