Source: Radio New Zealand
- Seven weeks of weeding work at Lake Horowhenua has finished
- More than 400 tonnes of invasive weeds collected
- No easy fix for lake that for decades had sewage pumped into it.
Raw sewage was pumped into Punahau Lake Horowhenua for decades, earning it the dubious reputation as one of New Zealand’s most polluted waterways.
A years-long cleanup project is now working to restore it back to health, although it is not possible to yet put a timeframe on when the lake, west of Levin, will be safe to swim in again.
A special harvester has operated there for the past five summers, chopping out invasive weeds to give native species the chance to flourish, and replenish the oxygen-deprived water.
After seven weeks of weeding the lake, Tuesday is the harvester’s last day for the summer. RNZ / Jimmy Ellingham
Mowing the lake
It was a crisp but calm day when RNZ visited Lake Horowhenua and headed out on the water on the paddle steamer-like $300,000 harvester – one of two in New Zealand.
Skipper Julian Everth explained how it worked.
“You have one horizontal set of teeth, which cuts horizontally, and then two vertical ones. And as you’re going along the weed gets cut out in a chunk.
“That comes out on a conveyer belt and then lands on another conveyer belt by your feet. Once it’s full there you can shift the conveyer belt backwards more and load more on to the boat.”
Over the past seven weeks Everth and another skipper have operated the harvester for 12 hours a day – mowing the 390-hectare lake bed in a grid pattern. Tuesday was their last day this summer.
Harvester skipper Julian Everth says they’ve collected more than 400 tonnes of weed this summer. RNZ / Jimmy Ellingham
They will collect more than 400 tonnes of weed, and the odd creature too.
“We gets lots of little perch and carp. They’re both invasive species, so we’re not too worried about them.
“Occasionally we will get eels that will swim up on to the harvester. When that happens we stop the cutting process, pull backwards a little bit and allow them to swim off.”
While the lake was safe for aquatic life, it was not recommended for humans due to the bacteria it contained.
The harvested weed was scooped into a truck and taken to Feilding for composting, rather than rotting in the lake.
“Essentially, it uses up lots of nutrients in the lake to grow. Eventually it will die and collapse,” Everth said.
“You end up with a blanket of dead weed on the bottom of the lake. When that happens it rots and makes an anoxic environment. A lot of the fish and eels can’t survive in that.”
Lake still getting poisoned – guardian
Tangata tiaki Deanna Hanita-Paki said the lake was for a long time a receptacle for effluent runoff, pesticides and worse.
“Back in the 1950s and 60s the council started putting raw sewage straight into the lake. That stayed like that until about 1985.
“They’ve had years of polluting – raw sewage straight into the lake.”
Tangata tiaki Deanna Hanita-Paki says the water quality is improving. RNZ / Jimmy Ellingham
Lake trustees fought court battles to halt this, but their win took years and the effects of the pollution were devastating, Hanita-Paki said.
But the iwi Muaūpoko and the Horizons Regional Council were now putting much effort into nursing the lake back to life.
“It’s a lot better. With the harvester being there we can see the water’s getting better – the quality of that water was getting better and so was the weed changing, and our creatures were coming back.
“Our fish were coming back – same with the eels.”
But it was not perfect, as stormwater was still routed there.
“We’ve found that on those weather events the lake starts to smell and it smells different.
“I go out every month. We do water testing with Horizons. It started changing about October, November last year. The smell inside the lake was really bad.”
She said as far as she was concerned, the lake was still being poisoned due to toxins in the stormwater.
Weeding part of the plan
Horizons Regional Council fresh water and projects manager Logan Brown said lakes were complex.
“There is no silver bullet for restoration of Lake Horowhenua. There are lots of little projects that go together and they piece together.
“For all lakes across the country when we’re doing restoration you have to do both in-lake interventions and catchment interventions.”
Logan Brown, from Horizons Regional Council, says there’s no quick fix for the lake. RNZ / Jimmy Ellingham
A wetland project would also begin shortly, while the weed harvester was about to get a deep clean and then go into storage ahead of next summer.
“I like to compare it to a lawnmower. Effectively, we’re mowing the weed of the bottom of the lake,” Brown said.
“We’re not trying to get rid of all the weed. Like cutting your lawn, you leave the lawn there for stability.
“We want the aquatic plants to stay on the lake bed. That helps with stability. We get really high winds here and it just stops that stirring up on the lake bed.”
The lake is only as deep as 1.8 metres now due to silt build-up down the years.
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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand
