M¯anuka a nectar for our waterways
‘‘E. coli died off much faster under ma¯nuka than pasture.’’ Dr Maria Gutierrez-Gines
Scientists are testing whether ma¯ nuka trees and woodchips can help clean the country’s waterways.
While important and complex public debates are being had about the state of our fresh water, scientists have been working behind the scenes on a range of practical solutions to improve water quality.
A field trial underway in the North Island is using ma¯ nuka trees to intercept and clean farm runoff.
Dr Maria Gutierrez-Gines, a scientist at ESR, says the antimicrobial properties of ma¯ nuka honey are already well established, but laboratory tests show that the root system of the trees also has a yet unexplained ability to reduce pathogens and nitrates.
‘‘In the laboratory-based tests, E. coli died off much faster under ma¯ nuka than pasture, and significantly reduced the leaching of nitrate when they were compared with both pasture and pine trees. We think it can also influence run-off from farms.’’
The ESR-led Centre for Integrated Biowaste Research is now running field trials looking at ma¯ nuka’s potential for reducing nutrients, sediments and pathogens from entering waterways in the Waikato farm catchment.
A trial at Lake Waikare is being run in partnership with local iwi, Canterbury University, EcoQuest, the Waikato River Authority and the Waikato Regional Council. The lake is the largest system of shallow lakes in the lower Waikato catchment and has extremely poor water quality.
Planting started last year in a four-hectare block near the lake that had been donated by local landowners – the Nikau Farm Trust. Gutierrez-Gines says preliminary results after the community planting of more than 40,000 trees already shows an encouraging increase in soil biodiversity.
The trial has funding for five years and researchers will measure the quantity and quality of the run-off as it passes through the biological barrier created by the trees.
Removing nitrate in groundwater is also a focus for ESR’s groundwater scientists, who are looking at solutions to reduce nitrate levels accumulating in gravel aquifer systems.
One project is examining whether woodchip bioreactors are a viable method for reducing nitrate loads in farm drainage water and also filtering nitrate from contaminated groundwater.
The woodchips remove nitrate in water by providing a carbon food source for bacteria, which convert nitrates to nitrogen gas.
Senior ESR groundwater scientist Dr Lee Burbery says gravel aquifers have no natural ability to reduce nitrate.
He says that while bioreactors are already being used to treat land drainage in countries like the United States, and are being used to treat industrial wastewater in New Zealand, it is not known how they might perform in the New Zealand agricultural landscape.
Burbery says a woodchip bioreactor is being installed on an artificial farm drain at a site near Geraldine, South Canterbury. Scientists at ESR are also finalising the design of a woodchip ‘‘wall’’ that they will be trialling in the Silverstream catchment, north of Christchurch.
The wall involves the ‘‘strategic placement of a woodchip mixture below the water table to intercept the flow of shallow groundwater tainted with nitrate. The woodchip acts as a porous reactive medium, passively removing nitrate from groundwater filtering through it.’’
There has already been some experimental work on this type of woodchip walls in New Zealand but this is the first time they will be trialled in a gravel aquifer.
Burbery says a lot of research has been done to understand the groundwater flow-paths at the experimental site, to help with the design of the wall.