Quick Dips

Tackling Toxic Waste with Trees

Soil contamination by toxic chemicals and waste water is an ever-increasing problem as populations and industries grow, but the answer could be as cheap and easy as planting some trees.

HortResearch scientists are working alongside farmers, orchardists and local authorities, developing ways to use new varieties of poplars and willows to clean up or stabilise some sorts of contamination. Heavy metals such as cadmium can build up over time until land becomes unsuitable for farming.

Dr Tessa Mills of the Environment Group says, "Elevated cadmium concentrations in soils pose a risk to human and animal health and the productivity of plants. They could even be used as a non-tariff trade barrier to exports.

"Conventional decontamination methods can cost upwards of millions of dollars per hectare and involve removal of tonnes of soil to landfill."

Pollution from excessive fertiliser use and the application of effluents to land such as municipal waste water, bio-solids and dairy-shed effluent, runoffs from roads, tips and landfills, mines and industrial sites can potentially be dealt with by poplars and willows.

Nitrate leaching into groundwater and waterways can be greatly reduced or even prevented. The tree roots can assist the breakdown of some organic pollutants such as PAHs (polycyclic aromatic hydrocarbons) and some pesticides.

The "secret" is that poplars and willows developed by HortResearch have the ability to take up, lock up or break down contaminants. They prevent pollutants leaching into groundwater by taking up large amounts of water via transpiration. Some pollutants can be broken down in the soil by the roots, or the microbes that live around the roots. In addition, some pollutants can be sucked out of the soil and "locked up" in the wood and leaves of certain trees. When the clean-up is complete the trees can be removed so that the pollutants do not re-enter the soil.

"Phytoremediation using poplars and willows is estimated at about half the cost of the cheapest conventional operations. Unlike some other methods the solution is permanent and the soil is left fertile," Mills adds.