Supersaturated Total Dissolved Gas (TDG)

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Total dissolved gas (TDG) is the total amount of gas, such as oxygen or carbon dioxide, dissolved in a water body. TDG concentration is usually represented by the ratio of total gas pressure (TGP) to the local barometric pressure (BP) and can be calculated as a percent of local barometric pressure: TDG (%) = TGP/BP × 100%. TGP is the absolute pressure of the sum of the partial pressures plus water vapor. If the partial pressure of these gases in water exceeds their atmospheric pressure, water becomes supersaturated, i.e., contains more than 100% of dissolved gas equilibrium at atmospheric pressure. TDG is a natural process where air is dissolved through the mass transfer between air and water. The local barometric pressure, humidity, and temperature would stabilize TDG with the saturation concentration. [1]

Supersaturated Total dissolved gas (TDG) has been identified as one of the key potential impacts of hydropower operations [1]. It occurs due to various factors related to water dynamics and environmental conditions. The occurrence of supersaturated TDG could be from the following:

• Hydraulic Sluicing: Supersaturated TDG can occur due to hydraulic sluicing, closely related to aeration, pressure, turbulence intensity, and water temperature. The strong aeration of high-speed water during dam discharging periods can lead to the supersaturated TDG in downstream water, causing gas bubble disease in fish and threatening their survival [2].
• Water Dynamics: Supersaturated dissolved oxygen (DO) and TDG can be generated by high dam discharge, excess oxygen production in photosynthesis, and increasing water temperature. Aeration positively promotes the release of supersaturated DO and TDG, while aeration aperture and depth can inhibit them [3]. Supersaturated TDG is transported and dissipated more slowly in reservoirs than in natural rivers due to higher water depth and lower turbulence [4].

The experimental studies indicate that pressure (water depth), aeration, bubble dissolution time, air-water contact area, and turbulence intensity are key factors affecting supersaturated TDG. The generation rate of supersaturated TDG is affected by the turbulence intensity and water-air contact area (bubble size). The smaller gas bubble diameter made it dissolve quickly because of the larger air-water contact area and the faster generation rate of supersaturated TDG. The more substantial turbulence also resulted in the higher generation rate of supersaturated TDG. Sufficient aeration and pressure (water depth) are required for supersaturated TDG generation when the high dam discharges water. A larger water depth and higher pressure can generate a higher Supersaturated TDG level. [5]

The impacts of supersaturated Total Dissolved Gas (TDG) resulting from dam operations are multifaceted and can have significant implications on the following:

• Gas Bubble Disease (GBD): Elevated TDG downstream of dams can increase the incidence of a non-infectious gas bubble disease in fish, leading to endothelial lesions and trauma. The presence of gas bubbles in fish has been associated with severe vascular and dermal lesions, affecting the health of aquatic biota [6]. 
• Fish behavior and health: Studies have shown that exposure to supersaturated TDG can significantly impact fish eggs’ hatching rate and juvenile fish’s tolerance to supersaturated TDG and suspended sediment [7]. The physiological effects of supersaturated TDG on fish are still not fully understood, highlighting the need for further research in this area [8]. Fish of different sizes and species have shown varying tolerance capacities to supersaturated TDG. The smaller fish sizes have been found to exhibit greater tolerance capacity at high supersaturation levels [9].

The saturated TDG can be mitigated through the following:

• Activated carbon dissipation: Supersaturated TDG can be reduced by introducing activated carbon and enhancing water turbulence when supersaturated TDG occurs. The higher activated carbon content can increase the dissipation coefficient of TDG. [10]
• Prediction model: The predictive model of TDG at hydropower facilities is considered an operation solution to deal with saturated TDG. It is a site-specific model that defines the fate of spillway and powerhouse flows in the tailrace channel. The model is based on the concept that the air in spillway releases and the subsequent exchange of atmospheric gases into solution through the stilling basin can cause elevated levels of saturated TDG downstream. The model is helpful as a reference for the tailrace channel management. [11, 12]

The saturated TDG has become an ecological environmental issue, and it is worth considering further studies and investigation, especially in regions where hydropower development is growing. 

By: Hendra WINASTU, SOLEN Principal Associate – IPC panel coordinator

Edited by: Nguyeng Duy Hung, SOLEN Director – IPC program director

Date: 21 November 2023

Article#: SOLEN-IPC-0029

References:

[1] Li, P., Zhu, D. Z., Li, R., Wang, Y., Crossman, J. A., & Kuhn, W. L. (2022). Production of total dissolved gas supersaturation at hydropower facilities and its transport: A review. Water Research, 223, 119012. https://doi.org/10.1016/j.watres.2022.119012 

[2] Chen, Y., Wu, X., Lai, J., Yan, B., & Gong, Q. (2023). Molecular mechanisms of physiological change under acute total dissolved gas supersaturation stress in yellow catfish (Pelteobagrus fulvidraco). Environmental science and pollution research international, 30(43), 97911–97924. https://doi.org/10.1007/s11356-023-29157-6

[3] Yao, Y., Yang, H., & Wang, Y. (2022). Research on the release relationship between DO and TDG in standing water. Water Supply.

[4] Jingjie, F., Li, R., Liang, R., & Shen, X. (2013). Eco-environmentally friendly operational regulation: an effective strategy to diminish the Supersaturated TDG of reservoirs. Hydrology and Earth System Sciences, 18, 1213-1223.

[5] Qu, L., Li, R., Li, J., Li, K., & Wang, L. (2011). Experimental study on total dissolved gas supersaturation in water. Water science and engineering, 4, 396-404.

[6] Speare D. J. (1991). Endothelial lesions associated with gas bubble disease in fish. Journal of Comparative Pathology, 104(3), 327–335. https://doi.org/10.1016/s0021-9975(08)80044-8 

[7] Li, N., Fu, C., Zhang, J., Liu, X., Shi, X., Yang, Y., & Shi, H. (2019). Hatching rate of Chinese sucker ( Myxocyprinus asiaticus Bleeker) eggs exposed to total dissolved gas (TDG) supersaturation and the tolerance of juveniles to the interaction of Supersaturated TDG and suspended sediment. Aquaculture Research.

https://www.sciencedirect.com/science/article/abs/pii/B9780128122112000664

[8] Moen, K.L., & Kirschbaum, D.S. (2009). Hydraulic Design of Total Dissolved Gas Mitigation Measures for Boundary Dam.

[9] Xue, S., Wang, Y., Liang, R., Li, K., & Li, R. (2019). Effects of Total Dissolved Gas Supersaturation in Fish of Different Sizes and Species. International Journal of Environmental Research and Public Health, 16(13). https://doi.org/10.3390/ijerph16132444

[10] Niu, J., Li, R., Shen, X., & Wang, L. (2015). Experimental Research on the Promotion of Supersaturated Total Dissolved Gas Dissipation by the Use of Activated Carbon.

[11] Pasha, F., Hadjerioua, B., Stewart, K., Bender, M.D., & Schneider, M.L. (2012). Prediction of Total Dissolved Gas (TDG) at Hydropower Dams throughout the Columbia.

[12] Hadjerioua, B., Pasha, F., Stewart, K., Bender, M.D., & Schneider, M.L. (2012). PREDICTION OF TOTAL DISSOLVED GAS EXCHANGE AT HYDROPOWER DAMS.