Consent sought for wall to protect Rotorua lake

A $12 million wall to stop polluted water from reaching Lake Rotoiti will be one of the most significant environmental remedies ever undertaken in New Zealand, says project co-ordinator Paul Dell.

Environment Bay of Plenty has lodged resource consents for the major engineering work which will divert nutrient-laden Lake Rotorua water from flowing into Rotoiti, sending it down the Kaituna River and out to sea.

Independent commissioners will be appointed to hear submissions to the consent application.

The 1250m long wall will stand on the lake floor about 75m offshore and rise to just above water level. Running parallel to State Highway 33 it will be 1250m long, starting below the Ohau Channel outlet which links the two lakes and extending to Te Akau Point. The main length of wall will be of solid sheet-pile material that should last for at least 50 years.

It is expected to reduce the seasonal blue-green algae levels in Lake Rotoiti over three to five years. This is one of the actions being taken to improve water quality in the Rotorua lakes - a clean-up estimated to cost $170 million over 20 years.

The Rotorua District Council, Environment BoP and Te Arawa Maori Trust Board have called for Government assistance and will put their case to Environment Minister Marian Hobbs when she visits Rotorua this month.

Last year the Government offered up to $4 million towards urgent remedial work for Lake Rotoiti.

The regional council's environmental investigations manager John McIntosh said the nutrients flowing from Lake Rotorua through the Ohau Channel were a major cause of Rotoiti's problems. "Over a year, about 40 per cent of the channel flow goes directly down the Okere Arm and into the Kaituna River," he said yesterday.

"The rest moves around the main body of Lake Rotoiti before exiting and going down the river. The wall is designed to block this flow path so all of the water goes directly to the ocean." The diversion wall would be supported by sewerage reticulation in some lakeside communities, upgrades to septic tank systems and restoration around lake and stream margins.

Mr McIntosh said Lake Rotorua's problems were more complex because the catchment's groundwater was laden with nutrients that would seep into the lake for decades to come.

A lot of work was being done to improve water quality there, including a multimillion-dollar sewerage reticulation programme and sewage treatment upgrade.