CONSTRUCTED WETLANDS WORK WONDERS FOR WATER QUALITY

A boggy spot on a Mourilyan cane farm has been converted into a high-performing “constructed” wetland, removing more than 90 per cent of nitrate from water in the system. 

The Wet Tropics Major Integrated Project team has worked closely with wetlands experts and a third-generation cane farmer to transform the low-lying and unusable strip of land into a water treatment system on his 101-hectare property. 

The 1.2 hectare wetland is proving that, with some remediation and construction, low-lying wet areas can effectively remove high levels of dissolved inorganic nitrogen from the water. 

Acting Project Manager Fiona George said this wetland shows the potential of wet areas on farms to improve the quality of farm run-off water. 

“This constructed wetland is evidence that a wet piece of marginally productive land can yield excellent water quality outcomes with the help of a tailored design and relatively minimal earthworks,” Ms George said. 

“We enhanced existing features that helped it to perform as a denitrifying wetland.” 

Vegetation uptake has been a key factor, with a thriving ecosystem of plant life establishing itself in a short timeframe and enabling the denitrification process to occur. 

The Wet Tropics Major Integrated Project has trialled a range of catchment repair systems in the Tully-Innisfail area, including seven different bioreactor designs, a landscape wetland, three in-drain wetlands, a high-efficiency sediment basin, three constructed wetlands and a riparian revegetation site. 

The constructed wetlands are yielding the best results, proving that, with improved hydrology and the appropriate amount of vegetation, they have high potential to reduce nitrate levels.  

Constructed wetland in Mourilyan

Both the landscape and constructed wetlands removed around 1,570kg of nitrate from DIN-rich water each year and they are the most cost-effective of all treatment systems trialled in the project, costing between $9 to $31 per kilogram of DIN removed. 

Ms George said the Mourilyan landowner loved the results on his property. 

“He used the topsoil cut during the construction for blocks that needed it, and said the water quality improvements were great to see, along with the bird life and fish this has attracted back into the area,’’ she said. 

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