Dissolved gas supersaturation, which is associated with powerhouse discharges due to increasing gas solubility with increasing hydrostatic pressure within hydroelectric project penstocks, received considerable attention in Canada and the United States. Elevated total dissolved gasses (TDG) can lead to a physiological condition known as gas bubble trauma in aquatic biota, which can be harmful or even fatal to aquatic organisms, as demonstrated by a number of significant fish kills in the Columbia and Snake rivers. As TDG rises from saturation to levels above 110-120 percent of saturation, there is a significant driving force of dissolved gasses into body tissues of fish and other organisms, leading to stress and mortality in some cases. Although many fish forage in or otherwise occupy deeper layers where this driving force is lower due to higher hydrostatic pressure, many resident fish use shallow waters to carry out their life cycles. Invertebrates also can develop bubbles in supersaturated water and lose the ability to swim normally, which may result in long term shifts in invertebrate community structure below powerhouse tailraces.
While modern runner and tailrace designs often can reduce the potential for TDG problems, monitoring is generally conducted in systems with hydrostatic head as little as 8 m (25 ft) because of the potential for attaining equilibrium gas concentration in excess of 110% of saturation. Reconnaissance-level surveys may be conducted using barometrically compensated dissolved gas pressure sampling devices (e.g., HydroLab and others) to determine total dissolved gas pressure. Depending on the initial levels found, the need for additional sampling or continuous monitoring during normal project operations may be assessed.
2.5.1.1 Selected references
Fidler, L. E. and S. B. Miller. 1994. British Columbia water quality guidelines for dissolved gas supersaturation. Report to BC Ministry of Environment, Canada Department of Fisheries and Oceans Environment. Aspen Applied Sciences, Ltd., Valemont, B.C.
ISAB (Independent Scientific Advisory Board). 1998. Review of the U.S. Army Corps of Engineers' Capital Construction Program, Part II, B: Dissolved Gas Abatement Program. Prepared for the Northwest Power Planning Council and the National Marine Fisheries Service. Portland, Oregon.
Weitkamp, D. E., and M. Katz. 1980. A review of dissolved gas supersaturation literature. Transactions of the American Fisheries Society 109: 659-702.
