Field Experiments to Determine Gas Transfer at Gated Sills

Field experiments were conducted to determine the oxygenation potential of gated sill structures in the Ohio River Valley. The objective was to determine operational procedures for water quality improvement. Oxygenation potential is typically characterized by gas transfer efficiency, with high transfer efficiency indicating a greater input of oxygen into the water. Direct oxygen measurement can be unreliable in determining transfer efficiency, and this is due to upstream stratification, relatively high background concentrations, and changes in saturation concentration when the bubbles are exposed to hydrostatic pressures within the stilling basin. As a result, in situ methane was used as an independent dissolved gas tracer. Methane is naturally present in measurable quantities with little stratification, and it does not experience an appreciable increase in saturation concentration with bubble depth. Therefore, methane measurements better reflect the true transfer efficiency of a structure. This paper focuses on the factors that influence transfer efficiency and how to achieve high quality field data through the use of methane and oxygen measurements. In addition, results from six-gated sill structures in the Ohio River Valley are presented.