Observing Nutrient Dynamics in Streams Draining Varied Land Uses Using In Situ Optical Sensors

In recent years, in situ optical water quality sensors have been applied to advance the understanding of biogeochemical changes in aquatic systems at temporal scales impracticable for traditional grab sampling. Optical sensor technology was used in northern Vermont to measure watershed response to seasonal- and event-based fluctuations in various water quality parameters using measurements every 15 minutes. An in-stream UV-Vis spectrometer measured turbidity and nitrate, total organic carbon, and dissolved organic carbon concentrations using turbidity compensated spectra at wavelengths from 200 nm to 700 nm. Sensors were placed in streams whose watersheds represent urban-suburban, agricultural, forested, and mixed land uses in order to understand varied chemical responses based on a variety of natural and anthropogenic influences. Salt dilution and velocity-area discharge measurements were made in concert with pressure transducer measurements to develop robust rating curves for streams where no US Geological Survey discharge data was available so that loads could be calculated. Findings will deepen the understanding of (1) the behavior of dissolved organic carbon, nitrate, and sediment across watersheds of different land uses, focusing on seasonal climatic drivers and response to extreme events, (2) the changes in the absorbance and fluorescence character of dissolved organic carbon over diurnal, event-based, and seasonal temporal scales, and (3) how high-frequency water quality data can be used to better calculate nutrient loads to receiving water bodies.