Revegetation of mine tailing impoundments and associated environmental issues

King, Annette Rachel.

Macraes mine is a hard rock gold mine and currently has New Zealand's largest identified gold resource (186 t). Ore from the mine is processed and pumped as slurry to large tailings dams: these are permanent repositories for tailings. Tailings are often enriched in metals and metalloids. At the Macraes mine site the metalloid arsenic is present naturally in the tailings. Arsenic is considered a major environmental contaminant often having detrimental impacts on human health. Revegetation is being investigated as a tool to stabilise the tailings, reduce dust production and leaching. A series of studies were conducted investigating the establishment of rye corn on the tailings, the takeup of arsenic by rye corn and various pasture species, the leaching of tailings material and the accessibility of geologic nitrogen to rye corn. A series of field trials were set up over a period of 3 months within a disused tailings dam (SPlO). Rye corn, a cereal, was successfully established directly in the tailings with the use of oxidised schist to anchor the seeds and superphosphate fertiliser. The rye corn was effective in stabilising the surface and collecting wind blown sand. Strong winds and desiccating conditions killed off some of the rye corn plants. Root structures from live and dead plants successfully anchored the tailings. The levels of arsenic in vegetation grown in the tailings trials were above the guideline values of 50 mg/kg for maximum tolerable levels of dietary minerals of cattle and sheep. An existing pot trial was sampled to ascertain the effect of different thicknesses of topsoil capping layers on arsenic uptake by pasture plants. Capping layers of 10 cm, 30 cm and 50 cm thickness produced rye corn plants with arsenic contents of 8.5-49.9 mg/kg, 6.4- 35.6 mg/kg and 4.2-11 mg/kg respectively. As capping thickness increased there was a decrease in the arsenic content in the pasture plants. Farmland soil and pasture species in the Macraes area were studied for arsenic content. Soil samples gave arsenic concentrations ranging from 18 to 595 mg/kg. The national guideline for allowable arsenic content (30 mg/kg) was exceeded by several of the samples. The arsenic contents of the vegetation growing in the farmland soils (0.9-2.6 mg/kg) were higher than typically found in New Zealand pastoral plants (0.09-0.12 mg/kg). There was no clear relationship between the arsenic content of soils and the arsenic content of the overlying vegetation. 11 A glasshouse study was set up to examine arsenic take up by rye corn in four different substrates: fresh tailings, farmland soil, tailings from an inactive tailings dam, and tailings from a lysimeter study. Average arsenic contents in the rye corn plants were very similar (2.7-5.6 mg/kg) even though substrate arsenic concentrations varied considerably (18-1620 mg/kg). This suggests that age and arsenic content of the substrates had only a small effect on the amount of arsenic taken up by the rye corn plants in the glasshouse study. Studies from this work and other researchers investigating arsenic take-up in rye corn growing in tailings from the Macraes mine were compared. The effects of confounding factors, superphosphate addition, plant structures, plant physiology and stress, study length and root interaction with tailings water were all examined. Of the factors considered, growing environment was thought to have had a substantial effect on arsenic take up by rye corn. The environmental conditions the plants were grown in affected root growth, water take up patterns, plant physiology. These in turn all affect the uptake of arsenic by plants. An existing lysimeter study was examined to observe whether there were changes in arsenic content of the tailings from the surface to a depth of 50cm. No trend emerged from the data. A leaching study using tailings was conducted over three different lengths of time 5, 15, and 30 days. Dissolved arsenic in water reached 0.275 gm-3 after 5 days, 0.730 gm-3 after 15 days and 0.672 gm-3 after 30 days. The results showed that water in contact with tailings contained dissolved arsenic and the amount of arsenic dissolved increased with contact time. 111 A glasshouse trial was set up to determine whether rye corn plants could access the geological nitrogen held within the schist. Rye corn growing in mineralised schist gave b15N values of -1.1 to 0.4 %o. Rye corn grown on quartz with urea fertiliser gave b15N values of -2.7 to 0.8 %o. b15N values for schist were 2.9 %o and urea --0.7 %o. The results indicated that rye corn plants could be able to access the geological nitrogen held within the mineralised schist.

xiii, 195 leaves : ill., maps ; 30 cm

2008King_A