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
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
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
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
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.
McDonald, John P.; Gulliver, John S..
Methane Tracer Technique for Gas Transfer at Hydraulic Structures.
St. Anthony Falls Hydraulic Laboratory.
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