Tailings Disposal - Mining and the Environment: From Ore to Metal - page 749

will be an important factor - in perpetuity. A spillway is usually required to accommo-
date excess water, with regular maintenance required to ensure that the spillway remains
functional - again, in perpetuity. Water supply reservoirs are usually considered to have
operating lives of 100 to 300 years. In practice, there have been many water retention
structures that have had to be substantially strengthened or re-built after periods much
shorter than this. In the case of tailings storages, it is unclear who will be responsible for
maintaining, strengthening or re-building them when required, or if anyone will even be
responsible for assessing their condition after decades or centuries of perhaps trouble-free
experience. More likely, the mining history of the land and its inherent risks will have
been largely forgotten by that time.
In rare cases, tailings are returned to the mine Pit(s) as part of mine closure. Normally, this
would be uneconomic unless it was factored in to the original feasibility study, and the neces-
sary funds accrued during the operating period. Such an outcome was a condition of develop-
ment of the both the Nabarlek Uranium Mine and Ranger Uranium Mine in the Northern
Territory of Australia. In the case of the Woodcutters base metals project in the same region
of Australia, the owner, Normandy Mining, decided during closure planning studies that the
long-term risks associated with the tailings which were known to be acid-generating, could
only be reduced to an acceptable level by below-grade storage in the mine Pit.
The above cases are exceptions. Rehabilitation of most tailings storages will involve a
series of steps such as the following:
1. Removal of surface water from the surface, repeated as necessary until the surface has
dried sufi ciently to support trafi c;
2. Once the surface is trafi cable, there is usually a requirement for earthmoving to pro-
vide drainage and to construct a landscape consistent with the intended land use,
rather than a l at basin, subject to inundation during each rainfall event;
3. Depending on the results of i eld trials, the selected crops may be established directly
on the dewatered tailings, with whatever soil amendments are indicated by the trials;
4. In other cases where tailings are found to be an unsuitable medium for growth of the
selected species, a layer of natural soil is placed over the tailings, prior to planting;
5. In yet more difi cult cases, establishment of sustainable agriculture or horticulture might
require capping of the tailings with clay, prior to placement of the soil growth medium,
or a capillary barrier may be required between the tailings and the natural soil layer;
6. Planting of one or more leafy crops which are subsequently ploughed in to provide
organic content to the soil;

7. Establishment of the selected crop(s);

8. Monitoring over at least several years to assess the sustainability of the selected land use.

In tropical areas, particularly
those where rainfall exceeds
evaporation, rehabilitation of
tailings storages is much more
diffi cult than in arid, semi-arid or
most temperate climates.

It should be emphasized that in tropical areas, particularly those where rainfall exceeds
evaporation, rehabilitation of tailings storages is much more difi cult than in arid, semi-
arid or most temperate climates. Rehabilitation in such high rainfall areas may take many
years, particularly if the tailings have been very i nely ground, in which case rates of con-
solidation will be very slow.

18.4 SUBMARINE TAILINGS PLACEMENT

Submarine Tailings Placement (STP) is an engineered technology developed as a disposal
alternative for mines in close proximity to coastlines. Application of STP is evaluated on a
case-by-case basis, and is an option where land-based disposal would have more substantial
environmental impacts or high risks. A detailed set of screening criteria has been developed