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3.0 SOURCES OF DISSOLVED GAS SUPERSATURATION

Dissolved gas supersaturation can result from a surprisingly wide variety of both man-made and natural causes. Hydroelectric and impoundment dams are known to cause high levels of DGS. Other sources include DGS associated with warm water discharges from cooling facilities (e.g., nuclear and fossil fuel power generating plants), oxygen production by aquatic plants (enhanced by nutrients associated with industrial effluents, municipal discharges, and agricultural runoff), solar heating of water bodies, ingestion of air into pumping systems, supplemental oxygen in hatcheries, and air lift re-aeration systems. In the following section, these and other causes of DGS are reviewed and some of the important physical and biological mechanisms involved in the processes are described.

3.1 Dams

Of the many possible sources of DGS, the discharge of water through dams has received the greatest attention in the literature. In these hydraulic structures, DGS is caused by the entrainment of air in water released over dam sluice ways, through low level ports and radial gates, or through turbo machinery associated with power generation. In dam sluice ways and radial gates, air is entrained in falling water which plunges to depth in pools at the base of the dam. There, under elevated hydrostatic pressure, air (in the form of bubbles) is forced into solution at pressures of several atmospheres. In the river downstream from the dam, delta P levels can range up to 400 mm Hg (Ebel 1969 and 1971, Ebel et al. 1973 and 1975, Meekin and Turner 1974, Clark 1977, Crunkilton et al. 1980, Weitkamp and Katz 1980, Maxwell 1985, Hildebrand 1991, White et al. 1991).

When water is discharged through turbines and low level ports, air is often entrained in vortices near the port or turbine intakes (Johnson 1988). Under conditions of elevated hydrostatic pressure near the face of turbine blades or in the discharge from low level ports, air is again forced into solution under hydrostatic pressures of several atmospheres. Low water conditions in reservoirs can enhance vortex formation and dramatically increase air entrainment.

Occasionally, air is injected into turbines to avoid problems of cavitation or mechanical stress under unfavourable loading conditions. This air, when discharged into deep tailwater regions downstream of the dam, can also contribute to high levels of DGS.

When DGS is produced by hydroelectric or impoundment dams, the ratio of the partial pressures of supersaturated nitrogen to supersaturated oxygen is often close to the ratio of nitrogen to oxygen in air (White et al. 1991). However, this can vary depending on the source of the discharge water. For example, water drawn from the hypolimnion of large reservoirs and discharged through low level ports may be very low in oxygen. Although oxygen is added as a result of air entrainment, it may not reach the same levels of saturation or supersaturation as the nitrogen. In other situations, water discharged over spillways may be drawn from the surface of a reservoir where high algae and plant productivity, combined with solar heating, have caused elevated background levels of DGS with oxygen more highly supersaturated than nitrogen.

As Fidler (1984) and Colt (1986) indicated, there is the potential to produce an additional delta P increment of 73 mm Hg for each metre of water depth in a plunge pool below a dam or water falls. Furthermore, the quantities of entrained air required to produce these levels of DGS can be quite small. Figure 6 shows the excess volume of air per litre of water (i.e., above that required to establish atmospheric saturation), needed to produce the indicated delta P levels.

Figure 6: Excess Volume of Air Required for Specified Levels of Delta P

Excess Volume of Air Required for Specified Levels of Delta P

The data of Figure 6 are for ideal conditions. In many situations, larger quantities of air and greater water depths may be needed to force the necessary gas volume into solution. For example, entrained air in the form of large bubbles may, due to bubble buoyancy, escape to the water surface before substantial quantities of gas are dissolved. On the other hand, if bubbles are very small, high surface tension forces may facilitate their collapse and lead to levels approaching those of Figure 6 (Harvey et al. 1944, Fox and Herzfeld 1954, Hlastala and Fahri 1973, Yount 1979, Fidler 1984).

The development of DGS and GBT in fish can be compounded when fish pass through turbines in dams (Bonneville Power Administration 1993). In these structures hydrostatic pressures can approach the vapour pressure of water (e.g., on the back side of turbine blades). The effective delta P in these regions can be as high as 750 mm Hg. The Bonneville Power Administration (1993) estimated that in the Columbia River of the United States, there is a 15% loss of migrating salmon smolts for each passage through a turbine structure. Tsvetkov et al. (1972) have shown that swim bladders of physostome and physoclyst fish rupture quickly in these low pressure environments. Because the pressures are so low, this condition may occur without pre-existing DGS. With pre-existing DGS the situation would be even more severe.

3.2 Industrial or Power Generation Cooling Water Effluents

When water is heated, the solubility of dissolved gases decreases (Figure 1). For water initially at
0 °C, an increase in delta P of about 20 mm Hg occurs for each degree rise in temperature (Colt 1984). At an initial temperature of 15 °C, the rise is approximately 15 mm Hg per degree increase in temperature. Water is used extensively as a coolant to reduce temperatures in industrial heat generating processes and in nuclear and fossil fuel power generating equipment. In the cooling of these facilities, the temperature of the coolant is increased substantially and the solubility of dissolved gases is decreased. The resulting DGS in the coolant water can be very high with delta P levels approaching 400 mm Hg (Miller 1974). As a result, elevated levels of DGS are often found in receiving waters into which waste cooling water is discharged (DeMont and Miller 1971, Adair and Hains 1974, Miller 1974, Marcello and Fairbanks 1976, Bridges and Anderson 1984). Unless the receiving waters are supersaturated with dissolved gases, the overall dissolved gas levels decline as a result of mixing between the two streams. However, the levels of DGS which are harmful to fish may not be reduced for many kilometres from the facility.

Waste cooling water discharges can pose a serious threat to aquatic and marine organisms independent of DGS (Becker 1973). It is well known that most aquatic and marine organisms have a maximum water temperature above which they cannot survive (Brett 1952, 1956, and 1976, Brett and Glass 1973, Brett et al. 1982). Thus, in addition to the effects of potentially high levels of DGS which could result from the discharge of waste cooling waters into rivers, lakes, or oceans, there is an added threat to survival caused by the elevated temperatures.

3.3 Solar Heating

For much of the year, streams and lakes are often subjected to high levels of solar radiation during daylight hours. As a result, surface waters can be heated rapidly with a corresponding rise in dissolved gas levels caused by reduced gas solubility (Colt 1986). If the water is clear and free of suspended particulate matter, the heating can penetrate ten metres or more into the water column (Harvey and Smith 1961). In some situations, the heating of surface waters may be enhanced by the presence of suspended particulate matter. Although the particulate matter reduces the penetration of solar radiation into the water column, it increases the solar absorbency of the water surface layers. Suspended particulate matter may also provide nucleation sites for bubble growth. If these nucleation sites are large, bubble growth could occur at delta P levels much lower than would occur in "clean" water.

The heating of the water column is also enhanced by shallow water depths and dark coloured stream or lake beds. Situations involving the heating of water by solar radiation are often accompanied by plankton and algae blooms (Section 3.5). These blooms contribute to high levels of DGS through the production of biogenic oxygen (Woodbury 1941, Renfro 1963, Stickney 1968, White et al. 1991). As with the discharge of heated cooling water, solar heating of lake and river water poses an added threat to most aquatic and marine organisms as a result of elevated temperatures.

3.4 Geothermal Heating

There is the potential for DGS in springs which are heated geothermally (Colt 1986). However, there have been no reported incidence of GBT in fish or other organisms inhabiting these springs. Bouck (1976) found that some alpine streams in Oregon had DGS with TGP % levels of 110%. Fish in the streams showed no signs of GBT, but fish in a trout hatchery which used these springs as a water supply died from GBT.

3.5 Nutrients, Primary Production, and Photosynthesis

Dissolved gas supersaturation can be produced in rivers and lakes which have high densities of plankton, aquatic plants, and algae. In these environments, dissolved oxygen levels can be significantly greater than would occur as a result of equilibration with the atmosphere (Woodbury 1941, White et al. 1991). Similar causes of DGS have been reported for coastal marine environments (Renfro 1963, Stickney 1968). During daylight hours, plankton, algae, and vascular plants produce oxygen through photosynthesis, a reaction described by:

CO2 + H2O = CH2O + O2

which is mediated by light energy.

During darkness, the reaction is reversed and oxygen is consumed from the water (Colt 1986). As a result, there can be large diurnal fluctuations in dissolved oxygen concentrations (Colt 1986, White et al. 1991). At night, in highly productive streams, dissolved oxygen can drop to levels which substantially reduce the amount of DGS (White et al. 1991). However, dissolved oxygen levels may also be critically hypoxic to many aquatic organisms (Macan 1974).

The discharge of chemicals and sewage into rivers, lakes, and the ocean by industrial, municipal, and agriculture operations can lead to nutrient enrichment of these environments. This in turn can contribute significantly to primary production and the development of DGS as a result of biogenic oxygen production.

In mariculture operations, fish and crustacean larvae are often raised in a "green water culture" of phytoplankton. In these environments, conditions of high light intensity and high algae density have produced GBT in mullet (Mugil cephalus) (Kraul 1983).

3.6 Ground Water

Many wells and springs have been identified as having DGS (Bouck 1984, MacKinlay 1984, Miller et al. 1987). However, the delta P of these water sources can be highly variable, depending on the conditions of the recharge area and temperature changes during recharge (Colt 1986). In the recharge process, bacteria can remove substantial amounts of dissolved oxygen and add carbon dioxide as water passes through soil and the unsaturated zone. However, gas may be added by the entrapment of bubbles in the capillary fringe of the boundary between the saturated and unsaturated zones (Herzberg and Mazor 1979, Heaton and Vogel 1981). The effectiveness of this mechanism would depend on the local subsurface conditions in the recharge area as well as the rate of recharge. In deep wells and springs, or in wells and springs close to geothermal zones, thermal heating may also result in significant temperature rises with corresponding increases in delta P.

Marsh (1910) reported a well water source as having dissolved nitrogen at 140 to 180% of air saturation. Well water supplying a hatchery at Leavenworth, Washington showed dissolved nitrogen levels at 144% of saturation (Rucker and Tuttle 1948). Matsue et al. (1953) found dissolved nitrogen in 15 artesian wells and two springs ranged from 118 to 159% of air saturation. In Canada, the federal Department of Fisheries and Oceans (MacKinlay 1984, Miller et al. 1987) listed elevated levels of DGS in several wells which are used as hatchery water supplies in the Salmonid Enhancement Program.

3.7 Air Lift Aeration and Gas Injection Systems

Injection air lift systems are often used to increase the dissolved oxygen content of lakes as well as hatchery water supplies (Fast et al. 1975, Colt and Westers 1982, McQueen and Lean 1983, Parker et al. 1984). In some hatcheries, pure oxygen is employed in an injector system to increase the carrying capacity of the hatchery (Edsall and Smith 1991). All of these systems involve the introduction of air or oxygen into the water column. This can take place at depths where elevated hydrostatic pressures can quickly force gas, in the form of bubbles, into solution with the potential for producing DGS. In shallow water environments, air can be injected (as very tiny bubbles) which allows surface tension to force gas into solution. In many ways, the mechanisms involved in producing DGS are the same as those encountered in dams and water falls.

3.8 Water Falls

Naturally occurring DGS may be caused by water falls along streams and rivers. DGS would be a characteristic of rivers downstream of water falls with deep plunge pools at their bases (Clark 1977, Alderdice and Jensen 1985a, Rowland and Jensen 1988). As with dam spillways, air is entrained in falling water and is driven to depth in the plunge pools where it is forced into solution by hydrostatic pressure.

3.9 Pumping Systems

Air entrainment into pressurized water systems is a common mechanism for the production of DGS in hatcheries (Marsh and Gorham 1905, Marsh 1910, Dannevig and Dannevig 1950, Westgard 1964, Serfling et al. 1974). The entrainment usually occurs on the suction or low pressure side of pumps. Often the problem is mechanical, involving leaks in the system (Marsh and Gorham 1905, Marsh 1910, Dannevig and Dannevig 1950). In other situations the problem may be seasonal and occur only when water levels are low at the inlet to the system or when the inlet system is poorly designed (Westgard 1964, Harvey 1967, Serfling et al. 1974). The over-pumping of wells has also been identified as a source of air entrainment and DGS (Serfling et al. 1974)

3.10 Ice Formation

In shallow lakes having a significant proportion of ice volume to total lake volume, solute freeze-out can produce lethal levels of DGS (Mathias and Barica 1985, Craig et al. 1992). During freezing, dissolved gases diffuse from the ice phase to the liquid phase. This raises dissolved gas levels in the liquid phase. During the winter, dissolved oxygen is gradually removed from the liquid phase through consumption by aquatic organisms. However, in the spring, photosynthesis may return oxygen levels to normal levels or even to supersaturated conditions. This increase in dissolved oxygen concentration, combined with the already supersaturated nitrogen resulting from the earlier solute freeze-out, can produce delta P levels as high as 560 mm Hg (Mathias and Barica 1985).

3.11 Barometric Pressure Changes

The delta P of a body of water is defined as the difference between the total gas pressure and the local barometric pressure. Thus, changes in barometric pressure would cause a change in delta P. If there is a sudden decrease in barometric pressure (usually resulting from storm activity), a body of water may become supersaturated with dissolved gases. Typical changes in barometric pressure resulting from storm activity are on the order of +5 to -17 mm Hg (Craig and Weiss 1971). Thus, a 17 mm Hg decrease in barometric pressure causes a 17 mm Hg increase in delta P. Normally this increase would not cause signs of GBT in fish. However if there is pre-existing DGS and a condition of incipient GBT present, the change in delta P may be sufficient to activate signs which would not otherwise appear.

3.12 Aircraft Transport

The transport of fish and other marine or aquatic organisms by aircraft can produce GBT as a result of reduced barometric pressures (Hauck 1986). Table 1 lists the US Standard Atmosphere (1976) pressures for various altitudes. Also shown are the delta P levels which would occur as a result of moving water which is equilibrated at sea level to various altitudes without re-equilibration during the transport process. For example, moving water quickly from sea level to 2000 metres results in a delta P change of 167 mm Hg.

The stocking of lakes and streams with fish is often done with helicopters (Hauck 1986). Because of the potential for rapid changes in altitude by these aircraft the effects on fish can be dramatic and occur more quickly than in situations involving DGS in rivers or lakes.

In the case of the swim bladder, the rapid change in altitude actually involves two separate but related processes. First there is the effect of decompression which leads to an immediate response in terms of swim bladder over-inflation and potential rupture. This is followed by the movement of the newly supersaturated dissolved gases from the blood and tissues into the swim bladder, causing additional over-inflation. If fish are in water which is supersaturated with dissolved gases prior to the altitude change, the swim bladder may already be over-inflated. Clearly, the effect of the change in altitude combined with the subsequent movement of gases into the swim bladder could greatly compound the potential for GBT in fish.

Table 1: Variation of Barometric Pressure with Altitude and Changes in Delta P

In addition to the rapid response of the swim bladder, the growth of intra-corporeal and extra-corporeal bubbles would also be enhanced. This is because supersaturated dissolved gases are already present at nucleation sites and there would not be the normal time lapse involving the transport of gases to the site. Again, pre-existing DGS would only intensify the problem.

3.13 Ocean Waves

The entrainment of air in breaking waves and the subsequent cycling of the air to depth can increase dissolved gas concentrations in ocean surface waters (Craig and Weiss 1971, Bieri 1974, Wallace and Wirick 1992). Kanwisher (1963) reported that in large bays, bubbles can be transported to a depth of two to three times the prevailing wave height, thus enhancing gas transfer into the water by way of hydrostatic pressure (Atkinson 1973).

Stickney (1968) reported an occurrence of GBT in fish held in the Booth Bay Biological Laboratory, Maine. It was discovered that Booth Bay itself was supersaturated with oxygen at 130% and nitrogen at 120% of air saturation. The author postulated that the cause of the supersaturation was wave action against a precipitous shoreline, with bubbles of entrained air transported to greater depths where they were forced into solution. Ramsey (1962a) showed that ocean water may be supersaturated with oxygen levels as high as 170% of air saturation. Some of this may be attributed to solar heating and/or photosynthesis; however, Ramsey (1962b) theorized that a large component may also come from bubbles taken to depth by wave action.

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