ABSTRACT: Elevated levels of total dissolved gas (TDG) may occur downstream of dams, leading to increased incidence of gas bubble disease in fish. Accelerating the dissipation of supersaturated TDG in the downstream river can mitigate this problem; however, data useful for modeling the dissipation of supersaturated TDG through the free surface in natural rivers are limited. Lacking data to the contrary, prior modeling studies have assumed (1) dissolved oxygen (DO) is a reasonable proxy for TDG; and (2) unsaturated reaeration is sufficiently similar to supersaturated dissipation such that the same rate coefficients may be applied to either process. To test the validity of these assumptions and motivate future research, laboratory experiments were conducted to estimate the first-order dissipation rate coefficients for supersaturated DO and TDG and the reaeration rate coefficients for DO under identical turbulence conditions. The results indicate the dissipation process is quantitatively different from the reaeration process, and TDG behavior is quantitatively different from DO. Comparison of laboratory results with prior field research leads to speculation that increasing river turbulence and/or decreasing in water depth may be possible methods for promoting the TDG dissipation rate and reducing the length of a river affected by supersaturation.

Fig. 5. Relative error (ε_t) in predicting time to concentration by using the incorrect gas or process, k_{Od}, k_{Or}, and k_{Or} to represent k_{Gd}, k_{Gd}, and k_{Od}, respectively; a negative value indicates the computed time to concentration; using the incorrect k would be shorter than expected with the correct k; error bars are ε_{tδ} from Eq. (5).