Waikato Wetland Showcase

Wetlands can be very effective in reducing nutrient losses from pastures and thus improving stream and river water quality. Much is known about the biological processes the trapping of P-bearing sediment and slowing of water flow to allow bacteria to deal with dissolved nitrates. However, less is known about cost effective ways in which natural wetlands can be protected and enhanced, former wetlands can be restored and new ones developed on individual farms.

The Karipiro catchment, which drains into the Waikato River and Lake Karipiro, is particularly sensitive and farmers in the area are under pressure to improve the ways they deal with nutrient runoff. In 2012 DairyNZ and Fonterra approached farmers in the catchment offering to help them document plans for nutrient reduction. The Baldwin family had farmed in the area for many decades and dairy farmers Gray and Marilyn Baldwin had won the Ballance Supreme Environmental Award in 2009 for their efforts in riparian planting and other measures to protect the environment. Gray says that rather than spend money on documenting what they were already doing, he suggested that they construct an experimental wetland to remove nitrates before they entered the river.

At one of the field days after we won the award in 2009, the late Gordon Stephenson pointed out a particular piece of drained swamp and commented that it would make a great wetland. When DairyNZ approached me in 2012 I suggested that same area for a really decent constructed wetland, says Gray.

They were persuaded that it was a good idea and Dr David Burger put together a detailed proposal involving a lot of science and analyses of nutrients and took it to the Waikato River Authority. They backed it with substantial funding along with NIWA, Hill Laboratories, Opus and the Waikato Regional Council.

The Baldwin Family Trust fenced off the wetland area and put it into a QEII Covenant. Other sponsors, particularly the Waikato River Authority, contributed to the excavations, plantings and all the scientific measuring instrumentation necessary. Construction took place in autumn 2015, instruments were put in place in winter and planting was carried out in spring by the Baldwin family along with many local volunteers.

The dairy farm involved is 270ha running a herd of approximately 800. Around 85ha of the property drains into the wetland area, which is 1.1ha in size. Just under half of the wetland is a natural seepage area that was drained in the past and has now been fenced, restored and replanted with typical swampland vegetation.

The remainder is a constructed wetland. DairyNZ scientist Dr David Burger who leads the research project describes it as being like a series of rice paddies but on a much larger scale.

We have created seven basins. The water flows into a basin and then overflows into the next one so that slows the water down. The water that we are treating is overland runoff that comes in during rainfall and contains some urine, faecal material and sediment. There is also some groundwater from small seepage springs, says David.

All the water passes initially through a filter to trap sediment material and the residual flow carries on into the wetland. Exposure to UV light kills off faecal coliform bacteria whereas natural bacteria will treat the nitrate.

Planting of species like rushes and reeds was necessary to create an environment for the bacteria to flourish, and the plants themselves also remove some of the nitrate and phosphate. Around 12,000 were planted by community groups in late winter and spring, says David.

We have now started monitoring the water flows into and out of the system and the difference between those samples allows us to estimate how much of the contaminant load is treated by the system, he says.

The dry summer has meant little water flow and few results but over the next 12 months the data will give us an impression of potential annual removal. We have also been focused on making sure the plants survive the dry season and establish well.

Much of the work so far has been around how to optimise the wetland design and also dealing with the practicalities of being on a farm. Often water flow paths intersect with races where cows move back and forth, resulting in a lot of sediment entering the catchment area. Power lines and a gas line had to be considered during the construction phase.

Another consideration was variable loading on the wetlands. The past summer has been very dry and initially there were no water flows at all. Then suddenly after rain there was a rush of water, so wetlands need to be able to cope with extended dry periods and sudden wet periods. That implies making sure that you have the right plants and the right design to hold the water back but not create flooding at the same time, says David.

What's different about this study is that we are monitoring the system in great detail continuously with automated flow recorders, and the system has also been set up with bottles that can be triggered by cellphone or by the computer to collect a series of samples during a high flow after rainfall. That will give us greater insight into nutrient removal under different flow rates - high flow, low flow, high temperature, low temperature, he says.

One of the aims of the project is to develop guidelines to help farmers do similar things on their own properties while acknowledging that every property is different. So it is about optimising the design, making these systems affordable, understanding their performance and developing tools to increase uptake of wetland management across all farms.

Marilyn Baldwin is part of the DairyNZ Environmental Leaders Forum and is committed to helping farmers mitigate the environmental impacts of farming operations.

Improving quality water is actually high on farmers agendas but you can tackle it at a slower pace than we have, you don't have to do it all in one go. We have got project partners who have all contributed and there is data being collected so that we can show that the system is working, says Marilyn.

It doesnt need to be on such a grand scale and it doesn't have to be done at the same pace but we all need to take action. We want our children to be able to keep farming here so we need to be proactive in solving these problems.

Gray Baldwin agrees and is adamant that in the case of the Karapiro catchment the potential effects on Lake Karapiro and the Waikato River quality mean that farmers in the area need to take action. However, cost is a major consideration.

At present a lot of dairy farmers are hunkering down just to survive but they are still expected to comply with changing environmental rules and societys expectations. We have the land available for this project but not every farmer has natural wetlands or a spare sloping area where wetlands can be constructed, he says.

An alternative is to pour a lot of concrete and put the cows in a barn, collect the solid and liquid effluent in a tank and then apply it to the land when you can and store it when you can't. That may be a more foolproof option but it may be a lot more expensive than constructing a wetland.

So the debate will be around how cost-effective a wetland is in terms of removing nitrate versus building concrete pads and collecting effluent to apply during the dry months. Hopefully the results from our project will give some insight into this.