Realtime fluorescence organic matter indicators for early warning of river pollution sources

• Real-time monitoring helps identify pollution sources. • Optical fluorescence sondes can be used for event monitoring. • The ratio of tryptophan-like DOM to humic-like DOM reveals water quality changes. Water quality monitoring is essential for effective river catchment management, yet traditional spot sampling often fails to capture short-term pollution events, particularly important in catchments impacted by multiple sources. This study explored the use of the ratio of tryptophan-like to humic-like dissolved organic matter (DOM) fluorescence as an indicator for distinguishing chronic and episodic pollution events, hereafter referred to as the T/C ratio. High-frequency monitoring across two tributaries of the River Evenlode in West Oxfordshire (UK) revealed the impacts of sewage treatment works (STW) on receiving rivers, with increases up to 350 % in tryptophan-like DOM concentrations during dry-weather flows relative to upstream conditions, with lower relative increases (250 %) in humic-like DOM. The T/C ratio showed clear seasonal dynamics, with seasonal shifts up to 21 % in one river and 7 % in the other, while diel cycles were found to be sensitive to seasonal changes in precipitation and temperature. In one of the rivers, regular episodic pollution events were identified based on sharp increases in the T/C ratio, which led to further management and monitoring actions. Logistic regression models based on the T/C ratio achieved an accuracy of 0.82 (AUC = 0.86) in distinguishing periods of STW spills from normal discharge conditions. Periods of low river flow (summer and autumn) exhibited the highest sensitivity to effluent inputs. Comparative analysis of two wastewater treatment processes (filter bed vs. oxidation ditch) demonstrated contrasting impacts on downstream T/C ratios, consistent with expected differences in DOM biodegradability. High-resolution monitoring of the T/C ratio therefore provides quantitative insight into pollution sources and their impact on receiving river water quality, highlighting the opportunities and challenges of sensor-based, real-time monitoring for informing water management and regulatory decision-making.