Until 1966, there was a legal requirement that all sheep sold in New Zealand were free of pest infestations such as lice, blowflies, ticks and mites. The most effective means of dealing with this problem was dipping the sheep in a pesticide solution. The active ingredients of these solutions were arsenic, organo-chlorines and organophosphates, the former two being persistent in the environment. Disposal of the pesticides after use resulted in areas adjacent to the sheep dip becoming contaminated. These areas pose a risk to human and animal health through groundwater contamination, as well as direct ingestion of soil. Dipping sites were often located near wells or streams, in order to prepare the pesticide solution. The exact numbers and locations of historical dip sites in NZ are unknown, but there are probably many tens of thousands on both private and public land.
A disused sheep-dipping site in an asparagus field was discovered near the city of Hamilton, North Island, following the measurement of elevated dieldrin concentrations in a nearby well. Soil analyses revealed dieldrin concentrations from 10 - 70 mg kg-1 over 100 m2. The Dutch Intervention Value for dieldrin in soil is 4 mg kg-1.
In late September 2001, the site was planted using HortResearch willow clones. In October 2003, the average height of the trees was over 3m (Fig. 1). Soil collected from the site before planting was homogenised and placed in 12 15-litre pots in HortResearch’s plant-growth facilities. Willow clones were planted in eight of the pots. All pots were watered and fertilised equally. After 5 months, soils from the pots were analysed for dieldrin, as well as biological (dehydrogenase activity) analyses. Substrate dehydrogenase activity is estimated from the rate of conversion of triphenyltetrazolium chloride to triphenylformazan (TPF). This is a measure of biological activity.
Fig. 2 shows the biological activity in the root-zones of grass and willows. The data shown in Fig. 2 may approximate the surface of the site before planting (i.e. when grass was growing on the site), and now after the planting of willows. Clearly, willows greatly enhance biological activity in the soil. Previous studies have shown that biological activity leads to a greater rate of the decomposition of some contaminants. This increase in biological activity is caused by root exudates, such as sugars and organic acids upon which bacteria and fungi can feed. Willows have a much greater biomass production than grasses, and consequently have a greater quantity of root-exudates. Willow roots also penetrate further than grass roots (up to 1 metre) and improve soil aeration because of their high water use.
The willows caused a significant decrease (p<0.05) in the soil dieldrin concentration over the treatment period (Fig. 3). This 20% reduction was achieved in only five months of growth. The dieldrin degradation effected by the willows was greater than that by the grass species that may first colonise many disused sheep dipping sites.
Robinson BH, Anderson CWN (2007). Phytoremediation in New Zealand / Australia. In: Phytoremediation Methods and Reviews (Ed. N. Willey). Humana Press, Totowa, NJ. pp 455 - 468.