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 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 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.