Use of High-Frequency, In-Stream, Ultraviolet-Visual (UV-vis) Spectroscopy to Characterize Organic Carbon and Nitrogen Species in Watershed Runoff

Natural or anthropogenic episodic events such as snowmelt, floods, fire, insect-defoliation, pollutant spills, etc. can result in sudden and unexpected changes in runoff water quality from watersheds. Depending on the magnitude and intensity of the change, such events which are also occasionally referred to as "hot moments", can have significant ecological and environmental consequences. Measuring and recording such rapid and unexpected changes in runoff quality has always been a logistical challenge. However, the advent of in-situ, UV- and fluorescence-based spectrometers that can continuously measure water quality changes at high-frequency (minutes to hours) show considerable promise. We implemented a UV-vis spectrometer (Spectrolyser, S::CAN Inc.) to characterize the stream water quality at every 30 minutes from a small (12 ha) forested watershed located in the Piedmont region of Maryland. The spectrometer recorded the UV-Vis spectrum (200-750 nm), turbidity (NTU), nitrate-N (mgN/L), and total and dissolved organic carbon (TOC and DOC, respectively; mgC/L). To evaluate the accuracy of the sensor values, water sampling was also performed simultaneously using automated ISCO samplers for multiple storms since November 2013. Water samples have been analyzed for suspended solids, particulate and dissolved forms of organic carbon (OC), and nitrate-N. In addition, water samples were also analyzed on laboratory spectrometers to develop a variety of UV and fluorescence metrics that characterize the lability and recalcitrance of DOC. Key questions that we address here are: How reliable and accurate are the spectrometer values for dissolved and particulate species of OC and nitrate-N? How does the magnitude of the storms and the amount of suspended sediment influence the accuracy of sensor readings? Can the sensor UV-vis data provide insights into DOC character/composition similar to those derived from lab-based UV and fluorescence metrics? Addressing these questions is critical to the long-term use of these sensors and their ability to provide critical insights into watershed and ecological processes.