A guide to leading practice sustainable development in mining

Topsoil treatments
For soils likely to be dispersive or acid generating, the use of amendments such as
gypsum or lime will be required. In some cases it may be necessary to inoculate with
symbiotic micro-organisms such as nitrogen-fixers and mycorrhizae. Ripping along
contour will usually be required to facilitate root penetration through compacted
spoil material and to reduce seed loss.
Fertilising will also be required in most cases to replace the nutrient bank lost
during vegetation removal and the mining process. It is essential that the types and
methods of application of macro-nutrients and micro-nutrients are carefully planned,
based on detailed soil characterisation studies and rehabilitation objectives and
targets. Inorganic fertilisers are most commonly used; however, organic fertilisers
such as sewage sludge or vegetation mulch can be a cost-effective alternative
provided care is taken not to introduce weeds and high concentrations of metals.
Where topsoil contains a viable native seed source, it should be conserved for reuse
following mining. This not only provides a cheap source of plants, but helps ensure
that they establish in relative abundances that reflect pre-mining densities, and
promotes establishment of species whose seed may be hard to obtain or difficult
to germinate. The bauxite mine rehabilitation program conducted by Alcoa World
Alumina Australia in the jarrah forest of south-western Australia is an excellent
example of how conservation of the soil seed bank can significantly enhance the
botanical diversity of the post-mining vegetation community. After vegetation is
cleared, the top 150 millimetres of soil, which contains most of the soil seed bank
and nutrients, is stripped prior to mining and then directly returned to a pit about
to be rehabilitated, wherever possible. Research has shown that the majority of
native plant species (72 per cent) on rehabilitated areas comes from seed stored in
topsoil. The importance of directly returning fresh topsoil has been demonstrated by
trials comparing this technique with stockpiling. These have shown that disturbance
associated with direct return of topsoil results in loss of less than 50 per cent of the
seed contained in the pre-mining forest seed store; by contrast, stockpiling results in
losses of 80 per cent to 90 per cent. Other aspects, such as the depth of respreading
topsoil, the season when the soil is handled and the timing of seeding, are also
important. Seed will not survive if buried too deep, and persists better when the soil
is moved during the dry season. Also, plant establishment from seeding is greater
when the seed is applied to a freshly disturbed surface. Together, the combined
use of fresh topsoil return, seeding, and planting of ‘recalcitrant’ plants have now
resulted in numbers of plant species at 15 months-of-age equal to those recorded in
equivalent-sized plots in unmined forest (see LP Rehabilitation p.40).
166 LEADING PRACTICE SUSTAINABLE DEVELOPMENT PROGRAM FOR THE MINING INDUSTRY