Assessment and Identification of Primary Factors Controlling Yangtze River Water Quality

Challenges for rapid dam construction remain, including pollutant trajectories after construction, how socio-economic developments drive long-term water quality and large spatial changes, and which indicators primarily control these changes. Here, high-density sampling and socioeconomic data were integrated to assess primary factors controlling Yangtze River water quality. Our results indicated that the pollutant trajectories in the upper and lower sections differ, owing to the Three Gorges Dam. From 2003 to 2020, the decreased TP, NH4+-N, and CODMn concentrations were strongly correlated to the per capita gross domestic product, drainage pipe length, number of wastewater treatment plants, and fertilizer consumption. Moreover, Se and Cd concentrations decreased, whereas Ni and Zn concentrations increased from 2007 to 2020. The water quality index (WQI) demonstrated that Yangtze River water quality varies from levels "good" to "excellent", is better in the winter, and deteriorates with decreasing distance from the estuary. Furthermore, an optimized WQI model consisting of six crucial parameters (TN, Pb, Cd, Zn, NO3--N, and As) was built using the random forest method, which exhibits excellent performance in water quality assessment. The approach proposed in the present study can significantly reduce the number of parameters required to assess water quality without the results.