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Wetland Wonders: cleaning up our freshwater

According to NIWA, wetlands have great potential to clean up polluted freshwater.

A four-year NIWA study found that constructed wetlands can reliably remove more than half of nitrate leaching from dairy farm pastures.

Excess levels of pollutants, such as nitrate-nitrogen, can leach into groundwater and enter freshwater, where they can cause toxic algae blooms, reduce oxygen and kill aquatic life.

NIWA Chief Scientist – Aquatic Pollution Dr. Chris Tanner says constructed wetlands are increasingly being used as a natural solution to reduce agricultural pollution.

Chief water pollution scientist Dr Chris Tanner monitors progress at the Awatuna-constructed wetland in Taranaki. (Photo credit/Stu Mackay, NIWA)

“Wetlands reduce runoff and drainage by creating a filter between agricultural areas and freshwater sources. They absorb excess pollutants and prevent them from entering streams, rivers and lakes. This can significantly improve the ecology of rural watersheds,” says Dr. Browner.

Owl Farm, a 160-hectare dairy farm at St Peter’s School near Cambridge, was the case study in question.

In 2016, a 0.34ha constructed wetland was established as part of a wider plan to reduce the load of pollutants flowing into the nearby Waikato River.

NIWA assessed the wetland’s ability to remove nitrogen from groundwater. Over four years they carried out flow and contaminant concentration measurements, which were then used to calibrate a mathematical model.

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“The wetland model simulated the movement of water and pollutants through three wetland areas, estimating the flow and pollutant loads entering and leaving the wetland. The model indicates that the constructed wetland was highly effective and between 55-80% of incoming nitrate nitrogen, by an average of 61%,” said Dr. Tanner.

Awatuna Wetland in Taranaki – one of six NIWA-designed artificial wetlands recently developed by local landowners and councils. (Photo credit/Stu Mackay, NIWA)

Removal was greatest when flows were low and during summer, when warm temperatures stimulate plant uptake and conversion of nitrate to inert nitrogen gas by microbes.

Although performance decreased as water flow increased, nitrate removal was still maintained at 20-40% even during higher flow periods.

The wetlands also provide a habitat for aquatic life, moderate flooding, improve the aesthetics of the landscape and provide a range of cultural benefits, such as materials for weaving.

“This study helps us determine whether constructed wetlands can be implemented on a larger scale as an effective long-term solution to water pollution. These results are promising and similar studies are underway across the country to collect more data and build the evidence base , said Dr. Tanner.

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