Environmental Engineering Reference
In-Depth Information
19.6
SEEPAGE FROM TAILINGS DAMS AND ITS CONTROL
19.6.1
General
As discussed in Section 19.2, many mine and industrial tailings have accompanying water
or “liquor” which contains dissolved salts, heavy metals and other residual chemicals
from the mineralogical processes. If this liquor escapes to the surrounding surface water
and groundwater in sufficient quantities, it can lead to unacceptable concentrations, mak-
ing the water unusable for drinking and affecting aquatic life. Therefore, there is often an
emphasis in the engineering of tailings dams on the estimation of seepage rates and, where
these prove unacceptable, to the provision of measures to reduce seepage.
Seepage cutoff measures can be very costly and are often not as effective as the propo-
nents would expect. They can affect the economic viability of a mining project and cer-
tainly the profitability of the operation.
This section presents an overview of the measures which are available and their effec-
tiveness in controlling seepage. In many cases it is more realistic to accept that tailings
dams will seep regardless of the measures adopted and to design collection and/or dilution
of the seepage to acceptable concentrations.
For a more detailed discussion on the topic readers are referred to the chapter by
Highland in Vick (1983). Ritcey (1989) and Fell, Miller and de Ambrosis (1993) discuss
chemical and geochemical aspects.
19.6.2
Principles of seepage flow and estimation
Many tailings storages (or “dams”) will be constructed on relatively flat land with a deep
existing groundwater table. This situation is discussed by Vick (1983) and
Figures 19.36
and
19.37
are reproduced from his book.
The following should be noted:
- The rate of seepage flow will be dependent on the permeability of the tailings, the
underlying soil and rock, climate, pond operation etc.;
- Contaminants in the seepage water will not all join the groundwater. Much will be
adsorbed in the foundation soil and rock. Hence, contaminant load does not equal
seepage flow rate
contaminant concentration in the storage;
- Further reduction of contaminant concentration may occur in mixing with stream
flows;
- It is contaminant concentration in ground and surface water which is generally critical,
not the total quantity. Hence adsorption, dispersion and dilution can result in accept-
able water quality in streams or well points, even though the original contaminant lev-
els in the storage may have been unacceptable.
It is important to realise that, in many cases, a partially saturated flow condition will exist
in the foundation at least at the start of operations, and possibly on a permanent basis if
the tailings permeability is low compared to the foundation permeability. Figure 19.37
shows an example of the stages in the development of seepage.
Note that flow in the tailings in Stages 1 and 2 will be essentially vertical, will not
emerge at the toe of the embankment, will be virtually unaffected by any foundation treat-
ment such as grouting and will not be intercepted by drains at the toe of the dam.
It may take years for the seepage mound to rise to connect to the tailings (Stage 3) or it
may never happen.
Figure 19.32
shows an example of a tailings storage which has been constructed at the
head of a valley. In this case the final development will consist of several embankments. When
