The Range of Environmental Concerns - Mining and the Environment: From Ore to Metal - page 521

of rock materials; a similar amount of rock needs to be disposed of - mining not only
removes mountains, it creates hills, mountains, and terraces of waste rock and tailings.
These physical changes in landform pose a unique set of problems relating to long-term
structural stability of the new landforms, whether great holes in the ground or mountains
of tailings and waste rock. Safety is of paramount importance, initially for workers and,
subsequently, after closure, for the public. In the latter case, a range of potential down-
stream impacts may result from mass wasting, erosion, or structural failure and conse-
quent uncontrolled release of contaminants.

Water Erosion

Erosion is a common problem associated with earthworks (excavation, transportation, and
placement of soil and/or rock). This applies particularly in mining where large areas are
cleared of vegetation, and huge volumes of soil and rock are removed and stored (envi-
ronmental impacts of erosion are detailed in Chapter Twenty). Also, the most erodible
material of all is topsoil, which, as discussed in Chapter Twenty One represents a valu-
able resource that usually warrants conserving. However, without application of stringent
safeguards, topsoil from disturbed land will end up in nearby streams causing short-term
or long-term degradation to aquatic habitats. Erosion of waste rock piles from runoff after
heavy rainfall also transports soil and rock materials into nearby streams. Increased tur-
bidity in natural waters will reduce the light available to aquatic plants for photosynthe-
sis. Increased sediment loads can also smother benthic organisms in streams, eliminating
important food sources for predators and decreasing available habitat for i sh to migrate
and spawn (Johnson 1997b). Some eroded materials are simply toxic due to their high metal
content, or become so after contact with air, which causes oxidation of sulphide minerals.
In addition, high sediment loads can lead to aggradation, decreasing the depth of
streams, with an increase in risk of l ooding during times of high stream l ow (Mason
1997) ( Case 13.2 ). Sedimentation from mining may modify stream morphology by disrupt-
ing a channel, diverting stream l ows, and changing the slope or bank stability of a stream
channel. All or any of these disturbances may, and probably will, reduce water quality
(Johnson 1997a).

Erosion is a common problem
associated with earthworks.

Slope Failure and Landslides

Mining projects result in formation of two categories of slopes: cut slopes and i ll slopes.
Cut slopes are created by the removal of waste rock and/or ore with new, usually steeper,
slopes surrounding the excavation or 'mine void' (see Figure 13.2 as one example). During

CASE 13.2
Log Jam or Tailings - Which one is the Culprit?

From the outset of mining its Grasberg deposit, PT
Freeport Indonesia adopted a riverine tailings disposal
scheme. This practice and associated environmental impacts
fi rst made headlines in the early 1990 when the original
tailings river overfl owed its banks during a severe storm

event, depositing tailings over large areas of natural
rain forest. At that time environmentalists claimed that
increased sediment loads due to past tailings disposal had
clogged the drainage system. Company offi cials on the
other hand ascribed the cause of bank overfl ow to logs

transported by the river during the storm event forming
a log jam. Whatever the initiating cause, the event led to
the construction of extensive levees along both sides of
the tailings river to reduce the risk of lateral uncontrolled
spread of tailings.