Mine Closure - Mining and the Environment: From Ore to Metal - page 815

it has not been used. One such example is the Ranger Uranium Mine in Australia's
Northern Territory, where native vegetation is being established directly on the surface of
waste rock dumps. Another example is the large tailings deposition area of the Freeport
Gold and Copper Mine in Papua where reclamation is proceeding without the use of
topsoil.
The advantages of using topsoil are:

●

It usually contains a 'seed bank', representing all or most of the plant species that pre-
viously grew on it, including both pioneer and climax species;

●

By providing seeds of the same provenance, application of topsoil also helps to con-
serve genetic diversity;

●

It contains a variety of micro-organisms, many of them benei cial to the growth of the
plant species with which they are associated;

●

It is a proven growth medium in terms of texture, moisture absorption, moisture reten-
tion and lack of phyto-toxins or growth inhibitors.

Application of topsoil is particularly benei cial for the restoration of natural vegetation
communities. In a rehabilitation programme, it is usually impractical to plant more than
about 10 species by seeding or planting of seedlings. However, topsoil may contain 50 or
more species, leading to the rapid re-establishment of diversity without the need to wait
for the slow spread of species from unmined areas.
It has been established that extended stockpiling of topsoil results in the progressive loss
of viable seeds as well as a reduction in populations of micro-organisms. Much of the ben-
ei t may therefore be lost if the topsoil is not used soon after it has been excavated. 'Bottom
up' waste rock dump construction and other approaches that enable progressive rehabili-
tation, overcome this problem by providing sites for placement of freshly excavated topsoil
as shown in Figure 19.7 in Chapter Nineteen.
It is not always practical or necessary to remove and use only the topsoil layer per se,
as its thickness may vary considerably over short distances, particularly in high relief
areas. The approach here is usually to establish a 'stripping depth' that removes most of
the topsoil, and to excavate to this depth, resulting in some sub-soil being mixed with the
topsoil. Unless the sub-soil has a particularly difi cult texture or other deleterious con-
stituent, this approach is usually successful, and helps to augment limited topsoil supplies.
Sometimes, to meet specii c requirements of particular plant species, up to three differ-
ent soil horizons are separately excavated and replaced on the rehabilitation area, in their
original coni guration. Similarly, soil proi les may be artii cially constructed to suit plant
requirements using, for example, tailings sand for topsoil and slimes to provide sub-soil.
Additives such as compost or sewage sludge may be mixed with the sand to provide
organic content.
Tailings are themselves soil materials which in many cases provide excellent substrates
for plant establishment, without the need for a topsoil capping. The main requirement is
that they be free of phyto-toxic substances. They may benei t from amendments such as
pH adjustment, fertilizing, and/or addition of organic matter as part of the rehabilitation
prescription. Simple nursery or i eld trials can be carried out to assess the need for and
effectiveness of such amendments. Husin et al. (2005a and 2005b) describe successful estab-
lishment on tailings from the Grasberg operations, of a variety of natural forest species as
well as horticultural species, with and without soil amendment.
In summary, while not always essential in mine land rehabilitation, application of top-
soil usually offers benei ts that more than justify the effort and costs involved. This is
particularly so for cases where mined areas are to be rehabilitated with the same natural
vegetation communities that occurred prior to mining.

Extended stockpiling of topsoil
results in the progressive loss
of viable seeds as well as a
reduction in populations of
micro-organisms.