Variability in water quality and chlorophyll a in the middle route of the south to north water diversion project

The Middle Route (MR) of the South to North Water Diversion Project of China (SNWDPC) is one of the most important cross-basin water diversion projects worldwide, leading to substantial attention on the water environment of the project. In order to evaluate the water environment status of the MR of the SNWDPC, the spatiotemporal variations and underlying causes in the water environment along the route are systematically revealed in this study. The data were analyzed using the water quality index (WQI) and Spearman correlation analysis. The results showed annual average WQI values ranging from 91.76 to 92.90 during the period of 2019 to 2022, indicating the water quality status of "excellent." However, permanganate index (COD_Mn) was increased along the route ranging from 1.83 mg/L to 2.27 mg/L, which is assumed to be primarily caused by the decomposition of dead planktonic algae. Chlorophyll a (Chl-a) showed significant spatial heterogeneity, increasing along the route with a range of 3.13 µg/L to 8.87 µg/L. The primary reason is the decrease in flow velocity and the increase in pH along the route. In summary, although the overall water environment of the MR of the SNWDPC is of high quality, environmental risks such as the high COD_Mn and Chl-a concentrations in the end of the route require sufficient attention from the management department. The findings provide guidance for ensuring the safety of water supply in cross-basin water diversion projects.