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.