CPC

Total Gas Pressure Brilliant Dam

Since the workshop on Ecosystem-Based Management, co-sponsored by Columbia Basin Trust (CBT) and other parties, was conducted at Castlegar in April of 1998, much interest has developed in water quality problems associated with Columbia River hydroelectric projects.

One problem identified by Dr. Allan Scholz of Eastern Washington University, in his presentation at this workshop, were incidences of gas bubble trauma observed in many different fish species in Lake Roosevelt in 1997.

As a consequence of the meeting, a group of concerned scientists, regulators, planners, tribal representatives, and industry representatives, formed an organization called the Transboundary Gas Group. Early analysis suggests that addressing the problem of dissolved gas supersaturation on the total Columbia system (Canada and U.S.), through either physical or operational changes, could cost hundreds of millions of dollars. The goal of this group is, on a systemwide basis, to reduce total dissolved gas in the most cost-effective manner for all aquatic life.

Gas super-saturation is a water quality condition that develops when water is mixed with air bubbles and then plunges into deep water below a spillway and the entrapped water goes into solution. As the water leaves a plunge pool below the spillways and begins flowing in shallower river water downstream from the dam, the supersaturated water becomes hazardous to fish.

A condition called 'gas bubble trauma' occurs when the gas becomes supersaturated in the bodily fluids of fish. When it comes out of solution, traumatic damage from the bubbles occurs in fish tissues, similar to the condition known as 'the bends' with human scuba divers. As with 'the bends' in humans, fish that remain at sufficient depth do not experience gas bubble trauma. Some of the easily observable symptoms in fish are 'pop eyes' and bubbles forming under the surface of the skin.

The weight of the water, known as 'hydrostatic pressure', allows gas to remain in solution in fish tissues and their blood stream when they are residing in relatively deep water. This characteristic allows supersaturated water to be transported downstream in deep rivers and lakes with little reduction in super-saturation as air bubbles releasing dissolved gas to the atmosphere are formed only near the surface. In shallow water, gas tends to come out of solution and form bubbles in fish tissue and in the blood stream. Fish that live in shallow water near the surface or need to be near the surface at certain times are in the greatest risk. For example, salmon tend to migrate near the surface and consequently are considered more at risk than resident fish that spend much of their time near the bottom of a river or reservoir.

Fish can usually tolerate supersaturated water of less than 110 per cent near the surface of the water. At one metre depth, most fish can tolerate 120 per cent with tolerance increasing about 10 per cent for each additional metre of depth. For example, fish experiencing supersaturated water of 140 per cent, require water of at least 3 metres in depth to ensure a safe refuge from developing gas bubble trauma. The British Columbia Water Quality Guidelines and U.S. Water Quality Standards are set at approximately 110 per cent of saturation.

Gas super-saturation is determined by measuring the Total Gas Pressure (TGP) of the gas dissolved in water and comparing it with the barometric pressure. For example, super-saturation of 110% will result when the measured TGP is 1.1 times higher than the measured barometric pressure.

Dissolved gas supersaturation is produced by spills at Brilliant Dam and persists downstream, with only minor dissipation, throughout the main stem Columbia River and into Lake Roosevelt. The limited dissipation is caused by hydrostatic pressure of the deep average water depth of the Kootenay and Columbia Rivers downstream of Brilliant. Downstream gas supersaturation levels result from a combination of gas supersaturated water that originates from upstream sources and passes through the existing Brilliant Generating Station relatively unchanged, combined with supersaturated water formed by Brilliant Dam spillways. As volumes of spill at Brilliant increases, downstream supersaturation levels also tend to increase, since now the Kootenay River below Brilliant Dam contains a higher proportion of gas supersaturated water.

The Brilliant Expansion Project will harness more of the dam's unavoidable spill and divert most of the gas supersaturation producing spills to the generating unit for electrical energy production. Increased powerhouse capacity at Brilliant will result in reduced gas supersaturation levels in both British Columbia and Washington State.