Wetlands can clean our water, but location is key

NIWA Media Release 20 September 2011

Wetlands can clean our water, but location is key

Wetlands are like a sponge - they take in large amounts of water and “clean” it by processing out nutrients carried in the water. These nutrients can include diffuse pollutants like nitrogen and phosphorus from farm runoff.

The location and structure of the wetlands is key to their success in this role. NIWA freshwater scientists have devised a new wetlands guide so farmers can make best use of them.

For wetland water filtering to succeed, NIWA scientists have discovered that you need between 1 and 5% of land in a catchment as a wetland. With that amount of wetland you’d expect to see a reduction of between 20 and 50% of nitrogen levels in waterways.

NIWA’s Dr Chris Tanner has also been involved in the first ever trials with floating treatment wetlands, a new innovation that is now being applied in Lake Rotoehu as part of the Rotorua Lakes restoration programme. Those wetlands are performing well.

“It’s new work internationally, to look at the performance of these wetlands that float on the water surface with their roots hanging down into the water below. Nobody has ever tried to quantify their performance. What we found, for stream flows into Lake Rotoehu, was between 45% and 77% nitrogen removal and 32% to 35% total phosphorus removal.”

Nitrogen is one of the key nutrients causing deteriorating lake water quality. Around 75% of the nitrogen and phosphorus runoff to the sea from New Zealand rivers originates from modified, mostly pastoral, land use.

Once there are high nutrient levels in the water it is very hard to remove them. Wetlands are a low cost natural system that can intercept and treat these nutrient-rich flows before they get into lakes and estuaries.

Tile drainage is an agriculture practice that removes excess water from soil subsurface. It can act as a significant route for nutrient losses, particularly of nitrogen, from intensively grazed pastures to waterways.

The New Zealand Guidelines for Constructed Wetland Treatment of Tile Drainage is “a handy new resource intended to guide farmers, farm advisors, rural contractors, and regional council staff to appropriately locate, size, design, and construct effective treatment wetlands,” says Dr Tanner.

Location is key to how effective constructed wetlands will be in removing nutrients from waterways. NIWA scientists have been pondering where the best place is to place a wetland. In doing so, they found that the answer changes depending on the landscape and the key pollutants you are trying to remove.

This research was funded by New Zealand Dairy Industry and the Ministry of Science and Innovation.

NIWA’s Chief Scientist Freshwater and Coasts, Dr Clive Howard-Williams, will lead discussion in an OECD-sponsored symposium on Wetlands in Agricultural Landscapes, at the DIPCON conference being held at the Energy Events Centre in Rotorua from 18 to 23 September 2011.

Dr Chris Tanner says the big question for the symposium is, ‘How much wetland should be retained in productive land-use areas to take advantage of these natural functions? For the conference programme, and other information, go to: www.dipcon2011.org

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