3.4 Tracer test
A single uranine tracer experiment was performed on June 26, 2019 to quantify transient storage between GS1 and GS2. Tracer concentration was chosen based on a previous study by Lautz and Siegel (2007) with similar flow conditions. A peristaltic pump (Solinst, 410) was set up at the top of the reach to release 0.3 g L-1 of the fluorescent dye, at 1.4 L min-1, for a total of 27 minutes. Fluorometers (Turner Designs, Cyclops-7F) were deployed at locations F1 and F2 (Figure 1), with shade caps, at the end of the reach and in the stream near P4 (Figure 1). To obtain an accurate relationship between sensor output voltage and dye concentration the fluorometers had previously been calibrated in the laboratory with five concentration standards: 0, 0.5, 4, 100 and 400 ppb of uranine. For the experiment, the fluorometers were connected to dataloggers (Campbell Scientific, CR1000) and set to take measurements at 5 second intervals. The resulting breakthrough curves were then used to calculate total tracer recovered to confirm complete mixing.
The one-dimensional transport model OTIS-P (Runkel, 1998) was used to estimate the transient storage parameters, dispersion coefficient, and stream cross sectional area (see electronic supplement for details) assuming that the flow is at steady state and that uranine is a conservative tracer (Smart & Laidlaw, 1977). The steady-state assumption was justified as there was a change in discharge of less than 6 % at GS1 during the experiment.