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 m(2) s(-1), and during the second period (days 8-13), it was 162 +/- 22 m(2) s(-1). A second gas, He-3, 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.
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