The owner-partners in Kaiwaiwai Dairies say their wetland – constructed on 0.75 hectares of boggy pasture in 2014 – is proving four times more effective in its reduction of nitrate loss from the farm than initial predictions using the Overseer Nutrient Budgets model.
“Levels of reduction vary from month to month depending on water flows and temperatures, but overall the wetland is far exceeding expectations of what it could pull out,” Aidan Bichan, one of the owners and a professional farm consultant, says. He and his six partners are keen to see how a wetland– carefully constructed, planted and managed – can become a more efficient denitrification “kidney” in the midst of a very productive dairying operation.
The Kaiwaiwai wetland has special significance for its long-term contribution to cleaning up Lakes Wairarapa and Onoke, and their wetland fringes. Lake Wairarapa is classified as “super-tropic” under the impact of a century’s agriculture.
Kaiwaiwai Dairies is one of about 90 farms in the catchment, many on land lifted by the earthquake of 1855 or recovered from the once-extensive lake-side wetlands through a huge network of drains.
The constructed wetland draws water from one of those drains and returns it to another downstream – and of course, the flow is ultimately into Wairarapa Moana (the collective term for the lakes and their fringes).
For this reason Kaiwaiwai Dairies has substantial support for its wetland as part of the Wairarapa Moana Cleanup Project.
Kaiwaiwai Constructed Wetland April 2015 from Groundtruth on Vimeo.
Of the $55,000 capital cost, three-quarters came from the Government’s Fresh Start for Freshwater Cleanup Programme and other public sources, including the Greater Wellington Regional Council (GWRC), through the Wairarapa Moana project. The Kaiwaiwai wetland and another on nearby Wairio Station won recognition as part of a Morgan Foundation River Story Award made to that project in November.
Kaiwaiwai Dairies, on the highway between Featherston and Martinborough, has a non-irrigated milking platform of 325ha with 900 cows in production at the seasonal peak. The seven partners bring together a unique set of dairy farming, engineering and business skills. Since 2005, they have made the farm a top performer among dryland dairy units in the region, with annual milksolids production of about 1100kg a hectare.
More than 80% of feed is grown on the platform or leased support land, including turnips, kale and other feed crops.
“We’re proud of being part of the dairy industry and part of Fonterra … and we’re going to keep making sure we’re best practice at everything we do,” Aidan says.
The partners have invested heavily over recent years in effluent storage and solids separation systems, and in the high use of recycled, green water for washing concrete areas around the dairy.
In fact, Kaiwaiwai Dairies has long been relatively low on nitrate loss because of its lack of irrigation, heavier soil type, low rainfall and the partners’ investment in feedpads. The property’s Overseer calculation for 2013-14 was 14kg of nitrogen per hectare per year compared with the national average of 35kg. Aidan says the wetland contributed to the Overseer figure coming down to 11kg in 2014-15.
Vern Brasell another of the partners, says they designed the wetland with clear objectives.
“We wanted to show what can be done by constructing an area that optimises the spread and flow of water, and then let plants and wildlife do the rest.”
It works by exposing the contaminated water to anaerobic bacteria –produced by organic matter as it builds on the wetland floor – that convert nitrates into nitrogen gas. The gas, which isn’t a greenhouse gas, constantly oozes into the atmosphere.
They chose a site that was winter-boggy and slightly lower than a crossfarm
drain from which water would be piped 180m into the wetland. They settled on a design of 12 parallel bays, each 6m wide with earth banks of uniform height. Water enters the top bay and flows down a serpentine course to an exit pipe at the far end.
The flow is slow, steady and shallow, with bay depth kept to only 0.3m so plenty of sunlight gets into the water. Maintaining that slow and uniform flow without diffusion or dead spots across a water area adding up to 0.5ha is considered critical to the denitrification process.
Biodiversity was also an objective for the wetland. The site includes a stand of remnant kahikatea and totara trees, some 145 years old. Development included ripping old-age blackberry out of the stand and planting for regeneration of low native species.
The wetland itself was planted with raupo, lake club rush and rautahi that slow and disperse the water. The banks were planted in flax, karamu, manuka and cabbage trees to hold them in place and promote biodiversity.
“Anything that lives is welcome here … as long as it doesn’t eat grass,” Aidan says. The wetland is, of course, fenced off from cows. So far, it has attracted pukeko, ducks, herons, shags and harriers – and on a summer afternoon, the frogs are very noisy.
In time, short- and long-fin eels are expected to find their way in from the South Wairarapa drain network, and perhaps also koura.
Water samples from both inflow and outflow pipes are analysed each month, giving a measure of the wetland’s efficiency. Results from April to December 2015 show sharp reductions, with monthly variations attributed to changing water flow, temperature and aquatic plant growth.
Kaiwaiwai Dairies, with expert input from National Institute of Water and Atmospheric (NIWA) Research and GWRC, uses the data and Overseer to calculate the wetland’s contribution to reducing the farm’s nitrogen “export” through the course of a year. The eight months’ data indicates the reduction would be close to 500kg – and Aidan says that is much higher than anticipated.
The figure depends on the size of the wetland and its catchment. In fact, the drain takes water from about 300ha, of which only 50ha are on Kaiwaiwai Dairies. During 2015, the piped inflow to the wetland was limited to 10 litres per second, which was1/6th of the average flow in the drain itself.
Phosphates are obviously of interest too. The monthly analysis in 2015 showed significant reduction of dissolved reactive phosphate in warmer months, although not in May when temperatures fell. The picture for E coli is less promising, with the wetland producing higher counts of these bacteria.
NIWA and GWRC attribute that to the slow flow and shallowness of the water, and its increased temperature as a result, and to the locality’s surge in birdlife.
The partners say the wetland will take at least two years to fully establish, and then be self-sustaining with its vegetation growing and drying in natural cycles. Meantime they will monitor water flow patterns and sediment buildup in each bay, along with the health of vegetation and wildlife.
In 2016, they have also increased inflow to more than 10 litres a second and are measuring resulting changes in wetland efficiency. A higher flow rate increases the volume of water passing through but reduces the time allowed for denitrification to occur.
The wetland’s capital cost and nitrogen reductions measured to date enable Kaiwaiwai Dairies to estimate a per-unit cost of nitrate export reduction. The figure is $118/kg nitrogen, which the partners believe compares favourably with costs of alternative methods of reduction around New Zealand.
“Our results so far show a wetland isn’t a silver bullet but it can be part of an integrated farm plan to mitigate nutrient losses,” Aidan says.
“The project demonstrates a very tangible action that farmers can take to reduce losses into the wider environment and give a significant lift to biodiversity.”
