John Gardner is Environmental
Manager for Mining, Alcoa World
Alumina Australia, Applecross,
Mine rehabilitation should establish land use values equal to or better than those existing before mining.
Mining of bauxite by tracked excavators and 105 tonne trucks
The science and technology of land rehabilitation and restoration ecology have advanced to a high level, allowing rehabilitation to meet complex and multiple objectives. Selection of post-mining land use objectives should be based on factors including compatibility with surrounding uses, community expectations, biodiversity protection and regulatory requirements.
This article presents the case of Alcoa World Alumina Australia's mine rehabilitation programme to meet multiple land use objectives in the jarrah (Eucalyptus marginata) forest of southwestern Australia. Advanced techniques for land preparation, soil handling, seed treatment and plant propagation are used to re-establish the native forest ecosystem.
Where mining takes place, the land is usually cleared of all vegetation, the landscape drastically altered and the ecosystem totally disrupted. If inappropriately managed, mining activities can also have significant off-site impacts, particularly from the discharge of drainage contaminated with sediments, chemicals, metals or altered acidity. Mining operations can also introduce pests, predators and diseases into natural ecosystems, and can open up isolated areas to further human-induced disturbances. In too many instances, mines have been abandoned in a highly disturbed condition, with limited or no rehabilitation treatment. They are eyesores, can have destructive environmental impacts, and are an unwelcome legacy for governments and communities to deal with. These "sins of the past" - and some continue today - severely damage the reputation of the mining industry. As a result, mining is often an unwelcome development, and mine developers are often denied access to land, especially where potential conflicts with nature conservation are foreseen. Yet mining and minerals have an essential role in future development, with a role in raising and maintaining standards of living throughout the world.
If the mining industry is to contribute effectively to future sustainable development, it must develop and consistently apply sound environmental management practices worldwide. Among these, it needs to minimize environmental impacts on and off site during the operational mining phase. It also needs to extract and use resources efficiently, and to encourage the efficient processing and use of its products. While minerals are a non-renewable resource, in many cases they can be efficiently reused and recycled.
Consistent with sustainable development principles, mining operations should be intended as a transient land use. This means that after mining, the condition of the land should be restored so that its value is similar to or greater than it was before disturbance.
The value of land can be measured in economic, social or ecological terms. There are many examples in which mined land has been effectively rehabilitated to agriculture, forestry, nature conservation or urban or industrial land uses (Laurence, 2001). In some of these instances the pre-mining land use was restored, while in others the land use was changed. Some of the changed land uses were carefully planned and implemented, while others have evolved, sometimes after the land has undergone a lengthy period as abandoned or waste land (Moffat, 2001). Mine rehabilitation (also referred to as reclamation) should be the process of converting mined land to its future valuable use - not a process of burying wastes, smoothing out the landscape and applying a green mantle of relatively valueless vegetation.
The first task in developing an effective mine rehabilitation programme is to set a clearly defined post-mining land use objective. It should be compatible with surrounding land use; it should support species diversity; it should be consistent with the expectations of the local community; and the landowners and regulatory agencies must agree to it. An understanding of future landownership is critical. Despite the best of intentions, it might not be worthwhile trying to establish a productive land use like fruit growing on undeveloped common land; when the company leaves, who will reap the benefits and who will take care of the maintenance? Who will be responsible for the land if it degrades through lack of maintenance?
When the appropriate land use objectives are set, then rehabilitation can commence. First and foremost, the disturbed mined areas need to be returned to a safe and stable physical state that is integrated with the surrounding landscape. Safety should be considered in terms of risks to humans, domestic animals and wildlife, but the rehabilitated site should also reflect the surrounding landscape; if natural cliff faces or steep and rocky slopes occur locally, these features may be acceptable for aesthetic or habitat values. On the other hand, while nature includes some unstable landforms, it is hard to imagine a justification for leaving or creating them. Stable soils are more likely to revegetate effectively and sustain productivity, and will maintain a protective cover over any hostile materials buried beneath them, such as acid-generating rocks or subsoils with toxic salt or metal concentrations. Stable soils will also avoid off-site impacts such as turbidity (muddiness) and siltation of watercourses.
Most rehabilitation programmes also involve some form of vegetation establishment (revegetation). Regardless of the land use objective, the chosen vegetation must be productive and sustainable. If the vegetation is for commercial use, then productivity levels need to be competitive with similar enterprises on natural soils. Where native vegetation is restored, productivity levels must be sufficient to establish and maintain a self-sustaining ecosystem. Restoration of species diversity can be a critical objective for rehabilitation programmes aimed at re-establishing native ecosystems. Success in this endeavour is often dependent on first establishing the appropriate habitat and ecosystem recovery processes that will subsequently encourage the full suite of flora and fauna to recolonize.
Much is known of the science and technology of mine rehabilitation and restoration ecology today. Books, journals, conferences and professional societies are devoted to the topic. This was not the case when Alcoa World Alumina Australia (referred to here as Alcoa) commenced mining bauxite in the jarrah forest of southwestern Australia in 1963. Initially, mining was at a relatively small scale, with about 30 ha of forest cleared and mined each year. The bauxite deposits occur at the surface, and mining involves removal of shallow topsoil and overburden (approximately 0.5 to 1 m depth) before excavation of ore to an average depth of 3.5 to 4 m.
Rehabilitation activities commenced in 1966 but were of a very rudimentary nature. Imported species of pine and eucalypt species from eastern Australia were established in monoculture plantations. The mine pits received no recontouring; overburden and topsoil were respread without any cultivation of the subsoil; and trees were planted with an arbitrary amount of organic fertilizer. The target land use for these areas was timber production - yet the trees generally grew very poorly and many were uprooted and toppled by wind as a result of nutrient deficiency and the failure of roots to penetrate the compacted mine floor soils.
The company recognized quickly that performance was substandard and that improvement was required. Work commenced immediately on a programme of research and field trials. Within five years improvements were incorporated into the mine rehabilitation programme. This work has continued for three decades, to a point where the quality of mine rehabilitation has reached a very high standard (Baker, Gardner and Ward, 1995).
Today's rehabilitation objective reflects the multiple land use management of the jarrah forest, the expectations of the community and the rehabilitation techniques and capability, which have mostly been developed locally. The jarrah forest covers some 1.8 million hectares, most of which is publicly owned and managed as State forest. The vegetation community is tall open forest dominated by jarrah (Eucalyptus marginata) and marri (Corymbia calophylla). This community is botanically diverse with an estimated 780 plant species occurring in the forest region. Less than 10 percent of the forest remains in an old-growth condition. About 15 percent of the forest is secure in conservation reserves that sample the range of forest plant communities and protect remaining old-growth areas. Mining is not permitted in conservation reserves and, to date, all areas mined have been selectively logged at least once.
Ecologically sustainable forest management principles are applied in the management of the forest for nature conservation, catchment protection, timber production, tourism, recreation, mining and wildflower industries (Department of Conservation and Land Management, Western Australia, 1994). The forest is at the back door of Perth, the capital of Western Australia, with a population of over 1 million people. This population places significant demands on the forest but at the same time takes a great interest in its welfare.
Winged ripping tine on the back of a bulldozer
Alcoa operates two bauxite mines at Huntly and Willowdale, approximately 90 km and 135 km southeast of Perth, respectively. A third mine at Jarrahdale ceased production in 1998 and has now been decommissioned and fully rehabilitated. Currently, about 550 ha are mined and rehabilitated annually. Since the commencement of mining, 12 560 ha have been cleared and 10 600 ha have been rehabilitated. Alcoa's rehabilitation objective is to "return a self-sustaining jarrah forest ecosystem that fulfils all of the pre-mining land uses". The specific conservation goal is "to encourage floral, faunal and soil characteristics similar to those of the indigenous jarrah forest ecosystem" (Nichols et al., 1991). A rehabilitation prescription, formally agreed between Alcoa and key regulatory agencies, was first developed in 1979 and is reviewed biennially. Completion criteria and standards for the eventual return of rehabilitated areas to full government management have also been developed and approved (Elliott et al., 1996). Regulatory, academic, conservation group, industry and community participants reviewed the criteria and standards before government approval was given.
Rehabilitation commences with re-shaping of the 2 to 5 m high pit walls to a maximum slope angle of 18º. Recontouring of the mined-out pits aims to mimic the original, natural landscape. The overburden, which is stripped separately from the topsoil and usually stockpiled nearby, is then respread. Topsoil is then returned from newly cleared areas (a practice called direct return) or from stockpiles of topsoil when there is no opportunity for direct return. Direct return of fresh topsoil enhances the return of viable seeds, nutrients, organic matter and beneficial soil micro-organisms. To maintain these important soil properties at the surface, the topsoil is stripped and returned in as thin a layer as possible, generally 10 to 15 cm.
Following topsoil return, a few tree stumps, logs and rocks are returned to the mined areas to provide habitat for fauna. The ground is then ripped to 1.5 m deep using a tine with wings. Ripping is carried out in summer and autumn to maximize shatter of the compacted subsoil. Contour lines at 3 to 5 m vertical intervals are surveyed and marked in the field and ripping accurately follows the contours. The ripping creates furrows approximately 0.4 m in height and 1.5 m wide. The contour furrows are critical for preventing rainfall runoff and soil erosion.
Immediately after ripping and before the onset of autumn rains, a seed mix of a wide range of local plant species (70 to 100 species) is broadcast on to the freshly cultivated ground. Seeding immediately after ripping maximizes plant establishment from the applied seeds (Ward, Koch and Ainsworth, 1996). Seed is either broadcast by hand or applied directly on to the freshly ripped ground by a seeding machine attached to the ripping bulldozer. The seed mix is applied at about 2 kg per hectare. Seeds of the dominant tree species, jarrah and marri, are included in the mix at rates that establish these trees in a proportion similar to that in the natural forest. Only indigenous species are included in the seed mix, and all the seed is sourced from within about 15 km of each mine to retain local genetic material in the rehabilitated areas.
Other plant species that are not easily re-established from seeds, because of low seed viability, germinability or availability, are propagated in containers at Alcoa's nursery and laboratory using tissue culture, cuttings or seed. Alcoa planted 215 000 of these "recalcitrant" plants at the mines in 2001.
Fertilizer is applied to the rehabilitated areas in late winter or early spring by helicopter. A mixed fertilizer (NPK and micronutrients) is applied at 500 kg per hectare.
Winter rainfall is generally reliable, and plants establish well in the first year. Provided contour ripping is carried out effectively and erosion is avoided in the first year, the sites stabilize and are not prone to erosion in subsequent years. Tree height growth from broadcast seed is usually less than 0.5 m in the first year but of the order of 1 m per year in subsequent years. The understorey also establishes rapidly.
Seeding rates are designed to establish a minimum of one leguminous plant per square metre and between one-half and one non-legume plant per square metre. With the good availability of nutrients from broadcast fertilizer, tree and understorey cover approaches that of the adjacent forest within the first five years. At this age, many short-lived plant species, especially acacias, start to grow old, contributing to the rapid development of a litter layer.
Deep ripping along the contour of landscape mine pit after return of overburden and topsoil
Various monitoring programmes are carried out in the rehabilitated areas. Initial monitoring at nine months checks that tree and understorey establishment requirements have been met. Rehabilitated areas are also inspected for erosion problems. At 15 months, plant species richness is measured.
Studies of plant succession, vegetation pattern, tree growth, biomass accumulation, nutrient cycling, water use and timber quality have been undertaken (Alcoa World Alumina Australia, 2001). Several fauna recolonization and succession studies have also been completed. These confirmed that the food and habitat requirements of animals are being met. They also identified shortcomings in the rehabilitation practices that required attention, such as the need to place more logs, tree stumps and rocks to restore ground habitat (Nichols and Gardner, 1998).
Alcoa's recent push for improvement in rehabilitation practices has aimed to increase species richness in rehabilitated areas to the same level as the adjacent forest. In 1992 the species richness in rehabilitated areas was just over 60 percent of the forest average. At the last measurement in areas rehabilitated in 1999, average species richness was 96.8 percent of the forest average. Improvements have come from improved topsoil handling methods, seed collection, treatment and application methods and the planting of nursery-grown recalcitrant species. By and large all monitoring indicates that the rehabilitated areas are developing towards the stated objective. However, given the level of disturbance from the mining activities and the age that the forest will need to reach to fulfil all of its functions, it will take some time to confirm this.
Completed rehabilitation with logs and rocks for fauna habitat and planted recalcitrant species; some species are guarded to prevent grazing by kangaroos during establishment
Rehabilitation site after one year
Rehabilitation site after 14 years
The Australian mining sector is fortunate to operate in a sound and stable legal, regulatory, policy, strategic planning and investment environment and to have clear environmental impact assessment, planning, implementation and monitoring guidelines to follow. Yet although this is undeniably a helpful foundation for the successful operation of the mines and the subsequent rehabilitation process, the performance of Alcoa has been largely driven from within, locally and beyond regulatory compliance. Indeed, Alcoa is working to transfer best practice and adopt the same rehabilitation principles and standards worldwide - and in so doing, to demonstrate that best practices are not only achievable in stable, developed countries. There is much opportunity for large international companies to help with the improvement of environmental standards for mining around the globe and to promote capacity building in the regulatory, technical and academic communities in developing countries.
Alcoa World Alumina Australia. 2001. Bibliography of environmental department publications. Alcoa World Alumina Australia Environmental Research Note No 11. Internet document: www.alcoa.com.au/environment/bib_internet.pdf
Baker, S.R., Gardner, J.H. & Ward, S.C. 1995. Bauxite mining environmental management and rehabilitation practices in Western Australia. In Proceedings of the Australian Institute of Mining and Metallurgy, World's Best Practice in Mining and Mineral Processing Conference. Sydney, Australia, 17-18 May 1995.
Department of Conservation and Land Management, Western Australia. 1994. Forest Management Plan 1994-2003. Kensington, Australia.
Elliott, P., Gardner, J., Allen, D. & Butcher, G. 1996. Completion criteria for Alcoa of Australia Limited's bauxite mine rehabilitation. In Proceedings of the 3rd International and the 21st Annual Minerals Council of Australia Environmental Workshop. Newcastle, Australia, 14-18 October 1996.
Laurence, D.C. 2001. Mine closure and the community. Mining Environmental Management, 9(4): 10-12.
Moffat, A.J. 2001. Increasing woodland in urban areas in the UK - meeting ecological and environmental standards. In Forests in a changing landscape. Proceedings of the 16th Commonwealth Forestry Conference, Fremantle, Australia, 18-25 April 2001.
Nichols, O.G. & Gardner, J.H. 1998. Long term monitoring of fauna in bauxite mined areas of the Darling Range. In Proceedings of the Australian Centre for Minesite Rehabilitation Research workshop on Fauna Habitat Reconstruction. Adelaide, Australia, 10-11 October 1997.
Nichols, O.G., Koch, J.M., Taylor, S. & Gardner, J.H. 1991. Conserving biodiversity. In Proceedings of the Australian Mining Industry Council Environmental Workshop, p. 116-136. Perth, Australia, October 1991.
Ward, S.C., Koch, J.M. & Ainsworth, G.L. 1996. The effect of timing of rehabilitation procedures on the establishment of a jarrah forest after bauxite mining. Restoration Ecology, 4: 19-24.