Tailings Disposal - Mining and the Environment: From Ore to Metal - page 738

intercept and recover leachate so that it does not enter the groundwater. Seepage can be
intercepted by a subsurface drainage system such as an upstream cutoff drain along the
upstream toe of the embankment. A perimeter drain around the TSF may also be installed
to collect runoff and seepage water, should it occur.
It is important that the water balance not only considers average conditions and planned
operating scenarios, but also includes unusual ('upset') operational scenarios and extreme
meteorological events. Water balances should be established for each stage of the opera-
tion, as different components may vary widely from one stage to another. For example, the
quantity of water produced by mine dewatering may be zero at the initiation of mining
but will increase as mining penetrates further below the water table.
Equally important are the water chemistry characteristics considering geochemistry,
sources of make up water, and chemicals used in benei ciation or tailings treatment. In
benei ciation processes that introduce heat, a thermal energy balance considering tailings
temperature and density may also need to be included.
Given the many uncertainties and high variability of inputs and outputs, establishing
the water balance is often a difi cult task (Wels and Robertson 2003). As mentioned above
it is important that the conceptual water balance of all inl ows and outl ows also includes
all potential worst case scenarios (e.g. decant failure combined with snow melt and high
precipitation) (DPI 2003). The simplest method to determine the water balance of a TSF
is to determine average annual water inl ows and outl ows. Annual averages, however,
are insufi cient for actual design and to establish day-to-day operating conditions. Systems
designed on average values may be grossly inadequate for extreme conditions. A more
complex method is to use hydrological modelling (WMC 1998) which may use long-term
rainfall records if they exist, or synthetic records formulated for the purpose.
Establishing a water balance as a basis for design of a TSF should prevent water man-
agement problems occurring during operation and closure, providing that upset conditions
such as pump failures are also taken into account. A TSF needs to be designed to handle and
control the routine inl ows and outl ows as well as any unusual l uctuations, either due to
unusual operational or meteorological events. Poor understanding of water l ow and hence,
poor design of water management infrastructure and control methods increases the risk of
problematic situations during operation such as uncontrolled upstream inrushes, low free-
board, or high seepage. Another consideration is that high volumes of water may require
storage during the dry season to maintain plant production. Ice and snow melt, upstream
inundation (valley impoundment) and severe storm recurrences (e.g. 1 in 100 year) should be
considered when designing a TSF so that minimum freeboard levels, normally determined
by legislation, company policy, or industry codes ( Case 18.3 ) will not be reached. Freeboard
is used to establish the elevation of the lowest point of the embankment crest relative to the
normal or maximum operating levels of the supernatant pond ( Figure 18.16 ; SANS 1998;
DME 1999). As is the case with water containment, the TSF design may allow for spillways,
diversion channels and/or emergency pumps as additional emergency mitigation measures.
Spillways may need to be re-established with each raising of the TSF embankment.

It is important that the water
balance not only considers
average conditions and planned
operating scenarios, but also
includes unusual ('upset')
operational scenarios and
extreme meteorological events.

Poor design of water
management infrastructure and
control methods increases the
risk of problematic situations
during operation.

Site Selection for Onshore Tailings Storage Sites

Selection of suitable on-land TSF sites is typically based on human and environmental
risk, distance and elevation relative to the process plant, storage capacity and topography,
embankment height that can be achieved, catchment area, and geology, including founda-
tion conditions and potential for seepage. Overall, the approach to siting is similar to that
for waste rock storage.