To investigate the variations in water quality of the Phase I of the Eastern Route of the South-to-North Water Diversion Project before and after water diversion, based on monitoring data of 18 water quality indicators from 30 monitoring sections along the route, this study systematically analyzed the spatiotemporal distribution characteristics and influencing factors of water quality indicators before and after water diversion using significance analysis, correlation analysis, non-metric multidimensional scaling (NMDS), and positive matrix factorization (PMF) and other methods. The results indicated a significant improvement in water quality after the diversion, with concentrations of chemical oxygen demand (COD_Mn), ammonium nitrogen (NH⁴⁺-N), and total phosphorus (TP) decreasing by 24.7%, 87.9%, and 39.6%, respectively , and the comprehensive water quality index (WQI) improved by 1.3%. The hydrochemical types remained as Ca²⁺-Mg²⁺-Cl⁻-SO₄²⁻ and Ca²⁺-Mg²⁺-HCO₃⁻ without change after diversion. Spatial analysis showed ‘excellent’ WQI levels in both Jiangsu and Shandong sections, whereas the inter-provincial boundary section exhibited relatively high total nitrogen (TN) and total phosphorus (TP) concentrations. Correlation analysis revealed that the significant positive correlations between COD_Mn and nitrate nitrogen (NO₃⁻-N) and total dissolved phosphorus (TDP) shifted from a significant positive correlation to no correlation after the diversion, reflecting the difference in water sources before and after the diversion. Furthermore, PMF analysis indicated that the increase in nutrients concentrations in the inter-provincial boundary section was related to agricultural runoff input, and the rise in phosphorus content in some sections was associated with disturbance of regulation and storage lakes.

Phase I of the Eastern Route of the South-to-North Water Diversion Project, positive matrix factorization (PMF), water quality indicators