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Vegetation for dust suppression on gold mine tailings
The Macraes mine in Central Otago produces about 5 million tonnes of tailings each year. These tailings are discharged to purpose-built storage facilities that are initially covered with water. However, when the water is recycled to the processing plant, and the tailings are allowed to dry, the surface becomes a large expanse of silty sand, up to 1 km across. This sediment can be picked up by strong winds to form a large dust cloud. This dust is a physical nuisance in the area, and also contains about 1000 parts per million arsenic, so formation of tailings dust clouds is a significant environmental issue at the mine site. Rapid establishment of a vegetation cover is one way of limiting dust mobility.
The tailings are an extremely harsh environment in which to establish vegetation. The tailings are crushed fresh rock, so no soil development has occurred, and so nutrients are sparse. The tailings have an open texture which permits water drainage and evaporation, so the surface zone is commonly dry. There is little cohesion, so the tailings readily blow around. Evaporation concentrates dissolved ions in any moisture in the tailings, causing precipitation of salts, especially gypsum (CaSO4.2H2O) and minor scorodite (FeAsO4.2H2O).
Establishment of rye corn after broadcast seed dispersal is difficult because wind erodes the tailings and blows the seeds around. Also, when seeds have germinated, wind erosion exposes the roots and the plants dry and die. Best success was obtained when broadcast seeds were covered with a thin layer of oxidised waste rock from the mine site. This thin cover layer anchored the seeds and helped to retain moisture so that seedlings could become established.
Even after this covering treatment, strong winds can erode the tailings surface and redistribute tailings. However, once the rye corn was established, it remained in place and was effective at capturing blowing tailings. The rye corn patches provided focus points for development of tailings sand dunes up to 50 cm high. This demonstrates the effectiveness of plant establishment in stabilising the tailings surface.
Complete coverage of the tailings impoundment by seeds and a thin anchoring layer is likely to be expensive and difficult. However, partial coverage with seeded and anchored strips is likely to be just as effective, as depicted in the diagram below.
Further information
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