In the third article of this series on the rehabilitation process, steps four to seven of the ten step rehabilitation process proposed by Toy and Daniels (1998) are discussed by Katina De Sousa, Astron’s Rehabilitation Principal Scientist. These steps cover the stages of preparing a landform for revegetation.
- Site characterisation
- Rehabilitation planning
- Material management
- Topographic reconstruction
- Replacement of topsoil and soil reconstruction
- Surface manipulation
- Addition of soil amendments
- Site monitoring and maintenance.
Landform profiling is a key step in the rehabilitation process as the resulting landscapes form the basis for all other rehabilitation activities and the surface for future land uses. Rehabilitation earthworks is also the stage at which there is a very high potential for budget over runs or design requirements not being met if not managed well. Hard learnt lessons from managing rehabilitation earthworks have highlighted the need for regular (daily if possible) on site management to achieve desired outcomes. Using skilled operators that have a good understanding of what is required is likely to result in the best outcome with the least stress and conflict. Rehabilitation earthworks is a specialist skill, and not every dozer operator will have the experience required to achieve the result required.
Effective rehabilitation is often measured by the attainment of long term goals, including vegetation cover and diversity, hydrologic response and the return of the site to productive use. These long term goals are achieved through daily conformance to material handling plans. Although the selection of appropriate vegetation is a critical component of rehabilitation plans, it cannot compensate for poor material handling and soil reconstruction practices.
Everyone knows that in most cases topsoil has the necessary physical, chemical and biological characteristics to support plant development. In general, topsoil layers are higher in organic matter, microbial activity and nutrients than the underlying subsoil and contain a seed bank. However, more topsoil is not necessarily better. Experience has shown that applying topsoil at depths of more than 10 to 20 centimetres can result in excessive erosion (as well as wasting what is a valuable and limited resource on most mine sites). Many mine sites in the Goldfields have had to rework rehabilitation areas to remove excessive topsoil where it has historically been applied too thickly.
On the other hand, as discussed in the second article in this series lack of topsoil is a common problem, especially for older mine sites. In this situation it is worth looking at the other materials available on site to see if there is an alternative that can be used as a soil substitute. One of the most important criteria for selecting a topsoil substitute is the cost and practicality of selectively handling the identified material within the mine plan. Lateritic subsoil has successfully been used as a substitute for topsoil at many sites in the Goldfields. In some cases its use can be more successful than topsoil as its rockier nature provides a degree of erosion protection on slopes.
Surface manipulation is generally conducted to facilitate water management and erosion control. Activities such as ripping on the contour break up soil crusts, roughen the surface and relieve soil compaction caused by equipment traffic. A heterogeneous soil surface can provide a variety of microsites for seed lodgement and seedling establishment (Harper et al. 1965). In climatic regions like the Goldfields and Pilbara where soil moisture limits plant growth, surface manipulations also expedite revegetation growth by increasing infiltration capacity of the surface, increasing soil moisture and decreasing surface runoff.
The final step in preparing the soil for revegetation is the application of amendments, such as gypsum, inorganic fertilisers and organic materials. Addition of soil amendments is generally not required in Australia as the native vegetation is adapted low nutrient conditions. Application of fertiliser can increase the growth of weed species (Norman et al. 2006) which compete with native vegetation for water, light and resources. Some native species suffer from luxury uptake of nutrients if they are applied at too high a level, particularly phosphorus (Lambers et al. 2013). However, in the jarrah forests of southwest Western Australia Alcoa consider fertiliser application to be a necessary component of the rehabilitation process to increase nutrient stores and encourage nutrient cycling in order to reach a productivity level equivalent to the premining forest (Norman et al. 2006).
Keep reading for the final instalment of this series, Part 4 – How do I get plants to grow on this thing? - where revegetation, irrigation, site monitoring and maintenance are discussed.
Loading of topsoil for rehabilitation
Rehabilitation earthworks, Goldfields WA
Application of topsoil on a waste rock dump
Harper, JL, Williams, JT, Sagar, GR 1965, ‘The behaviour of seeds in soil: I. The heterogeneity of soil surfaces and its role in determining the establishment of plants from seed’, Journal of Ecology, vol. 53, pp. 273-286.
Lambers, H, Ahmedi, I, Berkowitz, O, Dunne, C, Finnegan, PM, St J. Hardy, GE, Jost, R, Laliberté, E. Pearse, SJ and Teste, FP 2013, ‘Phosphorus nutrition of phosphorus-sensitive Australian native plants: threats to plant communities in a global biodiversity hotspot’, Conservation Physiology, vol. 1, pp. 1-21.
Norman, MA, Koch, JM, Grant, CD, Morald, TK and Ward, SC 2006, ‘Vegetation succession after bauxite mining in Western Australia’, Restoration Ecology, vol. 14(2), pp. 278-288.
Toy, TJ and Daniels, WL 1998, ‘Reclamation of disturbed lands’, in RA Meyers (ed) Encyclopedia of Environmental Analysis and Remediation, Wiley, New York.