50 | Horizons Regional Council COMMUNITY T T Kim Newth A big step towards a cleaner lake Reducing concentrations of nitrogen, sediment and phosphorus in catchment flows is a key goal. Polluted with sewage from Levin for decades and impacted by intensive land use practices, Punahau Lake Horowhenua has been in a very poor condition for a long time. That picture is now starting to change for the better with ongoing interventions seeking to restore Manawatū-Whanganui’s largest natural lake. Reducing concentrations of nitrogen, sediment and phosphorus in catchment flows is a key goal. The Arawhata Constructed Wetland Complex project, currently in its first phase, aims to improve water quality by converting farmland near the lake into a constructed wetland area. The first phase is on track for completion by the end of 2026, backed with funding support from Central Government ($11.2 million, Jobs for Nature Horowhenua) and Horizons Regional Council ($1.3 million). “This is only part of the puzzle [around lifting water quality in the lake] but this is a big piece of it and there’s been significant investment by ratepayers and taxpayers to make it happen,” says Logan Brown, Freshwater and Projects Manager, Horizons Regional Council. “We’re really proud to be delivering it and excited to be working with the Lake Horowhenua Trust and the Muaūpoko Tribal Authority. It’s a partnership approach that also includes Central Government; we would not have been able to do this work without their funding support.” The wetland site, west of Levin between Punahau and Lake Waiwiri, previously operated as a 142-hectare dairy farm. Following the granting of consent via the fast-track process, stage one got underway this year. It involves converting 15 hectares of farmland into wetland by constructing sediment basins, drainage modifications and planting species such as carex and rushes. Two further stages to complete the consented 75 hectare wetland area are planned, subject to further funding. While the council has previously been involved in other wetland projects, this is its first at this scale. Wetland experts from around Aotearoa New Zealand and beyond were consulted as part of project planning. “The site where this is happening used to be a wetland historically. It’s a perfect spot in the catchment. The land is naturally wet – so much so that we had to pause work in early June as it was too wet for planting. Construction is just starting again now that it’s drier.” The first large-scale sediment retention pond is due to come online shortly and is expected to be fully operational by Christmas. Water flowing through the site is being diverted into the pond with the goal being to reduce sediment loads. A constructed sediment wetland area is also being developed to help remove nitrogen from water. Two restoration wetland areas are due to be planted in coming weeks. During a previous constructed sediment trap project, rushes naturally regenerated very quickly and Logan says that will likely happen here too. “A really interesting aspect of a constructed wetland like this is there will be times when we will have to go in and do maintenance and effectively give the plants a haircut!” Meanwhile, other work to clean up the lake includes annual weed harvesting. Landowners in the catchment are proactively improving their properties and practices. As well, water quality is being regularly monitored in the lake and its tributaries. www.stringfellows.co.nz Proudly delivering civil works for Horizons Regional Council m (06) 354 4524 9 25 The Cutting Way, Palmerston North, Manawatu 4414, New Zealand ARCHAEOLOGY & HERITAGE MANAGEMENT SERVICES Proudly Supporting Horizons Regional Council Phone: 027 421 0803 Email: info@insitearchaeology.com
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