Tailings Disposal - Mining and the Environment: From Ore to Metal - page 755

Potential Impacts and Risks of Deep Sea Tailings Placement

Where bathymetric and oceanographic conditions are suitable, as in the cases listed above,
DSTP systems can be designed to completely avoid impacts on shallow marine ecosystems
such as mangroves, coral reefs and sea-grass meadows which constitute the most produc-
tive marine ecosystems. The deep water environments through which the tailings must
l ow and the very deep water environments where tailings solids accumulate, are char-
acterized by very low productivity. There is little or no interaction between organisms in
these deep habitats and those in the euphotic zone.

CASE 18.6
Comparison of Selected Tailings Disposal Schemes

It is instructive to compare the environmental outcomes
from four major, geologically similar projects in similarly
mountainous terrain. These are:
● Bougainville Copper Mine in PNG;
● Ok Tedi Copper Mine in PNG;
● Grasberg Copper Mine in Papua Province, Indonesia; and
● Batu Hijau Copper-Gold Mine in Sumbawa, Indonesia.

suffi cient capacity to transport the tailings (and, in the case
of Ok Tedi, also the waste rock) to the sea, where it would
deposit. In both these cases this proved not to be the case;
sediment deposited in the lower reaches of the rivers where
river bed gradients fl attened and fl ow velocities decreased,
leading to aggradation which eventually led to overfl ow of
the river banks and deposition of tailings over extensive
areas away from the rivers. In the case of the Grasberg
project, levees were constructed to confi ne the area of
deposition. In the case of Ok Tedi, the area of deposition
has not been managed; rather the approach adopted has
been to pay compensation for the resulting environmental
damage. In these three cases, the environmental damage has
been substantial, and at Ok Tedi and Bougainville contributed
to public outrage. At Bougainville, this led to the premature
closure of the project in 1989, and contributed to widespread
social turmoil on the island, that continues to this day.
It may be that the benefi ts of both the Ok Tedi and
Grasberg Projects outweigh the environmental costs. Both
provide direct and indirect employment for thousands of
people. The revenues from Ok Tedi comprise a major part of
PNG government receipts and represent a signifi cant portion
of GNP. How history will judge these projects will depend
largely on how rapidly and successfully the areas are reha-
bilitated, and whether or not there are secondary impacts, as
yet not apparent, such as oxidation leading to mobilization
of dissolved metals. It is clear that, had the magnitude of
direct impacts been recognized by either proponents or
regulators during initial project planning, alternative tailings
disposal schemes would have been sought. In fact, after
the impacts became apparent, alternative tailings disposal
schemes were intensively investigated for both projects.
However, the alternatives proved to be either impractical or
extremely costly and the costs could not be justifi ed, given
that the environmental damage had already occurred. It is
not clear whether or not the costs of implementing

environmentally acceptable tailings disposal schemes at the
outset, would have rendered the projects uneconomic.
Contrast this with the situation at Batu Hijau where
the total terrestrial footprint (combined areas of surface
disturbance) is an order of magnitude less than at the
other sites. Monitoring studies have confi rmed predictions
made in the environmental impact assessment that:
● No tailings would be present in the water column or on
the seabed above the 120 m outfall depth, which means
that there can be no impact of tailings on productive
shallow water ecosystems such as coral reefs;
● The tailings slurry would be largely confi ned within the
Senunu canyon to a depth of 1,400 m; and
● The main area of tailings deposition would be on the
seabed at depths exceeding 3,000 m.

All four projects involve large open Pit mines to exploit
large porphyry copper deposits containing in the order of
1 billion tonnes of ore. All reached the same conclusion
prior to or during construction - that conventional
on-land tailings disposal was either not feasible or entailed
an unacceptable risk of catastrophic failure. (The Ok Tedi
Project was designed to have an on-land tailings storage;
however, a major landslide occurred during foundation
preparation for the embankment, leading the owners and
the government regulators to the conclusion that a secure
tailings impoundment could not be developed).
The Bougainville, Ok Tedi and Grasberg operations all dis-
charged tailings into nearby streams or rivers, while at Batu
Hijau, a DSTP system was used. At Bougainville, which was
developed before environmental impact assessment became
established, tailings were discharged to a local stream, accu-
mulating on parts of the fl ood-plain and in a large deltaic
deposit beyond the mouth of the stream. A recent satellite
image clearly shows the onshore tailings accumulations,
and suggests that the deltaic deposits have been partly
removed, presumably by wave action and currents with
the remaining deposited tailings well vegetated. Prior to
the unplanned closure of this project, a pipeline was being
constructed to transport the tailings to a submarine outfall
further offshore, and plans for rehabilitation of the deltaic
area using mangrove vegetation were being implemented. For
both Ok Tedi and Grasberg, the initial sediment transport
modelling studies indicated that the receiving waters had

In fact, the footprint of tailings on the seabed at Batu
Hijau is somewhat less than the footprint of tailings on
land in the cases of Ok Tedi and Grasberg. These projects
also resulted in large areas of tailings deposition in shal-
low water marine environments, although these have not
caused anything approaching the same concern as onshore
deposition. The most important consideration relating to
Batu Hijau is that all the impacts of tailings disposal are
confi ned to water depths exceeding 120 m. This means
that the impacts all occur in the zones of very low bio-
logical productivity, below the euphotic zone.
This comparison of four large projects in the Asia-
Pacifi c region may lead to various conclusions; for example:
● Large porphyry copper deposits situated in the moun-
tainous, wet tropics in earthquake prone areas, should
not be developed; or
● Such deposits should not be developed unless the tail-
ings can be discharged by means of DSTP; or
● Other methods of tailings disposal need to be developed
before such deposits can be exploited.