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Treatment Walls Go Mainstream

A new method of blocking the flow of contaminants from road runoff is catching on in the northern North Island, and its designers are confident it will in time be adopted nationwide.

Road runoff is a toxic cocktail of contaminants including heavy metals and organic chemicals. The pollutants come from motor vehicle exhaust, road wear, and compounds deposited on roads as a result of tyre and break pad wear. They may also come from paint, and fuel and stock truck effluent spills. After rainfall, the resulting road runoff finds its way into the stormwater system, often ending up in rivers, lakes and estuaries where it may pose a threat to aquatic plant and bird life, and to fish.

A Landcare Research team in Hamilton led by Dr Surya Pandey has created permeable 'treatment walls' that can be inserted under or above ground along the edges of new or existing roads. These walls contain low-cost or waste products as media, including bottom ash from pulp and paper mills, sphagnum moss sweepings, crushed limestone, wool felt, sand and garden waste compost; which allow water to pass through, but trap contaminants. A new treatment wall has been put in place at a busy roundabout near Mt Maunganui, and talks are underway with several local authorities interested in building more.

Dr Pandey says the team worked with the Hamilton City Council, Waipa District Council, Waitakere City Council, Environment Waikato, and Auckland Regional Council to create the first three walls in Hamilton, Cambridge, and Henderson.

Transfund New Zealand and the Foundation for Research, Science and Technology also provided funding. More recently, Environment Bay of Plenty and the Tauranga District Council began work with the team to create the Mt Maunganui treatment wall on the basis of promising early results from the first three.

"Many other government agencies and local bodies are also closely observing the success of the existing walls," Dr Pandey says.

"Additionally, we have been in talks with the Ministry for the Environment, the Ministry of Transport and Transit New Zealand.

"I am confident that we will see many more treatment walls throughout New Zealand in years to come. They could be used wherever you have rainwater coming into contact with pollutants. They are especially useful in built-up environments, because they take up little space and can be built underground.

"A source control such as the treatment wall is desirable because once the runoff reaches the stormwater system, it is too diluted to treat at low cost."

Laboratory tests on the treatment wall materials found that lime and bottom ash removed more than 90% of contaminants from artificial road runoff. Sphagnum moss, wood waste and wool felt were effective at removing heavy metals, but not some other contaminants. However, using a mixture of the best media has overcome these problems. Dr Pandey says the four treatment walls currently in use are closely monitored, and have been registering similar results.

"There are differences between the sites, depending on the loadings of contaminants the walls are dealing with. However, the tests show that treatment walls are an effective tool to reduce contamination from road runoff, benefiting both local and surrounding environments.

"Ongoing monitoring will show how long the media can be kept in the body of the treatment wall before they become saturated with contaminants and need replacing. The oldest treatment wall, at the busy River Road roundabout in Hamilton, has been in place for more than two and a half years, and is still retaining a variety of contaminants."

Accolades for the treatment wall research so far include the Degussa Innovation Award at the 2001 Waikato Environmental Business Network Awards, and the Environmental Engineering Award (2002) from the Institution of Professional Engineers New Zealand (IPENZ) Waikato and Bay of Plenty branch. This project has also been nominated for the Ministry for the Environment Green Ribbon Award (2001 and 2003).


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