High frequency monitoring of dissolved organic matter dynamics in urban headwaters: implications of monitoring resolution for process inference

Submersible optical sensor technology provides new opportunities for high frequency observations of riverine dissolved organic matter (DOM) and nutrients that cannot be achieved from traditional discrete sampling. High frequency data are essential to reveal the DOM transport, processing and transformation changes during storm events. However, previous studies have tended to focus on DOM mobilization and transport in rural catchments, largely neglecting urban headwater systems despite DOM behaviour being highly variable given complex interactions between varying sources and pathways in urban catchments. The few studies that have explored urban DOM dynamics have done so over relatively short timescales (e.g., seasonal) and have not systematically explored the impacts of monitoring resolution on DOM process interpretation. This is surprising given the dynamic nature of urban hydrological systems. To address this research gap, we collected high frequency water quality and hydro metrological data (5 min resolution, 10/21-10/22) for an urban headwater stream (Bourn Brook, Birmingham, UK). An in-situ multi-parameter sensor (Proteus, Eureka) was deployed for monitoring tryptophan-like fluorescence (TLF, Ex 275 nm/ Em 350 nm) and humic-like fluorescence (HLF, Ex 325 nm/ Em 470 nm). High temporal resolution data (5 minutes) were aggregated into 10,15, 30, 60,120,180 minutes datasets to explore the impacts on concentration-discharge (C-Q) patterns and hysteresis, thus aim to understand the effects of monitoring rate on interpretation of solute pathways and determine the sufficient temporal resolution to capture the salient urban DOM storm-driven dynamics. Our results highlight that at coarser monitoring frequency (>30 min), the “first-flush” of liable DOM is hard to detect, but the recession dynamic (usually consisting of more humic-like compounds) is still adequately captured. At monitoring frequencies > 30 min, both HLF and TLF displayed clockwise hysteresis indices, suggesting proximal DOM sources were delivered through the urban drainage system on the rising limb. However, figure of eight hysteresis was most commonly identified for both fluorescence peaks when sub-15 min data were investigated, which suggested a more complex relationship with multiple sources of DOM being mobilised during storm events. Furthermore, chemodynamic behaviour for both HLF and TLF was observed at monitoring frequencies >30 min; yet at higher monitoring frequencies TLF displayed chemostatic behaviour at high discharge. Our works not only emphasised the importance of conducting high frequency monitoring when designing urban water quality studies as coarser resolution monitoring will not fully capture the urban DOM dynamic, but also provides new insight into the importance of carefully considering monitoring frequency and provides guidance for adaptive monitoring approaches if installations are constrained by power requirements or data storage.