A murky pond beside Auckland s busy Northern Motorway is the centre of groundbreaking environmental trial led by the NZTA's Auckland Motorway Alliance to produce cleaner water with the help of a garden growing on a floating island.
The pond, located near Silverdale 40 kilometres north of Auckland, is 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 pond's water for nourishment.
"The roots of the plants act as a biological filtration plant, some of the contaminants found in runoff water become part of the plants' diet; and there are other gains relating to water temperature and the settlement of sediments," says the AMA's Stormwater Asset Manager, Peter Mitchell.
Plants growing on the floating wetland like the wetlands grass carex vigarta have been selected for their thick and extensive mass of roots, an important factor in helping clean up polluted water. Their "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 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."
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.
"We have almost 90 of the ponds around the Auckland region and that number will grow," says Steve Mutton, the NZTA's acting States Highways Manager for Auckland and Northland.
Mr Mutton says the trial fits with a key NZTA objective to make better use of the country's existing transport system.
"Transport does have a big effect on the environment. 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," Mr Mutton says.
The stormwater pond at Silverdale has been split in two - environmentalists describe it as bifurcation - for the trial. On one side, the section of pond containing 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 says the two-year long trial should provide scientific clues leading to long-term environmental benefits.
"Our floating wetlands will give us more information about the way nature works to make stormwater cleaner - it's something we don't a lot about at present. We're also measuring the island against a conventional retention pond compared to see which is the most effective," Dr Fassman says.
Peter Mitchell says the the FVI pond is an example of the innovative work that the Alliance is undertaking with benefits for local and international communities.
In line with the NZTA's own Environmental Management Plan to improve the quality of water, the FVI Special Project was researched, designed, procured, and implemented by the AMA in partnership with the University of Auckland, Auckland Council and Kauri Park Nurseries at Silverdale.
"The results we produce are one part of an international trial to measure and understand the effectiveness of the Floating Vegetation Island to produce cleaner water and get rid of contaminants. At the end of the day, everything we learn will help contribute to an improved environment - not just beside motorways but in our communities as well," Mr Mitchell says.