Australia’s mining and resources project outputs for 2015-2016 was valued at approximately $158 billion, with $88 billion (56%). This makes WA’s mining and resource sector the largest in Australia with the State producing at least 50 different minerals from almost 1,000 current operating mines.
Western Australia has a long mining history of greater than 150 years. The abundance of mineral wealth in WA combined with its long development history has resulted in the State’s extensive understanding and experience with the industry, its social contribution, potential environmental impacts and the necessary regulatory framework for its management.
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 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. As a result, mining is often an unwelcome development, and mining developers are often denied access to land, especially where potential conflicts with nature conservation are foreseen.
Facts on Soil Degradation
Global demand for gold has led to a massive increase in mining activity around the world. During the last decade, gold mining grew significantly in Australia becoming a major driver for land degradation and heavy metal contamination. However, few studies have explored soil degradation, reforestation, and plant mercury accumulation after mining operations.
In this study, we established a reforestation field experiment in a gold mined area. We tested the outcome of planting seedlings of four native tree species previously grown in nursery polyethylene bags versus planting bare root seedlings, as well as the effect of three levels of biofertilization on seedling survival and growth.
Our results revealed that soil texture becomes disproportionately sandy, while organic matter content and cation exchange capacity were seven- and three-fold lower in the mined area than in the reference forest, respectively. Seedling survivorship and growth varied across planting methods, biofertilization intensity, and species. Even in the bare root planting technique seedling survivorship was highly acceptable (75%) and increased with transplanting (83%) and the addition of biofertilizer (92%).
What is Rehabilitation?
Rehabilitation, from the mining industry perspective, means putting the land impacted by the mining activity back to a sustainable usable condition. This definition (and implied intention) includes the concepts of minimization of loss of land use capability and of net benefit to society.
Mining takes many forms, but in its simplest division it either involves underground mining with minimal surface disturbance (e.g. board and pillar mining to specified safety factors or deep vein mining); underground mining with significant surface disturbance (e.g. shallow longwall mining or block caving); surface open-pit mining (where the pit overburden material is relocated to permanent overburden dumps); and surface strip-mining (where the pit overburden material is replaced in an adjacent open mined section).
From the rehabilitation perspective, the key factors to consider during the preparation for mining (construction) phase are to minimize the area affected by the development, minimize potential future contact of toxic or polluting materials with the environment, and to maximize the recovery and effective storage of those mining profile materials that will be most useful during the rehabilitation process after mining is complete.
Mine Rehabilitation Program
The first task in developing an effective mine rehabilitation program 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.
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 programs 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 enough to establish and maintain a self-sustaining ecosystem.
Restoration of species diversity can be a critical objective for rehabilitation programs aimed at re-establishing native ecosystems. Success in this endeavor 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.
Rehabilitation Step by Step
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 long shank. 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. 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 like 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.
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 understory also establishes rapidly.