Sulfur hexafluoride gas tracer studies in streams

The air-water mass transfer coefficients, dispersion coefficients, and mean residence times in two experimental streams and one natural stream are measured using a variation of the standard volatile tracer-dye technique. Sulfur hexafluoride (SF₆) is used as the volatile tracer and rhodamine WT is used as the conservative tracer. The low limit of quantification of SF₆ makes it possible to inject SF₆-rich water into many streams and avoid complications with dosing a stream with a gaseous tracer. The experimental methods are described in detail. The SF₆ measurements were extremely precise, producing smooth concentration time curves. The SF₆ measurements collected in side-by-side experimental channels yielded similar values of the gas transfer coefficient.