Tailings Disposal - Mining and the Environment: From Ore to Metal - page 757

not these metals may be more soluble in sea water, and if so, whether they may reach concen-
trations that could adversely affect marine organisms? This is simple to ascertain by mixing
tailings with sea water and analyzing for metals, before and after mixing. If this test shows
that the dissolution of one or more metals is occurring, more sophisticated testing is carried
out to measure the rate or l ux of dissolution for each metal of concern, from which model-
ling can predict the maximum concentrations that could develop under operating conditions.
Finally, the fate of sulphide minerals contained in tailings should be considered. Such
minerals are of concern when stored in onshore impoundments; on exposure to air they
can produce acid which can leach metals from the tailings solids. One of the strategies used
to avoid acidii cation of onshore tailings deposits is to maintain them in a permanently sat-
urated state. Clearly, DSTP achieves the same objective. The oxygen content of deep sea
water is so low that sulphides in the tailings will not oxidize. In fact, in many parts of the
deep sea, metals precipitate as sulphides. According to Environment Canada (1996) 'tail-
ings deposits on the ocean l oor tend to be a sink not a source for metals'.
DSTP systems are relatively simple, comprising only a De-aeration Tank, which is a
robust structure lacking any mechanical parts, and one or two pipelines, involving both sub-
sea and overland sections. Accordingly, the number of things that can go wrong is limited to:
●

Tailings deposits on the ocean
fl oor tend to be a sink not a
source for metals.

Overl ow or rupture of the De-aeration Tank;

●

Rupture of the onshore tailings pipeline; and

●

Rupture of the submarine pipeline.

The De-aeration Tank is a robust structure which is unlikely to rupture under any cir-
cumstances. The risk of overl ow is avoided in the hydraulic design.
The risk and consequences of rupture of an onshore pipeline apply equally to conven-
tional on-land tailings storage systems. Such risks are minimized by regular inspection and
maintenance programmes. Consequences may be minimized by building embankments on
both sides along the pipeline corridor.
Submarine tailings pipelines have ruptured, in one instance after blocking of the out-
fall and subsequent over-pressurization caused the pipe to part at a joint, and in the other
instance where a sea-bed landslide severed the pipe. In both these cases, temporary dis-
charge into shallow water occurred until the ruptures were remedied. However, in nei-
ther case was there signii cant environmental damage. This contrasts to the situation in
conventional on-land tailings disposal where many more risks need to be addressed and
where the consequences of malfunction are considerably higher.

Site Selection for Deep Sea Tailings Placement

An effective approach to site selection includes the following steps.

Review of Available Bathymetric and Oceanographic Data
Although such data are regional and unlikely to provide much local information, they
usually provide a good indication of whether deep water occurs close to the coast, and of
ultimate offshore water depths. In addition, useful data may be available on tides and sur-
face currents, and on the depths and strength of thermoclines in the water column. Other
relevant information may be obtained from daily sea surface temperature data generated
by remote sensing from satellites. In particular, a review of sea temperature records can
be used to check for evidence of upwelling in which colder water from deeper parts of
the water column is moved to the surface, despite its higher density. Upwelling, a natu-
ral phenomenon found in few parts of the world, can be a regional phenomenon, where