Bauxite Mining Rehabilitation

Alcoa of Australia’s aluminium process starts with the mining of bauxite ore in the Darling Range south of Perth.  We clear selected sections of jarrah forest to access bauxite, and are a recognised world leader in mine site rehabilitation. We were the first mining company in the world to achieve 100% plant species richness in our rehabilitated mine site areas.

In 2009, we adjusted the way we calculated our species richness statistics to account for recalcitrant plant survival.  The number in the table below under 2008 has been calculated to be consistent with previous years statistics, however if we take into account the recalcitrant survival, our figure is slightly lower at 98.8%.


Click image to enlarge.




* Alcoa is unable to provide a species richness figure for 2009 at this stage because we do our monitoring 15 months after the rehabilitation has been planted. The 2009 species richness figure will be provided in the 2010 Sustainability Report.

Rehabilitation Research and Trials in 2009
Research by Alcoa, over a number of years, has shown low similarity of the natural soil seedbank between areas with similar vegetation composition. This finding led Alcoa’s rehabilitation scientists to ponder whether mixing screened soil from two sites would increase species richness in rehabilitation.  Soil screening involves removing the inert gravel fraction of topsoil, thereby concentrating seed and reducing the required application rate, making transport costs lower. An operational trial, established in 2008, took place within our rehabilitation areas. The results after one year indicate that mixing screened soil from two donor sites significantly increased species richness in rehabilitation. We can therefore apply less screened soil in future rehabilitation areas, without impacting the performance of the rehabilitation. This presents a cost saving in processing and applying screened soil for rehabilitation areas.

Salicylic Acid naturally occurs in plant tissue and, among other functions, it plays a vital role in a plant’s defence against infection and drought stress.  It was thought that when sprayed on plants, Salicylic Acid may activate a stress response which may increase the survival of planted recalcitrant species.  Alcoa conducted a Salicylic Acid (Benzoic Acid) trial on 2008 rehabilitation which tested this theory - however, testing showed no significant survival response.  One year after planting, survival of plants sprayed with Salicylic Acid was 50.8% compared with 55.6% for untreated plants.

Fertiliser application can affect plant species’ composition, within rehabilitated areas, by favouring the growth of fast growing vigorous species, or in some cases by directly inhibiting the growth of sensitive species such as the Banksia family (or Proteaceae). A research project using jarrah forest soil found that Proteaceae species are not inhibited by high phosphorus fertiliser rates, but did not show increased growth when supplied with phosphorus.  In contrast, the fast growing eucalypts and acacias responded strongly to phosphorus fertilizer and out-competed the slower growing Proteaceae.  As a result of these findings, the fertilizer application rate was decreased in the 2009 rehabilitated areas to reduce the competitive advantage of the fast growing species and encourage a higher diversity of slower growing Proteaceae species.

In the Western Australian jarrah forest, where Alcoa mines bauxite, more than 80% of the rainfall is transpired by vegetation or lost into the atmosphere by evaporation. The remainder finds its way into streams that sustain aquatic ecosystems and flow into Perth’s water supply dams.  In 2009, we trialled areas of “Hydrologically Considerate Rehabilitation” (HCR) at the Huntly and Willowdale mines.  This involved rehabilitating with a deliberately reduced density of trees and a lower biomass of understory plants, compared with the standard rehabilitation prescription.  In addition, the deep ripping operation was modified to try to yield a higher proportion of water runoff from the areas. It is expected that a greater proportion of the rainfall that falls onto these rehabilitated areas will end up in streams and find its way into water supply dams.  This research is part of Alcoa’s commitment to ensuring the re-establishment of a sustainable multiple-use jarrah forest after mining.

Within the 2009 rehabilitation area, mechanical seeding of Macrozamia riedlei (zamia) seeds was trialed using the screen soil spreader device for one pit at both Huntly and Willowdale. Results of the field establishment for the initial trial will be available in mid-2010. Initial observations indicate that mechanical seeding using the screen soil spreader works well.  Hand sowing of zamia seed is expensive, physically demanding and can lead to injuries.  Significant improvements in safety and cost efficiencies will be realised if this operation can be mechanised.

Read more at www.alcoa.com.au/bauxitemining.




Click image to enlarge.




Click image to enlarge.




Click image to enlarge.




Click image to enlarge.