Horowhenua (Waiopehu) Water Quality Interventions

​This page covers work by Horizons Regional Council and partners to improve water quality and aquatic health for the Horowhenua-Waiopehu Freshwater Management Unit (FMU).

Community meetings

Community meetings were held in November 2022 to discuss the conceptual designs for the constructed wetland complex near Lake Horowhenua. If you were unable to attend a meeting but would still like to have your say, you can do so by emailing us at horowhenua.wetlandcomplex@horizons.govt.nz or calling us on freephone 0508 800 800 by Sunday 4 December 2022.
 
The presentations from these meetings are also available below.
Community engagement presentation 6 November 2022 Waitārere
Community engagement presentation 15 November 2022 Taitoko Levin
Community engagement presentation 17 November 2022 Hōkio
Community engagement presentation 20 November 2022 Waikawa
Community engagement presentation 21 November 2022 Landowner meeting

Current State of the Horowhenua (Waiopehu) FMU

The Horowhenua (Waiopehu) FMU includes the Lake Horowhenua, Wairarawa (Waitārere), Waiwiri, Ōhau and Waikawa catchments in the south of the Horizons Region. Waterways in this FMU face pressure from intensive land use activities such as agriculture and urban development, wastewater and stormwater discharges, water abstraction for human use, and invasive freshwater weeds.
 
Lakes and estuaries are particularly sensitive to pollution. Lake Horowhenua (Punahau) is a shallow coastal lake in the FMU that receives water from several streams, groundwater and drains that run through and under the Levin township and agricultural land.
 
In pre-European times, Lake Horowhenua was a clean water supply, local taonga and valued fishery for the Muaūpoko iwi who lived in the coastal forest that surrounded the lake. However, the clearance of coastal forest, draining of swamps, intensification of land use, urban expansion, and the disposal of treated effluent in the lake between 1962 and 1987, led to significant degradation.
 
Lake Horowhenua continues to receive large amounts of nutrients and sediment in addition to the ongoing internal processes influenced by past discharges into the lake. Water quality is poor as a result and the lake is regularly closed for recreation in the summer due to the potential presence of toxic cyanobacteria.
 
The lower reaches and estuaries of the Ōhau and Waikawa streams are also vulnerable and suffer from eutrophication as a result of upstream land use activities.

What we're doing to help

The current interventions work around Lake Horowhenua was boosted by the Mahi mō te Taiao – Jobs for Nature Horowhenua (Waiopehu) Freshwater Management Unit Water Quality Interventions project (2021-2024). This support comes from Central Government funding and is complemented by Horizons Regional Council (a total of $12.5 million; $11.2 million from Central Government and $1.3 million from Horizons).
 
One of the key objectives for this project is the construction of a wetland complex in the Arawhata sub-catchment of Lake Horowhenua. The project also aims to further characterise the surface and groundwater of the FMU to improve understanding of the flow paths to Lake Horowhenua, connections between catchments within in the FMU and the state of the Ōhau and Waikawa estuaries.
 
Another objective of the project has seen a Governance Group established which includes Horizons Regional Council, Horowhenua District Council, Muaūpoko Tribal Authority, Te Rūnanga o Raukawa, Lake Horowhenua Trust, and Ministry for the Environment representation.
 
The wetland complex will be constructed to improve the mauri and water quality of Lake Horowhenua with Te Mana o te Wai at the forefront of the project. It will also consider economic, social and recreational opportunities in future development. Horizons has purchased a 142ha dairy farm in the Arawhata for this purpose.
 
A Wetland Design Group and a Mātauranga Rōpu (Kāhui Ārahi) have been established to develop the wetland design and a Community Stakeholder Group has also been formed with input from this group to be considered in the process.
 
This work leads on from the Lake Horowhenua Accord established in 2013, where five parties representing Muaūpoko owners, community interests and statutory bodies agreed to work together to provide leadership and halt degradation of Lake Horowhenua and Hōkio Stream. Past and ongoing initiatives, and funding, include investments in sediment traps, lake weed harvesting (see the video below), fish passage around the Lake Horowhenua weir, riparian fencing and planting, working with horticultural growers in the Horowhenua District to maximise nutrient use and reduce leaching to waterways, and developing sediment and erosion control plans for horticultural growers.

Horowhenua constructed wetland complex conceptual design

The initial conceptual design for the Horowhenua constructed wetland complex has been completed and the project team is now seeking community feedback.

The initial wetland complex construction will help to reduce the concentrations of nitrogen, sediment and phosphorus within the water and therefore the amount of contaminants that enter Lake Horowhenua from the Arawhata catchment. This combined with the other interventions both within the catchment and the lake itself will improve the water quality within Lake Horowhenua.

The conceptual design includes a master plan for the constructed wetland complex which is then split into three phases.

The first phase aims to convert at least 15 hectares of farmland into a constructed wetland complex by December 2024.

As further funding becomes available the following two phases will be considered. These phases will extend the constructed wetland complex  

The constructed wetland complex will incorporate plants historically found in this area prior to its clearance, species such as Harakeke (flax), carex, and rushes.
Horowhenua constructed wetland complex conceptual design

Environmental data

Horizons measures a variety of water quality parameters across the FMU. This data is provided to the public via the Land, Air, Water Aotearoa (LAWA) website. Click on a link below to explore the data for that catchment.

Lake Horowhenua

Lake Horowhenua is a shallow coastal dune lake and the largest natural lake in the Manawatū-Whanganui Region. In 2013, Horizons Regional Council installed a continuous water quality monitoring buoy in the lake, and since then water samples have also been collected by boat or helicopter, usually on a monthly basis.

Lake Horowhenua Catchment Streams

Lake Horowhenua has a single outflow to the sea, the Hōkio Stream, and receives water from several waterways, including the Arawhata, Pātiki and Mangaroa Streams.

Ōhau

The Ōhau River flows from the confluence of two short rivers, the North Ōhau River and the South Ōhau River. The Ōhau initially flows north, turning west to the southeast of Levin. It reaches the Tasman Sea 10 kilometres to the southwest of Levin.

Waikawa

The Waikawa Stream meets the sea at Waikawa Beach in Horowhenua. It is a popular spot for recreation. There is weekly summer season monitoring at some spots.

Reports and related documents

Ōhau and Waikawa Estuaries – Nutrient Limit Setting
The Waiopehu FMU Water Quality Model
Phosphorous and Nitrogen Budgets for Lake Horowhenua
Lake Horowhenua Accord (PDF - 2MB)
Lake Horowhenua Accord Action Plan (PDF - 2MB)
Restoration Plan for Lake Horowhenua (PDF - 3MB)
Lake Horowhenua Catchment Report Card (PDF - 4MB)
Arawhata Wetland - Initial 2021 Conceptual Design Report (PDF - 5MB)
Assessment of Opportunities to Address Water Quality Issues in Lake Horowhenua (PDF - 2.5MB)
Assessment of Fish Populations in Lake Horowhenua (PDF - 4MB)
Informing a Weed Harvesting Strategy at Lake Horowhenua (PDF - 4MB)
Integrated Sediment, Nutrient and Drainage Management Plan for the Arawhata Catchment (PDF - 12MB)
Kākahi Survey of Lake Horowhenua (PDF - 3MB)
Lake Horowhenua Groundwater Model (PDF - 5MB)
Lake Horowhenua Pest Fish Monitoring 2018 (PDF - 2MB)
Lake Horowhenua Sediment Legacy (PDF - 3MB)
Lake Horowhenua Water Balance Assessment (PDF - 6MB)
Life-History of Lake Horowhenua Common Smelt (PDF - 1MB)
Ōhau Estuary Synoptic Intertidal and Subtidal Survey 2017-18 (PDF - 3MB)
Synoptic Subtidal Monitoring of Ōhau Estuary - 2020 (PDF - 5MB)
Synoptic Subtidal Monitoring of Ōhau Estuary - 2021 (PDF - 6MB)
Waikawa Estuary Synoptic Intertidal and Subtidal Survey 2017-18 (PDF - 3MB)
Synoptic Subtidal Monitoring of Waikawa Estuary - 2019 (PDF - 5MB)
Synoptic Subtidal Monitoring of Waikawa Estuary - 2020 (PDF - 4MB)
Synoptic Subtidal Monitoring of Waikawa Estuary - 2021 (PDF - 6MB)