Comparison of SF₆ and flourescein as tracers for measuring transport processes in a large tidal river

We present the first large-scale comparison of a fluorescent dye [fluorescein (C₂₀H₁₀O₅Na₂)] and a gas [sulfur hexafluoride (SF₆)] as tracers of advection and longitudinal dispersion from a dual tracer release experiment conducted in the tidal Hudson River. At the beginning of the experiment, 36 kg of fluorescein and ∼4.3 mol of SF6 were injected into the Hudson River at an averaged depth of 9.5 m, ∼1 m above the bottom, near Hyde Park, N.Y. After injection, fluorescein distributions were surveyed for 4 days (until it became undetectable) and SF6 distributions were surveyed for 10 days. The dye resolves initial vertical mixing on the day of injection, and then net advection and longitudinal dispersion, whereas SF₆ provides information on net advection and longitudinal mixing on larger spatial scales and longer time scales. Quantitative estimates of transport processes (net advection and longitudinal dispersion) calculated from the two methods are consistent for the first three days, and start to deviate on the fourth day when the signal-to-noise ratio of the dye deteriorated.