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Constructed wetland guidelines

Constructed wetlands are a water quality restoration tool that can reduce levels of sediment, nutrients and microbes such as E. coli. This can significantly improve the water quality exiting the wetland, and the ecology of downstream water bodies.

Local and international evidence shows that well-designed wetlands are capable of significantly reducing nitrate and contaminant concentrations. In warm areas of New Zealand, removal of nitrate will range from 25 up to 50 percent as wetland size is increased from 1% to 5% of the contributing catchment area. In cool areas nitrate removal will be a little lower, in the range of 20 to 40 percent.

Constructed wetlands can also buffer flows, decreasing downstream flooding, and provide a valuable habitat for native species.

The guidance was developed in association with DairyNZ and 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.

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These guidelines will be updated as final documents become available.

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Principal Scientist - Aquatic Pollution
Lake Okaro constructed wetland in the Bay of Plenty. [Photo: Chris Tanner, NIWA]