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, He-3 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 He-3 and SF6 observed during each transect decreased from about 6500 x 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 He-3/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.