Seasonal Variations of Dissolved Organic Matter (DOM) in Urban Riverine Outfall Water and Its Association with Water Quality: A Case Study of the Nanfei River and Banqiao River in Hefei
Urban river water quality is critically influenced by outfall discharges, yet the seasonal dynamics of dissolved organic matter (DOM) and its linkage to water quality remain poorly constrained. This study collected outfall water samples seasonally during 2023–2024 along the Nanfei River and Banqiao River in Hefei, Anhui Province. Parallel factor analysis of excitation-emission matrices identified three fluorescent components: fulvic acid-like C1, tryptophan-like (protein-like) C2, and terrestrial humic-like C3. Seasonal variations were pronounced: protein-like C2 dominated in winter and spring, whereas summer and autumn showed lower C2 proportions due to rainwater dilution and urban nonpoint source runoff inputs. Water quality indices decreased in summer and autumn, primarily attributed to dilution by rainfall runoff. Fluorescence index (FI > 1.9) and biological index (BIX > 1.0) indicated predominantly autochthonous DOM sources. During summer and autumn, humification index (HIX) and specific UV absorbance (SUVA) increased, while spectral slope ratio (SR) decreased, suggesting enhanced terrestrial and urban runoff influence. Significant positive correlations were observed between protein-like C2 and terrestrial humic-like C3 with water quality parameters, indicating their utility as precise indicators of pollution sources and seasonal water quality variations. These findings provide a scientific basis for integrated management of urban outfalls.