Fine-Scale Dissolved Organic Matter Fluorescence Fingerprints Reveal First-Flush Transition Dynamics in Urban Drainage Overflows

Urban drainage overflows can release a large fraction of event-scale pollutants during the early stage, yet current control remains largely driven by hydraulic signals rather than pollutant-release dynamics. This study created a dissolved organic matter (DOM)-based fluorescence fingerprint method to precisely identify the shift from pollutant flushing to dilution or ongoing input, helping determine the timing of first-flush transitions and potential interception. Fourteen wet-weather overflow events from seven drainage systems in Shanghai and Changzhou were investigated using excitation–emission matrix fluorescence spectroscopy, combined with non-negative matrix factorization, random forest feature screening, principal component analysis, mass–volume (M(V)) curve analysis, and Pettitt change-point detection. Five macro-scale fluorescence fingerprints were resolved, representing protein-like, fulvic-like, and humic-like components. Protein-like fingerprints dominated rapid event-scale variations, while fulvic-like and humic-like fingerprints reflected continuous surface-derived input and stable background contribution, respectively. Peak-shift trajectories revealed three fluorescence-evolution modes: directional red-shift migration, peak-position stability, and weak, non-directional variability, reflecting different source-release dynamics and DOM compositional adjustments during overflow. Random forest screening identified 20 high-importance fine-scale fluorescence fingerprints, with 90% concentrated in protein-like regions linked to sewage-derived and labile DOM. Compared with macro-scale fingerprints and conventional water quality indicators, fine-scale fluorescence fingerprints showed clearer stage separation, stronger consistency with M(V)-based cumulative response patterns, and more distinct first-flush interception timing. This timing marked the transition from early concentrated pollutant release to dilution or sustained input, whereas macro-scale fingerprints indicated broader transition intervals and conventional indicators showed delayed responses. These findings highlight the potential of fine-scale fluorescence fingerprints to support future fluorescence-assisted overflow control by improving transition identification and targeted interception decisions.