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

• • Three fluorescent DOM components (C1 fulvic-like, C2 tryptophan-like, C3 terrestrial humic-like) were identified via parallel factor analysis; C2 dominated in winter/spring (>50% of total fluorescence) while summer/autumn showed reduced C2 due to dilution and increased C3 from urban runoff. • • Fluorescence index (FI) > 1.9 and biological index (BIX) > 1.0 across all seasons indicate predominantly autochthonous (microbial) DOM sources, with implications for carbon cycling and pollutant transport in urban rivers. • • Summer/autumn exhibited increased humification index (HIX) and specific UV absorbance (SUVA) at 254 nm, alongside decreased spectral slope ratio (SR), signaling enhanced terrestrial and urban nonpoint source inputs during high-flow periods. • • Protein-like C2 and terrestrial humic-like C3 showed significant positive correlations (p < 0.05) with water quality parameters (e.g., COD, TN, TP), enabling precise tracing of pollution sources and seasonal water quality fluctuations for outfall management.