Aeration project for Lake 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 has 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, contributing to the release of nitrogen and phosphorous. These are the 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) occur during summer and autumn,” Bay of Plenty Regional Council Lake Operations manager Andy Bruere said.

The technique has been used successfully overseas, and could 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.

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.

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.