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Total dissolved gas (TDG) when supersaturated can cause chronic or acute “gas bubble trauma” in fish. A criterion of 110% saturation has been adopted into most water quality standards regulations. TDG is generated when a dam spills water, and water entrains air and plunges to depth, forcing gas into solution by hydrostatic pressure. Conditions in the natural environment also affect TDG levels, such as waterfalls, wind, water temperature, and barometric pressure. Dams spill because river flows have exceeded powerhouse capacity. This can occur because of high river flows or because turbines are off-line due to maintenance, repair, or lack of a power market. Developing a total maximum daily load (TMDL) for TDG presents several unique challenges. Allocations are set in terms of percent saturation or pressure, not loading. TDG is highly variably in space and time and monitoring poses some unique challenges, so representative monitoring and determining an appropriate averaging scale and point of compliance can be challenging. TDG TMDLs also offer some opportunities. Since TDG is not cumulative, it may be possible to set allocations equal to criteria at each dam. TDG TMDLs are implemented at a dam through measures that reduce spill or the gas generation of spill. System-wide management of river flows, spill, and power generation can also minimize spill and TDG generation.
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Document Type: Research Article

Publication date: 2005-01-01

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