Methane tracer technique for gas transfer at hydraulic structures

Using a simple headspace gas chromatographic technique, gas transfer at several hydraulic structures were investigated. Naturally occurring methane was used as a tracer gas to measure gas transfer, and was found in sufficient quantities for accurate measurements in all but one reservoir, where sulfate reduction inhibited methane production. Methane was generally unstratified upstream, except under ice cover, providing an excellent tracer for gas transfer. Oxygen and methane transfer efficiencies, after correction for diffusivities, were comparable at a given structure except when the entrained air bubbles were pulled to a depth in the tailwater. Bubbles under the water surface experience a higher oxygen and nitrogen saturation concentration, increasing the concentration difference and therefore the gas transfer rates. This problem was solved by determining an effective saturation concentration that incorporates the higher saturation concentration of bubbles under the water surface.