Phytomanagement of mine tailings
The Tui mine tailings near Te Aroha, was considered New Zealand’s worst environmental disaster caused by mining activities. The site consisted 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 resulted in a pH < 3 for the surface material. The low pH mobilised heavy metals that leached out of the tailings dam into a nearby stream. Analyses of the stream water and sediments reveal that both were 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 had 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 was blown around, contaminating nearby areas. Adjacent to the tailings dam was a car park used by hikers. Children were observed playing on the tailings, their parents unaware of the risk of heavy metal poisoning.
The goal of remediation at Tui was to mitigate heavy metal leaching, prevent erosion and dust movement, and to return the area to native vegetation. Plant accumulation of heavy metals was 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 needed to be modified so that vegetation could be established. Biowastes (municipal compost) and lime were added to raise the pH of the material and reduce the plant-availability of toxic metals. 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 were the result of surface contamination of the leaves with dust from adjacent non-vegetated areas. Greenhouse experiments with the Tui tailings, where there was no risk of dust contamination, demonstrated that leaf lead-levels never exceeded 50 mg kg-1. Microbial activity (Fig. 2) was significantly higher under the vegetated areas compared to the bare tailings.
In 2012, the Tui mine site was re-engineered to reduce the risk of the dam collapsing. The tailings are now stabilised with cement and the site capped with topsoil.
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.
