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The Arawhata wetland complex near Levin is set to be one of the largest in the country.
Earlier this month, councillors from Horizons Regional Council received an update on the Arawhata wetlands, a programme designed to improve water quality in the Punahau (Lake Horowhenua) catchment.
The project involves creating a constructed wetland complex on a former dairy farm next to the lake.
It will be designed to filter phosphorus bound to sediment and reduce the concentrations of nitrogen through de-nitrification in ground and surface water before it reaches the lake.
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The wetland complex is expected to be one of the largest constructed wetlands in the country, creating improved habitats for native bird and fish species, along with better environmental outcomes through nutrient reduction in the Arawhata Stream, and in turn Punahau.
A governance group has been established to oversee the project, with representation from the Lake Horowhenua Trust, Muaūpoko Tribal Authority, Te Rūnanga o Raukawa, Horowhenua District Council, Ministry for the Environment and Horizons Regional Council.
The group was aware of the concerns of neighbouring property owners around existing drainage.
Co-chairperson Rachel Keedwell said these issues have been at the forefront of the planning and would continue to be, as the project moved into the consent design phase.
“The current conceptual design contains a number of measures to manage groundwater effects and surface water flows within the wetland complex property. These have been and will continue to be further refined as we receive feedback from surrounding landowners and members of the community.
“In addition, the project team have engaged with groundwater experts to liaise with neighbouring farmers and landowners to identify any risks and source remediation measures if necessary.”
Horizons freshwater and partnerships manager Logan Brown said while surface water levels had been high over the past 18 months, this was due to rain and runoff, high lake levels in Punahau, and highly elevated groundwater levels pushing water up to the surface.
“Groundwater levels in the catchment are ultimately controlled by the lake level, as there is only so much water that can leave the lake via the Hōkio Stream. As the lake level rises due to high rainfall, so too do the surrounding groundwater levels.
“Furthermore, the Ōhau River feeds the groundwater into the lake and therefore the capture zone for water to the lake is massive.
“So overall, when we have higher levels of rainfall – as has been the case over the 18 months – the only option is for the groundwater levels to rise.”
The good news is groundwater can be managed within the wetland complex property once the wetland is constructed.
“The wetland will raise shallow groundwater levels within the property. However, drains around the outside of the complex will keep the water contained,” Brown said.
“The design also has the potential to reduce existing drainage issues outside the property – one thing is for sure, it certainly won’t make it any worse. Horizons have also not undertaken any construction to date which could have contributed to the groundwater levels experienced in the past year.”
Keedwell said in addition to water quality improvements, the project sought to enhance the cultural and ecological values of the lake and its surrounds.
“It’s important to note that we are doing our utmost to ensure positive solutions for the ecology in the area, landowners and our wider community.”
Dean Wilson of Lake Horowhenua Trust wanted to see the lake’s mana restored.
“This taonga holds pride of place for iwi and the Horowhenua community. Muaūpoko Tribal Authority are onsite assisting with monitoring and some of the current works required to advance and improve the Arawhata wetland project overall.
“Muaūpoko are providing the mātauranga for the Arawhata project which is based on their ancestral lands and support the wetland design, monitoring and maintenance planning required. They also have a representative on the project team.”
Horizons have submitted an application to be able to use the fast-track consent process, and if successful, will submit an application later this year.
In the meantime, the lease for the land purchased for the wetland has recently been through a tender process and awarded to a cut and carry and dry stock operation.
“Even in the short term, changing the land use from dairy to dry stock reduces the level of nutrients entering the catchment,” Keedwell said. “This is a positive step to achieving our long-term goals.”
Further information on the project is available at horizons.govt.nz/managing-natural-resources/horowhenua-water-quality-interventions.
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