Continuous field estimation of dissolved organic carbon concentration and biochemical oxygen demand using dual‐wavelength fluorescence, turbidity and temperature

Abstract

Dissolved organic matter (DOM) quality and quantity is not measured routinely in‐situ limiting our ability to quantify DOM process dynamics. This is problematic given legislative obligations to determine event based variability; however, recent advances in field deployable optical sensing technology provide the opportunity to address this problem. In this paper, we outline a new approach for in‐situ quantification of DOM quantity (Dissolved Organic Carbon: DOC) and a component of quality (Biochemical Oxygen Demand: BOD) using a multi‐wavelength, through‐flow fluorescence sensor. The sensor measured tryptophan‐like (Peak T) and humic‐like (Peak C) fluorescence, alongside water temperature and turbidity. Laboratory derived coefficients were developed to compensate for thermal quenching and turbidity interference (i.e., light attenuation and scattering). Field tests were undertaken on an urban river with ageing wastewater and stormwater infrastructure (Bourn Brook; Birmingham, UK). Sensor output was validated against laboratory determinations of DOC and BOD collected by discrete grab sampling during baseflow and stormflow conditions. Data driven regression models were then compared to laboratory correction methods. A combination of temperature and turbidity compensated Peak T and Peak C was found to be a good predictor of DOC concentration (R2 = 0.92). Conversely, using temperature and turbidity correction coefficients provided low predictive power for BOD (R2 = 0.46 and R2 = 0.51, for Peak C and T, respectively). For this study system, turbidity appeared to be a reasonable proxy for BOD, R2 = 0.86. However, a linear mixed effect model with temperature compensated Peak T and turbidity provided a robust BOD prediction (R2 = 0.95). These findings indicate that with careful initial calibration, multi‐wavelength fluorescence, coupled with turbidity, and temperature provides a feasible proxy for continuous, in‐situ measurement of DOC concentration and BOD. This approach represents a cost effective monitoring solution, particularly when compared to UV – absorbance sensors and DOC analysers, and could be readily adopted for research and industrial applications.

High‐frequency monitoring of catchment nutrient exports reveals highly variable storm event responses and dynamic source zone activation

“…It is likely that different DOM pools of varying quality and composition contribute to the DOC flux causing time-variable relationships between absorbance and DOC quantity. Hence, coupled in situ monitoring of humic-and protein-like fluorescence could improve understanding of DOC behavior under stormflow conditions [Khamis et al, 2017].…”

“…Moreover, antecedent soil moisture, temperature, and groundwater conditions can alter the potential for transformation (e.g., mineralization) and accumulation of nutrients in shallow subsurface flow paths [Agehara and Warncke, 2005]. In particular, extreme flow conditions caused by episodic storm events and seasonal snowmelt have been shown to exert major influences on nutrient export patterns and dynamics [Pellerin et al, 2012;Saraceno et al, 2009;Basu et al, 2010;Khamis et al, 2017]. During these events, changes in surface and subsurface flow paths can modify riparian connectivity to the river catchment and lead to the activation of distant solute and particulate source zones that would not usually contribute to catchment nutrient export under base flow conditions.…”

Exploring Tracer Information and Model Framework Trade‐Offs to Improve Estimation of Stream Transient Storage Processes

“…The last decade has seen an explosion of novel techniques for collecting data used to characterize dynamic hydrologic systems. Tools and techniques that fall under this umbrella include the burgeoning field of hydrogeophysics (e.g., St Clair et al, ; Ward et al, ), the use of unoccupied aerial vehicles (e.g., Brenner et al, ; Vivoni et al, ), high space‐time resolution sensing systems (e.g., Blaen et al, ; Khamis et al, ), and the growing use of smart and conservative tracers in the environment (e.g., Blaen et al, ; González‐Pinzón et al, ; Haggerty et al, ; Knapp et al, ; Runkel, ). Observational data obtained from these techniques have been used to reveal new process dynamics and to refine current understanding of hydrological systems.…”

Using In‐Situ Optical Sensors to Understand the Biogeochemistry of Dissolved Organic Matter Across a Stream Network

“…The T280 peak is often referred to as ''tryptophan-or tyrosine-like'' fluorescence since the indole ring of tryptophan fluoresces in a similar region (Coble et al, 1998;Fellman et al, 2010). The T280 peak has been previously used to effectively predict biological oxygen demand (BOD) in urban environments and in wastewater effluent (e.g., Hudson et al, 2008;Khamis et al, 2015Khamis et al, , 2017. Since tryptophan and tyrosine are both amino acids and forms of DON, the T280 peak might be a strong predictor of ambient DON concentrations.…”