Determination of longitudinal dispersion coefficient and net advection in the tidal Hudson River with a large-scale, high resolution SF₆ tracer release experiment

Physical processes such as advection, dispersion, and air-water gas exchange play important roles in determining the movement and change in concentration of contaminants discharged into rivers. In the following, we report results from a large-scale SF₆ tracer release experiment conducted in the tidal Hudson River to examine longitudinal dispersion and net advection. SF₆ was injected into the Hudson River near Newburgh, NY, and surveyed for 13 days using a new, fully automated, high-resolution SF₆ sampling and analysis system. Net down river advection of the water body originally tagged with SF₆ was slow, averaging mean displacement rates of about 0.5 ± 0.2 km d^-1. In contrast, spreading of the tracer was driven by tidal movement, causing rapid mixing of the water up and down river. By examining the change in the second moment of the tracer distribution with time, we determined the mean longitudinal dispersion coefficient to be 70.1 ±4.3 m² s^-1. Temporal evolution of the SF₆ inventory indicates an average gas transfer velocity over the period of the experiment of 6.5 ± 0.5 cm h^-1 (1.56 ± 0.12 m d^-1). Vertical profiles show that mixing into the bottom layers of the river, in places reaching more than 53 m, seemed to be rapid.