Gas exchange rates for a first‐order stream determined with deliberate and natural tracers

Gas transfer velocities have been determined for a first‐order stream by performing a 3‐hour release of the volatile tracer sulfur hexafluoride, SF 6 , and the nonvolatile tracer tritiated water, 3 H 2 O. The average gas transfer velocity for the 292‐m reach was 29 cm/h which corresponds to a reaeration coefficient for oxygen at 25°C of 134 day −1 . Groundwater inflow along the stream was corrected for by measuring the downstream dilution of the 3 H 2 O spike. Downstream discharge increased from 0.5 L/s, 2 m downstream of the point of tracer release, to 19.3 L/s at a point 292 m downstream. As an alternative to using (radioactive) 3 H 2 O, we investigated the possibility of using natural radon, 222 Rn, as a groundwater tag and using the variation of SF 6 and 222 Rn along the stream to determine gas exchange rates and groundwater inflow. The method yielded an average transfer velocity of 21 cm/h and underestimated the groundwater inflow by a factor of 3. This large discrepancy is attributed to a doubling of stream discharge between the time the stream was sampled for radon and the tracer experiment and the limited number of radon samples.