Lake Okaro in the Rotorua district is popular with water skiers, but experiences persistent blue-green algal blooms most summers. The regional council, Environment Bay of Plenty, defines the lake as ‘super-eutrophic’ which is the second-worst lake water quality category. Now a major wetland has been built to filter nutrients from farmland to help improve the lake.
The constructed wetland is the first in New Zealand to be designed specifically to protect a lake from natural runoff (as opposed to effluent). It is part of a community action plan for the lake, with strong input and backing from Environment Bay of Plenty, Rotorua District Council, and Te Arawa Maori Trust Board.
NIWA managed the wetland construction project throughout, from initial consultation with stakeholders, to conceptual and detailed design, resource consent application, construction, and planting. The wetland designer, Dr Chris Tanner of NIWA, says the project "required a team approach, integrating a range of ecological, environmental, and civil engineering skills."
Once mature, the wetland is expected to remove about 165–210 kilograms of nitrogen per year, which is roughly one-fifth of the nitrogen reduction target set in the lake action plan. Environment Bay of Plenty Environmental Investigations Manager John McIntosh says the council is committed to monitoring the performance of the wetland: "If it turns out to be worthwhile, we will consider doing it elsewhere."
The Lake Okaro Constructed Wetland was officially opened on 3 February this year by John Cronin (Chair of EBOP), Kevin Winter (Mayor of Rotorua & Chair of The Rotorua Lakes Protection and Restoration Committee), Mr Anaru Rangiheuea (Chairman of the Te Arawa Maori Trust Board), and the Hon. Nanaia Mahuta (Associate Minister of Local Government & Environment).
New guidelines for constructed wetland planting
Need practical information on plant selection, establishment, and maintenance and weed and pest control? The guidelines focus primarily on wetlands treating secondary and tertiarytreated domestic sewage, and were written by Chris Tanner and Paul Champion of NIWA, and Vivian Kloosterman of VK Environmental Engineers, with support from the NZ Water & Wastes Association.
Download the new guidelines: www.nzwwa.org.nz/Constructed_Wetland_Planting_Guidelines.pdf
Guidelines for wetland treatment of farm dairy wastes: Guidelines for constructed wetland treatment of farm dairy wastewaters in New Zealand