Floating island on trial

GREENER SOLUTIONS: Auckland University senior lecturer Elizabeth Fassman with international stormwater management expert Bill Hunt of North Carolina State University working on the Floating Vegetated Island.

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A Little floating island has been formed in Silverdale.

It is beside the northern motorway, and is an environmental trial by NZ Transport Agency's Auckland Motorway Alliance to produce cleaner water with the help of a garden growing on the island.

The pond is used to catch stormwater runoff from State Highway 1, and is now home to the Floating Vegetated Island, FVI.

FVI is a hydroponic mat-like platform which allows plants to grow on its surface while their roots dangle into the pond's water for nourishment.

"The roots act as a biological filtration plant, some of the contaminants found in runoff water become part of the plants' diet," Auckland Motorway Alliance stormwater asset manager Peter Mitchell says.

"And there are other gains relating to water temperature and the settlement of the sediments."

Wetlands grass carex vigarta have been selected for their thick and extensive mass of roots, helpful in cleaning polluted water.

The plant's diet includes contaminants found typically in a motorway environment – oils, zinc, chromium, and copper – the residue of tyres, brake linings and fuel flushed into the pond when it rains.

"It doesn't sound like the tastiest of diets, but the plants on the FVI are flourishing," Mr Mitchell says.

"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."

It is unlikely the patch will be used to grow commercial vegetables because of potential contaminants posing a health hazard.

For the trial, the pond is split in two. One side of the island is a section of pond containing the FVI, while the other section contains untreated stormwater.

This allows environmentalists to see how effective the floating island is in improving water compared to the standard stormwater pond.

The two-year trial should provide scientific clues leading to long-term environmental benefits, Auckland University civil and environmental engineering lecturer Elizabeth Fassman says.

The FVI is a project between AMA, Auckland University, Auckland Council and Kauri Park Nurseries at Silverdale.

- © Fairfax NZ News