Quantifying Reaeration Rates in Alpine Streams Using Deliberate Gas Tracer Experiments

Gas exchange across the air-water interface is a critical process that maintains adequate dissolved oxygen (DO) in the water column to support life. Oxygen reaeration rates can be accurately measured using deliberate gas tracers, like sulfur hexafluoride (SF6) or xenon (Xe). Two continuous release experiments were conducted in different creeks in the Sierra Nevada of California: Sagehen Creek in September, 2009, using SF6 and Martis Creek in August, 2012, using both SF6 and Xe. Measuring gas loss along the creek, which was approximated with the one-dimensional advection-dispersion equation, allows for the estimation of the SF6 or Xe reaeration coefficient (K-SF6, K-Xe), which is converted to DO reaeration (K-DO or K-2) using Schmidt numbers. Mean K-SF6 for upper and lower Sagehen and Martis Creeks were, respectively, 34 day(-1), 37 day(-1) and 33 day(-1), with corresponding K(DO)s of 61 day(-1), 66 day(-1) and 47 day(-1). In Martis Creek, K-Xe was slightly higher (21%) than K-SF6, but the calculated K-DO from SF6 agreed with the calculated K-DO from Xe within about 15%; this difference may be due to bubble-enhanced gas transfer. Established empirical equations of K-DO using stream characteristics did a poor job predicting K-DO for both creeks.