Rangitīkei District Council prepares to divert treated wastewater away from Lake Waipu

Rātana Pā's treated wastewater currently ends up in Lake Waipu. Photo / NZME

Pipes have been ordered to divert Rātana’s treated wastewater away from Lake Waipu near the pā but there is no start date for the $6.35 million project.

Earlier this year, Rangitīkei District Council purchased land for a storage pond near Whangaehu Beach, with a resource consent application now before Horizons Regional Council.

The 7ha lake is rated as supertrophic, meaning it has very poor water quality with high amounts of phosphorus and nitrogen.

Three public submissions were received on the application, with John Bent and the Water Protection Society opposed to it and the Rakautaua 9 Ahu Whenua Trust indicating conditional support.

A report from project manager Blair King last month said planners WSP should meet with the submitters to see if concerns could be addressed without new conditions being added to “what is already an expensive project for the district”.

Building the access track and pipeline was recommended this construction season (during the warmer months), as well as a design and application for building consent on the 30,000 cubic metre storage pond, it said.

“Contractors are advising there is a shortage of approved works so this should enable us to get sharper pricing than if we leave construction till after January when more work will be available in the market.”

A spokesperson said council representatives would meet with the Rātana community later this month to update them on progress and notify them of the next steps.

“We will keep the community updated as this project progresses,” they said.

In 2017, the Ministry for the Environment put $875,000 towards the project as part of its Freshwater Improvement Fund, with the council committing $950,000.

The budget was set at $2.4m in 2019 and the council increased it to $6.35m in September last year.

A resource consent application was lodged in December 2022 but Horizons requested further technical analysis, including soil sampling.

Minor upgrades to Rātana’s wastewater treatment plant and the design of an irrigation system for the Whangaehu Beach land are also part of the project.

A council report in August said pipes, air valves and fittings had been ordered but it was holding off delivery to ensure valves did not degrade on-site while the council was waiting for consent approval.

The Lake Waipu project first received funding in 2017.

Treated wastewater has been going into the lake since 1977.

According to Land Air Water Aotearoa (Lawa), Lake Waipu has a trophic level (TLi) of 6.3, down from 7 in 2019.

Lakes with good water quality have a TLi rating of between 2 and 3, with very good quality between 0 and 2.

Horizons will begin work on a restoration plan once the treated wastewater is removed.

The council’s freshwater and partnerships manager, Logan Brown, said the lake was far from a lost cause.

“Reducing nutrients going in is the big win to start with but we haven’t seen those bright green algal blooms in there, like you see in other lakes around the region,” he said.

“Lakes will cleanse over time if we stop inputs going in and there are other interventions you can do - capping being one of them.

“You add a chemical agent to the water that binds some of those nutrients and speeds [cleansing] up quite a bit.”

Brown said there were still good populations of kākahi (freshwater mussels) and tuna (freshwater eels) in the lake.

A 2023 report from independent science organisation Cawthron Institute said removing treated wastewater would reduce annual nutrient loads by an estimated 17% for nitrogen and 25% for phosphorus.

“However, given the presently very high loads of nutrients from the catchment, legacy sediment sources and abundant waterbird populations on the lake, a significant further reduction in nutrient loads will be required if it is to meet national and regional water quality standards,” it said.

“It is anticipated that significant further investment will be needed in the management of on-farm activities, lakebed sediments (legacy loads) and waterbirds to meet NPS-FM [National Policy Statement for Freshwater Management] limits and achieve a healthy ecological state.”

The institute’s suggestions to improve water quality included riparian management, modification of existing farm practices and fertiliser usage and diverting inflows to sediment traps or purpose-built wetlands.

Rangitīkei Mayor Andy Watson said Regional Development Minister Shane Jones had initiated a meeting between himself and Māori Development Minister Tama Potaka to discuss possible financial assistance for general infrastructure projects in Rātana.

“I would welcome those discussions,” Watson said.

Rātana Community Board chairman Charlie Mete could not be reached for comment.

Mike Tweed is a multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily the Whanganui District Council.