Tailings Disposal - Mining and the Environment: From Ore to Metal - page 737

Water Balance

As with water dams, understanding the water balance of a TSF is essential to successful
operation. In fact water is cited as the underlying source of virtually all failures of tailings
storage facilities between 1980 and 1996 (Fourie 2003). All these failures occurred at facili-
ties that store their tailings by conventional impoundment methods. Fourie (2003) also
notes that the presence of large quantities of stored water is the primary factor contrib-
uting to most of the recent tailings storage failures. The risk of failure of a conventional
tailings facility can be reduced by having good drainage and little (if any) ponded water.
Simply put, 'no water, no problem' (Engels 2006).
However, as discussed, water management is not only required to ensure long-term
integrity of the TSF, but also to protect the local and regional natural water regime. When
developing water management measures for protecting natural water resources from the
potential effects of tailings disposal, the appropriate initial response is to dei ne the natural
resource that needs protection. Accordingly, both the receiving waters and the benei cial
water uses that could be affected by tailings disposal should i rst be identii ed. This includes
identifying locations of water resources and potential users for both groundwater and surface
water, average and extreme l ow rate for both wet and dry cycles, water quality variation
over time, surface water biota, depth to groundwater table and aquifer characteristics.
The next step is to assess the water balance of the TSF and to design water manage-
ment measures. Several elements contribute to the water balance of a TSF ( Figure 18.15 ):
(1) process water that easily drains; (2) net precipitation (rainfall minus evaporation);
(3) overland l ow of surface water into the TSF (run-on); (4) upward directed l ow of pore
water which occurs as tailings consolidate (resident water content in tailings); (5) loss (or
gain) of groundwater; and (6) seepage through embankment.
Process water that readily separates from tailings solids, together with net precipitation
comprise the decant water. Depending on its quality, decant water is directed to silt ponds
or water treatment plants for further treatment prior to i nal release to the environment
or prior to reuse as process water. Diversion drains at the upstream end of TSF minimize
overland l ow of surface water into the TSF. Surface water in-l ow from upstream catch-
ment areas is commonly either channelled away from the TSF or passed under the TSF via
culverts. If leachate poses a potential risk, water can be contained in several ways. First, the
underlying geological formation may be of sufi ciently low permeability to form a natural
barrier to leachate. A second method is to use the inherent low permeability of i ne tail-
ings themselves to minimize seepage. Third, one or more liners may be installed to prevent
seepage. Finally drainage systems can be installed to facilitate tailings consolidation and to

The risk of failure of a
conventional tailings facility can
be reduced by having good
drainage and little (if any)
ponded water.

FIGURE 18.15
Water Balance of a Conventional
On-land Tailings Storage Facility

Decant/Reclaim
Surface Runoff

Evaporation Precipitation Tailings

Discharge

It is important that the water bal-
ance not only considers average
conditions, but also includes extreme
operational or meteorological events.
Source:
UNEP 1996

Decant Pool

Dam Wall
Seepage

Groundwater
Inflow

Seepage