Methane Tracer Technique for Gas Transfer at Hydraulic Structures

Abstract

Hydraulic structures have an impact on the amount of dissolved gases in a river system, even though the water is in contact with the structure for a short time. Bubbles become entrained wilen water flows over a spillway, creating more area for gas transfer. Because of this, the same transfer that normally would require several miles in a river can occur at a hydraulic structure. It would seem natural to use oxygen to measure gas transfer at a structure. However, there are problems associated with using oxygen for measurement. Many times the dissolved oxygen (D.O.) levels are near saturation. The uncertainty associated with estimates of D.O. saturation concentration and with D.O. measurements then results in a large uncertainty in the gas transfer measurement. Also, if the reservoir is stratified, it is difficult to predict withdrawal from the various layers with the required accuracy. Methane is produced in the sediments as a by-product of the anaerobic decomposition of organic material. Methanogenesis is the terminal process in a chain of decomposition processes and represents a major mechanism by which carbon leaves the sediments. Although methane is oxidized by bacteria to form carbon dioxide and water, the oxidation rate is insignificant over the short residence time of a hydraulic structure. If methane is present in measurable quantities, it may prove to be an excellent in-situ tracer of gas transfer. This report investigates using methane as an in-situ tracer of gas transfer at hydraulic structures. There were two major objectives: first, to develop a measurement technique to determine methane concentrations accurately; second, to perform field investigations to determine the applicability of using methane as an in-situ tracer of gas transfer. During an investigation, samples were gathered upstream and downstream of the structure and the transfer efficiencies were calculated for oxygen and methane. The mid-winter sampling technique of Rindels and Gulliver (1987) was used to assure accurate oxygen transfer measurements. Thene and Gulliver (1989) developed a headspace measurement technique while using propane as a tracer gas for measuring transfer efficiency at hydraulic structures. This measurement technique was adjusted to compute methane concentrations, and is also presented in this report. Methane was found in sufficient quantities for accurate measurements in all but one river/reservoir, where sulfate reduction inhibited the production of methane. Methane was generally unstratified upstream except under ice cover, providing an excellent tracer for gas transfer. The exception was under ice cover, when methane tends to be stratified and accurate transfer efficiency measurements were difficult. Under ice cover, however, the mid-winter oxygen measurements will produce accurate transfer efficiency measurements. The stratification of methane under ice cover created difficulties with the comparison of oxygen and methane measurements because the field conditions required to accurately measure the transfer of the two gases seems to be mutually exclusive, except at a few structures. A technique using oxygen, methane, and temperature measurements with a selective withdrawal routine (Davis, et aI, 1987) was developed to compare oxygen and methane transfer measurements. Oxygen and methane transfer efficiencies, after adjustment for diffusivities, were comparable at a given structure except when the entrained air bubbles were pulled to a depth in the tailwater causing the bubbles to experience a higher pressure. Since the partial pressure in air determines saturation concentration of atmospheric gases, the saturation concentration of oxygen is higher as the bubbles are pulled through the stilling basin. Thus, an II effective saturation II concentration must be determined for oxygen at hydraulic structures with a tailwater.