SF₆-³He tracer release experiment: A new method of determining longitudinal dispersion coefficients in large rivers

Longitudinal dispersion coefficients have been determined in the tidal Hudson River using sulfur hexafluoride. For this purpose, SF₆, a nonreactive synthetic gas, was injected into the Hudson south of Albany, NY, and used in a fashion similar to a fluorescent dye. Three days after injection, SF₆ distributions along the main axis of the river were approximately Gaussian. The change in variance with time showed two periods of linear growth. During the first period (days 3-8), the longitudinal dispersion coefficient was estimated to be 47 ± 6 m² s^-1, and during the second period (days 8-13), it was 162 ± 22 m² s^-1. A second gas, ³He, which was injected simultaneously with SF₆, allowed for the determination of gas exchange rates across the air-water interface. Gas transfer velocities normalized to a Schmidt number of 600 ranged from 2.8 to 9.2 cm h^-1 and correlated well with mean wind speed.