New insights on the transport of solutes in a large estuary revealed with sulfur hexafluoride tracer

Traditional approaches to the observation of solute transport in estuaries and coastal areas depend on numerical models calibrated with Eulerian measurements of velocity and depth. This approach suffers from inaccuracy, particularly over large spatial scales, and yields little useful information about mixing. Tracer injections of SF₆ (an inert gas) were pursued as an alternative, Lagrangian, approach in a set of experiments covering the Hudson-Raritan Estuary. A boat-mounted, high-resolution, automated SF₆ measurement system was developed (in collaboration with other scientists), able to detect SF ₆ concentrations spanning 5 orders of magnitude, at a measurement interval of 60 s. This technique achieves large-scale (50+ km2), direct visualization of solute transport and fate at an unprecedented spatial and temporal resolution.; Results of three SF6 injection experiments are reported: a study of freshwater transport on the upper Hudson River in October 2001, and studies of tidal flushing and solute fate in Newark Bay (a heavily used industrial area) in July 2002, and in the East River (a tidal strait connecting New York Harbor with Long Island Sound) in June 2003. Specific findings include: on the Upper Hudson River, a relatively small longitudinal dispersion coefficient (17.3 m2 s−1) and large tracer losses from gas exchange at dams; in Newark Bay, a relatively long mean residence time (8 days) and identification of the Kill van Kull as the strongly preferred flushing pathway; in the East River, mean residence times of 3.3 and 1.6 days for injections on the flood tide and the ebb tide, respectively, illustrating the potential for environmental remediation via tidally phased contaminant discharge.; More generally, the process by which solutes are transported through a strait by tidal motion is isolated, labeled as “tidal transfer”, and quantified for the first time in these waters. Tidal transfer flux rates of 890 m3 s−1, 430 m3 s −1, and 275 m3 s−1 were measured at the south and north ends of the East River, and at the east end of the Kill van Kull, respectively, dominating sub-tidal circulation. Parameterization of tidal straits based on the ratio of the tidal excursion to the length of the strait is proposed to predict transport properties.