Case study: Phytomanagement trial on the Tui mine tailings    

The Tui mine tailings near Te Aroha, is considered New Zealand’s worst environmental disaster caused by mining activities. The site consists of a 1.5 ha tailings dam containing 100 000 cubic metres of toxic mining waste, principally sulphide minerals with high concentrations of lead (0.5%), cadmium (26 mg kg-1) and mercury (8 mg kg-1). Continual oxidation of the sulphide produces sulphuric and sulphurous acids that result in a pH < 3 for the surface material. The low pH mobilises heavy metals that leach out of the tailings dam into a nearby stream. Analyses of the stream water and sediments reveal that both are above the allowable limits for lead and cadmium set by the World Health Organisation.

Although the site has been abandoned for more than 30 years, no vegetation has established on the tailings due to a low pH and the high concentration of heavy metals. During the summer months, dust containing high concentrations of heavy metals is blown around, contaminating nearby areas. Adjacent to the tailings dam is a car park used by hikers. Children have been observed playing on the tailings, their parents unaware of the risk of heavy metal poisoning.

The goal of phytoremediation at Tui is to mitigate heavy metal leaching, prevent erosion and dust movement, and to return the area to native vegetation. Plant accumulation of heavy metals is not desirable because this may provide an exposure pathway for metals to enter the food chain via herbivore browsing.

The pH and plant nutrient status of the tailings must be modified so that vegetation can be established. Experiments conducted at Massey University determined the optimal rate of liming and organic matter addition to permit plant-growth on the Tui tailings. A 100 m2 plot was established on the Tui tailings in April 2001. Several indigenous species were planted as well as lupin to fix nitrogen. The plants established rapidly (Fig. 1), with other plants and animals colonising the plot area over time.

Chemical analyses of harvested on plot leaf-material indicated little risk for bioaccumulation of Tui metals (Table). Elevated lead levels may be the result of surface contamination of the leaves with dust from adjacent non-vegetated areas. Greenhouse experiments with the Tui tailings, where there is no risk of dust contamination, demonstrated that leaf lead-levels never exceeded 50 mg kg-1. Samples of surface material on and off the vegetated plot were analysed for TPF, an indicator of microbial activity (Fig. 2). Microbial activity was significantly higher under the vegetated areas. The measurable TPF concentrations off the plot may be due to the presence of Thiobacillus bacteria that are responsible for sulphide oxidation.

Clearly, it is possible to establish vegetation on the Tui mine tailings. Full-scale phytoremediation would eliminate lead-laden dust, reduce the visual impact of the mine and reduce leaching by re-evaporating some of the rainfall through transpiration. Phytoremediation of the Tui tailings can be considered a revegetation operation. Quantification of the environmental benefit afforded by phytoremediation can be made through analysis of the biological activity of the substrate.

Table. Average (5 samples) metal concentrations (mg/kg) in Tui tailings and supported plant species. Values in brackets are the standard error of the mean.

Figure 1. Experimental plot on the Tui mine tailings, Sep 2002.

Figure 2. Tailing TPF (triphenylformazan) concentration, a measure of biological activity, on and off the phytoremediation plot.

Related publication

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.