Water Quality Dynamics and Their Natural and Anthropogenic Controls Under the National Protection Strategy in the Chongqing Section of Yangtze River, Southwest China

This study aims to evaluate the spatiotemporal dynamics of water quality in the Chongqing section of Yangtze River (YRCQ) under the implementation of Yangtze River Protection Strategy (YRPS). Leveraging daily records for nine indices from 15 monitoring stations during the hydrological years from 2020 to 2024, multivariate statistical analyses were employed to identify the dominant pollutants and elucidate the driving mechanisms. The results demonstrated that the proportion of Classes I–III complying with the Chinese Environmental Quality Standards for Surface Water (GB3838−2002, abbreviated as ‘Chinese Standard’), remained above 95% over the study period. The water quality index (WQI) indicated an overall ‘good’ water quality, with no ‘poor’ or ‘very poor’ classifications. Seasonal variations were evident for indices, with pH, dissolved oxygen (DO) and electrical conductivity (EC) higher in the dry season, and water temperature (WT), permanganate index (COD_Mn) and total phosphorus (TP) elevated in the wet season. Generally, water quality was markedly superior in the dry season compared to the wet season. The WQI trends indicated by the Mann–Kendall test divulged a positive trend in water quality at most upstream and midstream stations, whereas a slight deterioration was observed at most downstream stations. Water quality characteristics exhibited significant spatial heterogeneity. Based on the temporal patterns of WQI, stations were classified into four distinct categories: superior, good, fair and poor. Principal component analysis (PCA) pinpointed TP and total nitrogen (TN) as primary drivers of water quality variations, predominantly linked to urban domestic sewage and agricultural runoff. Air temperature and precipitation amount tremendously influenced water quality temporally. An increased rate of surface runoff generation from urban development and nonpoint source pollution from cropland exerted negative effects on water quality. This study underscores critical water environmental challenges in the YRCQ, providing robust theoretical and practical insights for regional water protection and sustainable management.