Gas exchange rates in the tidal Hudson river using a dual tracer technique

Gas exchange rates have been determined in the tidal Hudson River by injecting 2 inert gases, ³He and sulfur hexafluoride (SF₆) and monitoring their decline with time. Their distributions along the main axis of the river were approximately gaussian and maximum concentrations of excess ³He and SF₆ observed during each transect decreased from about 6500 × 10⁻¹⁶ cm³ STP g⁻¹ and 250 ppt (part per trillion by volume), respectively, to values close to atmospheric equilibrium concentrations over a period of 16 days. Throughout the experiment, vertical gradients in tracer concentration were observed. After 3 days of mixing, tracer concentrations in bottom samples were 0–19% greater than in surface samples. Gas transfer velocities were calculated from the temporal change in the depth averaged excess ³He/SF₆ ratio from stations having maximum tracer concentrations. They ranged from 1.5 to 9.0 cm h⁻¹ and correlated well with mean wind speed.