Aeration project planned for Rotoehu

A new intervention technique to prevent sediment releases of nitrogen and phosphorous from the beds of Rotorua's lakes will be tried in Lake Rotoehu in late January.

The Rotorua Te Arawa Lakes Strategy Group last week approved the de-stratification aeration project, which takes water from the deepest part of the lake and pumps it to the surface to eliminate pockets of low oxygen water, which contributes to the release of nitrogen and phosphorous. These are key nutrients which lead to poor water quality and algal blooms.

"This release [of nutrients] occurs at times when a lake has low oxygen levels in its bottom waters. For shallow lakes such as Rotorua and Rotoehu, short periods of lake stratification (separation between the surface and bottom waters due to temperature differential) occurs during summer and autumn," Bay of Plenty Regional Council Lake Operations Manager Andy Bruere said.

The technique has been used successfully overseas, and if successful here could be scaled up to be used in larger lakes such as Lake Rotorua to improve water quality. The prototype trial aims to refine the technique in a large shallow lake first to gather information on design and any ecological impacts before deciding if it is applicable to Lake Rotorua.

Mr Bruere said the Lakes Water Quality Technical Advisory Group had found that nutrient inputs from sediment could be a significant contribution to water quality, and the process of restoration could be accelerated by addressing both catchment nutrient inputs, as well as nutrient recycling from lake sediments.

The nutrient release could be dealt with by aeration, applying a capping agent such as alum or dredging the sediments. The Regional Council and University of Waikato had undertaken considerable research into capping agents and there had been a high level of success on small lakes or bays. Dredging had been used overseas but cost estimates for New Zealand had shown much higher costs and more complex disposal issues due to geothermal contamination with mercury and arsenic.

He said the community had been very supportive of the project, especially Māori and the Tautara Matawhaura Māori Trust Farm had provided on-going access close to the new floating wetland on the lake. A public open day had been held at Ōtautū Bay which included discussion on this project.

The $524,000 project will use a lightweight plastic draught tube system and compressed air lines which would run from the southern shore of the lake, and it is cost effective option compared to capping options, he said.

The project would be comprehensively monitored for water quality impact, ecological effects, pump performance and sediment monitoring.

Rotorua Te Arawa Lakes Strategy Group Chairman, Rotorua Mayor Kevin Winters said the project was a whole new initiative on the lakes.

"If it works we could scale it up even bigger for use on other lakes," he said.