Untreated pine woodchips — a waste product on many farms — could be an affordable, low-tech solution in reducing nitrate levels in Canterbury waterways.
Brandon Goeller, a PhD student at the School of Biological Sciences, the University of Canterbury, is testing the possibility that untreated pine woodchips will reduce nitrate levels as well as improving the habitats for stream dwellers.
As part of a three-year project, he has constructed woodchip-filled trenches called denitrification bioreactors and placed small string bags of the woodchips alongside, in or under waterways.
Mr Goeller’s woodchip bioreactors are some of the first of their kind to be tested in agricultural waterways across New Zealand.
The woodchips have been placed in a low-lying wet area on land alongside a drain and in the flowing drain 800 metres downstream on Warren Harris’ dairy farm at Coldstream. One site has tile drains, while the other does not.
Another woodchip bioreactor is planned to intercept a tile drain on his brother Graeme’s neighbouring arable property. The fourth trial is on a Methven dairy farm.
‘‘Streams would often flow through native bush where leaves and branches would fall in them and decompose,’’ Mr Goeller said.
‘‘On the farmland of the plains, we are missing that carbon source.’’
Woodchips could be beneficial in two ways — by providing the energy for microbes, which could remove nitrate, and by improving the habitat for in-stream invertebrates and fish.
As a low-cost, low tech method with little maintenance which would not interrupt farming activities, the woodchips could prove attractive to farmers, Mr Goeller believes.
With four 800m lengths of waterways to monitor on at least a monthly basis, he has spent a lot of time in water during the past two years and there is still a year of the project to run.
Water chemistry is monitored upstream, in the woodchip area and downstream, measuring changes in nutrients, carbon and invertebrate and fish abundance.
Experimental results from North America and Europe indicate that the woodchips could remove up to 70% of the nitrate, Mr Goeller said.
The project is another tool being trialled in the Canterbury Waterway Rehabilitation Experiment — Carex — conducted by biologists at the University of Canterbury. They are testing practical tools to address aquatic weeds, sediment and nutrient management issues in lowland Canterbury waterways.
The project involves 23 landowners from Rangiora to Lowcliffe, 40 stakeholders and four councils, and covers nine kilometres of waterways across 14 farms.
The project is a collaboration with landowners, industry, government agencies and the community, with research funding coming from the Mackenzie Charitable Foundation.
Measures introduced have included the planting of thousands of native plants along waterways, adding rocks and logs to the water, trialling various methods of controlling aquatic weeds, sediment traps, and installing denitrification bioreactor sites.
Also on site were researchers from the University of Auckland who are looking at greenhouse gas emissions and riparian management practices.