Council has invested $4 million on aerating the Maitai Dam to increase the security of Nelson’s safe drinking water and to improve the ecological health of the dam reservoir and river.
The project, to be commissioned next month, has involved installing 75kW air compressors with associated infrastructure. A 90mm-diameter sparge pipe has been laid along the base of the reservoir. The new compressors will pump up to 240 litres of air per second through discharge nozzles in the pipe to create bubble plumes that will increase the mixing and oxygen saturation of the stored water. Council has partnered with Nelson’s Cawthron Institute to monitor the water quality over the next two years and help us determine the most efficient air flows to achieve the targeted dissolved oxygen levels.
Stratification of the dam water that occurs over summer is the reason for this aeration project. The upper layers of the reservoir warm in late spring and, with warm water being lighter than cold, form a separate layer. The lower reaches are deprived of oxygen and have increased concentrations of natural contaminants from sediment at the bottom of the reservoir. This makes the lower layers of water more difficult and expensive to treat for the city’s supply and less healthy ecologically for release into the Maitai River. Dissolved oxygen in rivers and lakes is essential for fish and other aquatic organisms.
The stratification problem solves itself each winter when the upper layers cool and naturally mix with the lower layers. However, it is in summer when we most need to use water from the dam. The aeration project means we will have access in summer to more better-quality water for the city water supply and for release into the river.
I love practical environment initiatives like this, based on good science and innovative engineering. Whether it be our all-electric bus fleet, the efficient cleanup of the contaminated wood waste at Tāhunanui Beach, our gas wells reducing greenhouse gases released from the York Valley Landfill or this Maitai aeration project, Council is doing a lot of good environment stuff.
