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Tracers allow for the investigation of reach-scale physical properties of streams. Typically, a tracer is released upstream and its concentration is measured downstream, producing a break through curve (BTC). The BTC contains information about the physical properties of the reach through which the tracer has passed, such as transient storage and residence time distribution. These properties are important in dictating stream chemistry and biology. A stream’s physical transport properties can change under different flow conditions. Application of tracers can quantify these changes. In March of 2105, we applied a pulse injection of Rhodamine WT to the headspring of the Silver River (Silver Springs, FL, USA) and measured BTCs at nine fixed locations downstream. Additionally, roving vessels collected 318 samples to characterize differential mixing along the channel’s width, depth, and eco-geomorphological features. The observed BTCs were “fitted” to the OTIS model, a one dimensional transport and mixing model, using non-linear regression optimization and Bayesian inference. The resulting parameter values were compared to fitted OTIS parameters from a 2009 Silver River Rhodamine WT dye trace, when the river discharge was approximately 22.5% lower. The comparison suggests that the Silver River’s hydraulic transport properties are substantially different for the two discharges. For the larger discharge, the mean residence time of the spring run decreased and the transient storage increased. In addition, the tracer experiment results were used to calibrate and validate a hydrodynamic EFDC model being developed by the St. Johns Water Management District. The EFDC model was used to simulate the dye trace. The results of the simulation were compared with observed BTCs. The model was able to capture most of the major features of the BTCs and provide insight where the match was poor. Continuing work will include additional dye trace experiments timed to different flow conditions and further EFDC calibration. |