Cold water pollution is a serious environmental issue affecting the river systems downstream of large storage dams. Dams are often fitted with offtakes that withdraw water from deep parts of the reservoir. In summer, increased temperatures warm the water in rivers and at the surface of dams.
This warming does not extend down to the deep waters at the bottom of the dam, resulting in cold water being released into the downstream river environment. This cold water significantly impacts the migration, breeding and growth of fish for many tens of kilometres downstream from a dam.
WRL was engaged by the NSW Department of Primary Industries, Fisheries, to complete an extensive literature review and investigation into options to mitigate cold water pollution. An obvious option is to release water from the surface of the dam, however this can require additional, expensive infrastructure. As well as this, reservoir surface waters can often experience algae blooms in summer, which can be a concern for both human and aquatic health if released downstream.
WRL investigated a variety of systems that could artificially mix the dam water, resulting in a constant temperature from the surface to the bed. Our study concluded that bubble plumes (created through pumping air to the bottom of the dam) are a feasible option for this strategy, if coupled with renewable energy.
Bubble plumes also have the potential to alleviate other water quality issues commonly observed in dams, such as dissolved oxygen depletion, release of nutrients and soluble metals from sediments and potentially limit toxic algae growth. WRL is continuing our research into successfully implementing bubble plume mixers in very large, deep dams.