Source: Radio New Zealand
A digger removes peat from the wetland area. RNZ / Jimmy Ellingham
One of New Zealand’s most polluted waterways could be on the way to losing that title thanks to a milestone reached in a years-long clean-up.
The first phase of a new wetland to help turnaround Punahau Lake Horowhenua’s water quality is just weeks away from completion.
For years Levin’s sewage was pumped into the lake. That stopped decades ago, but the effects from that are still felt, as are the effects of excess nitrogen flowing into the lake, degrading its water.
Shovels hit the ground
Work at the Arawhata Constructed Wetland Complex near Lake Horowhenua began in mid March, but it’s been a long wait.
It took more than five years for construction to begin once central government funding of $11.2 million for the initial phase of the work was assigned as part of the Jobs for Nature Covid response.
Ratepayers are contributing $1.3m.
For years Levin’s sewage was pumped into Punahau Lake Horowhenua which has degraded its water quality, despite the fact it has not now happened for decades. Wikimedia Commons
Getting there hasn’t been smooth sailing. There was a High Court challenge to the resource consents granted to Horizons Regional Council for the work, but that was dropped last year.
Council fresh water and projects manager Logan Brown said the initial phase had to finish by the end of June, when the Jobs for Nature money ran out.
“The main reasons behind the wetland are to help improve Lake Horowhenua – the water quality and aquatic health,” Brown said.
“For a constructed wetland, there’s heaps of different drivers. Some main ones are some reductions in the amount of sediment that makes its way through to the lake. There’s some big sediment wetlands and sediment traps.
“The other one is a reduction in nitrogen that’s making its way down to the lake.”
Logan Brown, from Horizons Regional Council, says the constructed wetland will help improve water quality at Lake Horowhenua. RNZ / Jimmy Ellingham
This comes from underground waterways that flow towards the lake, including through the Arawhata Stream, and surrounding farmland.
Horizons bought a farm at the lake’s southern end for the constructed wetland – about 70 hectares of which will eventually be developed to act as a filter for the lake in phases 1 and 2 of the development.
“It’s the perfect spot for a wetland. It used to be a wetland until it was drained to put into production.
“We’re putting it back into a wetland, although slightly different to what it was,” Brown said.
The original wetland was drained about 60 years ago.
One of the sediment reduction ponds is under construction. RNZ / Jimmy Ellingham
Plants to get regular haircuts
When RNZ visited the site on Tuesday there was plenty of heavy machinery being operated by the project contractor.
It was moving tonnes of earth for the ponds that catch that sediment, and creating an area for nitrogen-sucking plants.
“There’s multiple phases within the project. Phases 1 and 2 are fully consented through the fast track legislation process we followed for that.
“This phase 1.1 – we’ve broken it into multiple ones, which are all funding dependent – the major focus is reduction on sediment flowing into the lake and setting up some of the wetlands for the future phases, so they can start growing,” Brown said.
In coming years the sediment-reducing ponds will require regular maintenance to clear them. The plants will need a trim too.
“It sounds a little weird, but some of the wetland plants will need a haircut every now and then to keep them functional and doing that nitrogen removal.
“It is very much a constructed wetland so there’s elements of it that will need to be cleaned out occasionally.”
Wetland under way
Project manager Aydin Maxfield said on Tuesday top soil was getting stripped from one of the restoration wetlands.
“They’re pulling up peat and they’re lowering the contours. That will eventually receive the treated water that’s had most of its sediment removed, but to get water to travel through there at a slow enough pace it needs to be quite long and a lot of material needs to come out of it.”
Project manager Aydin Maxfield is hoping the weather holds over the next few weeks during construction. RNZ / Jimmy Ellingham
There are plants ready to go at a nursery, but when they’ll be in the ground depends on how much rain falls in the coming weeks. Planting them into water wouldn’t work.
Logan Brown said all this work helped the long process of restoring Lake Horowhenua back to health.
“It’s a part of the jigsaw, part of the puzzle. We’ll monitor… to see those sediment reductions that these interventions make, but we’re not building all of phases 1 and 2.
“We’re constrained by budget and we’ve done the interventions that will have the greatest benefit for the least cost.”
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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand