Author(s): Ida Vartia; Ulrich Pulg; Martin Enqvist; Rowan Hamper; Sebastian Stranzl

Linked Author(s):

Keywords: Total Dissolved Gas Supersaturation; Hydropower; Environmental risk assessment; Freshwater; Gas Bubble Disease

Abstract: Total dissolved gas supersaturation (TDGS) is an under-recognized environmental impact of hydropower operations, caused primarily by air entrainment in power plant intakes and by spillway-induced turbulence. When aquatic organisms are exposed to TDGS, they may develop gas bubble disease, which can lead to mortality and population declines. Although severe TDGS-related impacts have been documented in several regions worldwide, awareness, monitoring, and management remain limited. To address this gap, NORCE LFI developed risk assessments for TDGS at hydropower plants and dam spillways in Norway and Sweden. These assessments identify facilities with high potential for producing TDGS and guide monitoring priorities. A two-stage monitoring approach is recommended: Stage 1 maps the occurrence and degree of gas supersaturation, while Stage 2 examines its longitudinal spread and associated biological impacts. Monitoring is currently underway in Norway, Sweden, Spain, and Greece, using both conventional TDGS equipment and a newly developed smart sensor system designed for long-term, real-time ecological monitoring. Results show potentially harmful TDGS levels (>110% saturation) at a majority of monitored sites. In Norway, TDGS maxima ranged from 120–229%, while preliminary data from Sweden, Spain, and Greece showed values of 110–131%. Most documented cases were linked to air entrainment at creek intakes, with additional instances associated with dam spills and aeration of Francis turbines. These findings confirm that TDGS occurs more frequently, and over wider geographic areas than previously assumed. The study highlights the importance of systematic monitoring for detecting TDGS, understanding its drivers, and supporting effective mitigation. Continued refinement of risk assessments and broader adoption of TDGS monitoring are essential to protect freshwater ecosystems and achieve good ecological status.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.293

Year: 2026