Science Centres: Freshwater

Wetlands are like a sponge - they take in large amounts of water and "clean" it by processing out nutrients carried in the water that flows through them. These nutrients can include diffuse pollutants like nitrogen and phosphorous 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 for farmers who want to make best use of constructed wetlands.

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 NIWA's Dr Chris Tanner.

Tile drainage can act as a significant route for nutrient losses, particularly of nitrogen, from intensively grazed pastures to waterways so the resource book is very timely.For wetland water filtering to succeed, NIWA scientists have discovered that you need 1-5% of land in a catchment as a wetland. With that amount of wetland you'd see from 20-50% reduction of nitrogen levels.

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. Up to now, nobody has ever tried to quantify their performance. We found that for stream flows into Lake Rotoehu, the removal of nitrogen was between 45% and 77% and for total phosphorus removal was 32% to 35%."

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

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.

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 Ministry of Science and Innovation. 

For comment, contact:

Dr Chris Tanner

NIWA Principal Scientist, Freshwater

Mob: +64 21 1100 118

Or care of

Jayne Cooper- Woodhouse

Mob: +64 275 651 308

Dr Clive Howard-Williams

NIWA Chief Scientist, Freshwater

Mob: +64 27 431 5037