Reaeration measurements using a methane tracer at six dams in the Ohio River basin

The reaeration process across hydraulic structures on the Ohio River was evaluated using in situ methane as a tracer. Reaeration rates determined from dissolved oxygen (DO) measurements are often unreliable when upstream DO concentration is near saturation. In such cases, the increase in DO as the flow passes the structure is small, leading to relatively large measurement uncertainties. Naturally occurring methane can be used as a tracer for DO because the difference between upstream and downstream dissolved methane concentration is sufficiently large for current analytical techniques. The relationship between methane and oxygen transfer is well documented. The reaeration effectiveness of various gate and gate openings at these structures was investigated. The results obtained suggest that the highest gas transfer, among the structures studied, takes place across gated spillways. In addition, the gas transfer efficiency was higher than expected, in some cases, for hydropower facilities on the Ohio River.