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It’s better known for honey but manuka may be a key to helping keep NZ waterways clean.

The humble manuka or New Zealand tea tree, long valued for the part it plays in the regeneration of native bush and for its honey-making properties, may have found a new role – helping to clean the country's waterways.

More than 40,000 manuka trees have been planted near the shores of Lake Waikare in the Waikato as part of a field trial being run by the Institute of Environmental Science and Research (ESR) to determine whether they can reduce nitrate run-off from farms entering the lake.

An ESR scientist, Dr Maria Gutierrez-Gines, says laboratory-based tests have shown the root system of the tree has an as yet unexplained ability to reduce pathogens and nitrates.

Tests have found they have significantly reduced the leaching of nitrate compared to pasture and pine trees and Gutierrez-Gines says "we think they can also influence run-off from farms."

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The project is being run by ESR in collaboration with Canterbury and Massey universities, Nga Matahuru marae, Waikato Regional Council and Ecoquest Education Foundation.

The trial, which began last year, has funding for five years; researchers will be measuring the quantity and quality of the run-off as it passes through the barrier created by the trees, along with manuka's potential for reducing nutrients, sediments and pathogens from entering waterways.

Stats NZ data shows that nationwide 137 million kg of nitrogen leached from the soil in 2012. Of this total only 19 per cent was directly from fertiliser, the remainder coming from livestock including sheep, goats, dairy herds and beef cattle.

Gutierrez-Gines says Lake Waikare, the largest of a system of shallow lakes in the lower Waikato farm catchment and which has poor water quality, is one of two sites being used.

The other is on farmland near Lake Wairarapa in the lower North Island where around 1500 manuka trees have been planted on two blocks of 1.5 hectares donated by local landowners in collaboration with the Greater Wellington Regional Council and Ngati Kahungunu ki Wairarapa.

She says this land is flatter than that around Lake Waikare and researchers will assess if it will work as effectively: "Run-off will likely be less on flatter land because the water won't run through the plants so much; we are looking for variety in the topography."

Preliminary results from Lake Waikare show an encouraging increase in soil biodiversity but Gutierrez-Gines says it will be some time before they can measure the full effect of testing.

Manuka, which acquired the name tea tree after its bark was used for tea by early settlers, is useful in a variety of ways; the tree is favoured for ecological restoration purposes, is well-known for its role in providing a shelter canopy for regenerating native bush and is suitable for smoking fish (especially kahawai). It is also internationally famous for the honey produced by bees that pollinate its flowers.

Meanwhile in another trial aimed at reducing nitrate levels in waterways, ESR scientists have installed a 25m wide, 3m deep wall in a north Canterbury shallow groundwater system. The trial, at the Silverstream Reserve near Kaiapoi, has been running for just over two months and has already achieved significant results.

ESR principal scientist Murray Close says nitrate levels in the groundwater at the reserve have dropped from 7.1mg per litre to 0.5mg per litre after passing through the wall.

The denitrification wall, made of (non-manuka) woodchips mixed with gravel, acts as a groundwater filter by removing nitrate from water as it passes through the structure. The woodchips de-nitrify the water by providing a carbon food source for bacteria which convert nitrates to harmless nitrogen gas. The test site was selected because of its shallow water table and high nitrate levels.

Close says he believes the concept will ultimately be able to be used by farmers, although the key question will be around the costs of installation.

"The sheet piling we are using represents the single biggest cost but this could change if we can come up with an alternative," he says. "But we can't yet say how cost-effective it will be."

ESR is also working on a project examining whether woodchip bioreactors are a viable method for reducing nitrate loads in farm drainage water. A reactor is being installed on an artificial farm drain at a site near Geraldine in South Canterbury.

Senior ESR groundwater scientist Dr Lee Burberry says while the bioreactors are already being used to treat land drainage in the US and to treat industrial wastewater in New Zealand, it is not known how they might perform in the New Zealand agricultural landscape.

The projects are part of ESR's work over the last 10 years investigating ways to improve water quality. The trials are funded by the Waikato River Authority, the Waikato and Wellington regional councils and ESR Strategic Science Investment Funding.