New guidance on the performance and design of constructed farm wetlands draws on local and international evidence to show how well-planned wetlands can reduce nitrates by 20 to 50 percent.
NIWA, supported and co-funded by DairyNZ, has developed the new guidance and performance estimates, which show that wetlands are capable of significantly reducing nitrate and contaminant concentrations.
NIWA principal scientist – aquatic pollution, Dr Chris Tanner, says wetlands play a number of important environmental roles, and several factors need to be considered in order to construct a high performing wetland.
“Wetlands can reduce levels of nutrients and microbes such as E. coli, and filter sediment. This can significantly improve the water exiting the wetland, thereby improving water quality in downstream water bodies.”
Dr Tanner says a well-designed farm wetland which is appropriately sized for its catchment can remove 25 to 50 percent of nitrate in warm areas of New Zealand, and 20 to 40 percent of nitrate in cool areas.
Performance differs across regions because rates of microbial activity and wetland plant growth are higher in warm regions, which result in greater nitrogen removal.
DairyNZ environment manager Aslan Wright-Stow says through the Dairy Tomorrow strategy, the dairy sector has committed to improving water quality outcomes, and protecting wetlands is an important part of this.
“This new analysis provides farmers with greater certainty about how wetlands can improve water quality, alongside the biodiversity benefits they provide. This will support farmers who are considering whether to invest in developing wetlands as an on-farm environmental mitigation.”
The guidance was developed following a review of international and national studies. It includes how to size and site a wetland, construction, planning and sequencing of works, plant selection, wetland effectiveness in removing nutrients and sediment, and ongoing maintenance.
The guidance will assist people who design, review plans for, and construct wetlands, including rural contractors, environmental consultants, and regional council land management advisors. It will also give councils more confidence to recognise how constructed wetlands can help meet requirements to reduce contaminant losses.
DairyNZ will draw on the new guidance and other information to develop a practical resource for farmers. Existing resources on riparian management are already available at dairynz.co.nz/environment/waterways
The guidance and performance estimate calculations went through a robust development process. A technical advisory group of experts from eleven regional councils, the Ministry for the Environment, NIWA, DOC, DairyNZ, Beef + Lamb NZ and an environmental consultant reviewed, and by a consensus, supported the document. It was also shared with nearly 100 council stakeholders and environmental experts who critiqued the draft document.
The guidance has emerged from NIWA and DairyNZ’s ongoing collaboration, including the monitoring of constructed wetlands in different New Zealand farm landscapes and climatic regions. As new data becomes available, this will be used to build understanding and update the performance estimates included in the new guidance.
The new guidance is available at dairynz.co.nz/wetlands