Insights from in-situ, UV-based, high-frequency sensor for characterizing storm-event particulate organic carbon in stream runoff

While dissolved forms of organic carbon (e.g., DOC) make up a large portion of the runoff load during baseflow and small storms, large storms can erode and mobilize significant amounts of particulate organic carbon (POC). Large storms yield sudden and rapid changes in POC which occur at minutes to hours and typically early in the storm event. Capturing these "hot moments" of POC is critical for understanding watershed processes, developing accurate budgets of solute flux, assessing the impacts on receiving aquatic ecosystems and developing sustainable mitigation strategies. The recent availability of in-situ, high-frequency, electronic sensors has shown considerable promise for characterizing dissolved forms of solutes (e.g., DOC, nitrate-nitrogen), but their ability to measure POC has yet to be rigorously evaluated. We evaluated the accuracy of a UV-based sensor to measure POC concentrations using a combination of field and laboratory based studies. Stream water POC concentrations were studied for multiple storms over a 2-year period (2015-2016) in a 79 ha forested watershed (second-order stream) in the Piedmont region of Maryland. Storm sampling was performed using ISCO samplers and POC (% OC content) was determined for suspended sediments (SS) retained on a 0.7 micron filter. POC values measured by the in-situ stream sensor are being evaluated against those determined for suspended sediments from stream runoff. Sensor versus lab-determined POC concentrations will be evaluated for: magnitude, intensity, and seasonal timing of the storms; values on the rising versus falling limb of the hydrograph; and potential sources of POC. Simultaneously, a laboratory experiment was performed where sensor versus lab-determined POC were examined for varying POC concentrations; variety of POC sources including stream banks, stream bed, forest floor, upland A horizon; and four particle size classes (2000-1000 µm; 1000-250 µm; 250-63 µm and < 63µm). This study will provide important insights into how stream water POC evolves through storm events and how storm event conditions, POC sources, and particle sizes may influence POC concentrations and measurements. Knowledge of POC along with DOC chemistry is especially critical in light of climate change predictions that indicate increasing intensity of large storms.