Waste rock disposal areas are usually located as close to the mine as
possible to minimize haulage costs. Although the waste rock may contain
metals, such as lead, zinc, copper, or silver, the rock is still considered a
waste, because the cost to process it would exceed the value of the metals
it contains. If not properly managed, erosion of mineralized waste rock
into surface drainages may lead to concentrations of metals in stream
sediments. This situation can be potentially harmful, particularly if the
metals are in a chemical form that allows them to be easily released
from the sediments into stream waters. When this occurs, the metals
are considered to be “mobilized” and “bioavailable” in the environment.
In some cases, bioavailable metals are absorbed by plants and animals,
causing detrimental effects. Although current U.S. mining and reclamation
practices guided by environmental regulations minimize or prevent waste
rock erosion into streams, disposal of waste rock in places where it could
erode into surface drainages has occurred historically. These conditions
still exist at some old or abandoned mines

Acidic and Metal-Bearing Soils and Waters
Although the character of waste rock varies with the type of ore, many
waste rocks contain sulfide minerals associated with metals, such as lead,
zinc, copper, silver, or cadmium. An important sulfide mineral common in
waste rock is pyrite, iron sulfide (FeS2). When pyrite is exposed to air and
water, it undergoes a chemical reaction called “oxidation.” Oxidation of
pyrite results in the formation of iron oxides that typically impart an orange
or red “rust” color to waste rock (Fig. 10). The oxidation process, which is
enhanced by bacterial action, also produces acidic conditions that can
inhibit plant growth at the surface of a waste pile. Bare, non-vegetated,
orange-colored surface materials make some waste rock areas highly
visible, and they are the most obvious result of these acidic conditions.
If water infiltrates into pyrite-laden waste rock, the resulting
oxidation can acidify the water, enabling it to dissolve metals
such as copper, zinc, and silver. This production of acidic
water, is commonly referred to as “acid rock drainage.”
If acid rock drainage is not prevented from occurring