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Cleaner ponds, with the help of a floating garden

A murky pond beside Auckland's busy Northern Motorway is at the centre of an environmental trial to produce cleaner water with the help of a garden growing on a floating island.

A murky pond beside Auckland's busy Northern Motorway is at the centre of an environmental trial to produce cleaner water with the help of a garden growing on a floating island.

Photo: NZTA

The pond, located near Silverdale, 40 kilometres north of Auckland, is being used to catch stormwater runoff from nearby State Highway 1.

It is now home to what is known environmentally as a floating vegetated island (FVI) – a hydroponic mat-like platform that allows plants to grow on its surface while their roots dangle into the water for nourishment.

The initiative is being spearheaded by the NZTA's Auckland Motorway Alliance.

Plants growing on the floating wetland have been selected for their extensive mass of roots, an important factor in filtering polluted water. Their diet includes contaminants found typically in a motorway environment, according to the AMA's stormwater asset manager, Peter Mitchell – oils, zinc, chromium and copper, the residue of tyres, brake linings and fuel flushed into the pond when it rains.

"It doesn't sound the tastiest of diets, but the plants on the FVI are flourishing," Mitchell said. "How well they adapt to the 'diet' will help us discover if there is a way to improve on the work we're already doing to clean up water in the ponds."

The Auckland Motorway Alliance was established by the NZTA four years ago to maintain and operate 220 kilometres of motorway – a role that also includes environmental accountabilities. It is responsible for the highest concentration of stormwater ponds in New Zealand.

Steve Mutton, the NZTA's acting state highway manager for Auckland and Northland, said the trial fits with a key NZTA objective to make better use of the country's existing transport system.

"In the next few years, there will be more than 100 stormwater treatment ponds besides Auckland's motorways. If the result of this trial is positive, we could retrofit other stormwater ponds to help improve the city's landscape and reduce the environmental impact of transport," Mutton sad.

The stormwater pond at Silverdale has been split in two – environmentalists describe it as bifurcation – for the trial. On one side lies the FVI and on the other, the stormwater remains untreated. The split allows environmentalists to see how effectively the floating island helps improve water compared to the standard stormwater pond.

Auckland University's Dr Elizabeth Fassman said the two-year trial should provide scientific clues leading to long-term environmental benefits.

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