Hydropower turbine-generated total dissolved gas supersaturation during synchronous condense operations

Total dissolved gas (TDG) supersaturation in rivers is a well-documented environmental concern, as exposure to levels above 110 % can increase mortality risk for fish and other aquatic organisms. While supersaturated TDG is primarily generated during spillway releases where entrained air dissolves in plunge pools, hydropower turbines operating in synchronous condense mode may also contribute to TDG generation. In this study, field measurements were conducted at the Seven Mile Dam on the Pend d'Oreille River, Canada, under different synchronous condense scenarios to evaluate the magnitude and mechanisms of turbine-driven TDG supersaturation. A simplified mathematical formulation was developed to describe mass transfer across the draft tube free surface and simulate TDG transport. The influence of operational parameters including compressed air pressure, cooling water discharge, mass transfer coefficient, and operation duration on TDG generation was examined. TDG levels up to 146 % were observed downstream of the unit, demonstrating that turbines operating in synchronous condense mode can generate biologically significant supersaturation. Results showed that operating synchronous condense mode in conjunction with generation flow reduced TDG levels by about 20 %, and that supersaturation effects were largely confined within 0.5 km downstream of the turbine. These findings highlight the importance of turbine operation as a potential source of TDG supersaturation and its associated ecological risks, and provide guidance for operational strategies to mitigate impacts on river ecosystems. • Hydro turbine synchronous condense operations generate supersaturated dissolved gas. • Operational factors like air pressure and discharge strongly affect dissolved gas levels. • Total dissolved gas risks mitigated by optimized operations to protect aquatic life.