Geology Reference
In-Depth Information
When open-pit mining ended in the Clark Fork River Complex in
1982, pumping was discontinued, with the result that water began filling
underground shafts and tunnels and the 390 m deep Berkeley pit.
Contaminated acid water containing individual metal and sulfate con-
centrations thousands of times those in uncontaminated water could
flow into an adjacent alluvial aquifer and eventually over the rim of
the pit. Large-scale hard-rock mining in the western USA, especially
Nevada, has greatly increased in recent years, with the result that deep
'pit lakes' are likely to form as open-pit metal mines intersecting
groundwater are depleted and shut down.
64
Pit lakes in high sulfide
rock will tend to have poor quality acidic water, although oxidized rock
that contains significant carbonate will produce better quality, near-
neutral pit lake water. Most of the larger existing pit lakes currently
contain water that does not meet standards for drinking water, agricul-
tural water quality, or aquatic life.
64
Factors such as the oxygen status
of the lake, pH, the hydrogeologic flow system, composition of the
wallrock, evapo-concentration, biological activity, and hydrothermal
inputs are all important to the modelling of future water quality and
impact.
Remediation approaches in circumstances like those described here
are based on a variety of physical, chemical, and biological systems.
These include the construction of ponds where sucient organic matter
is available to establish anaerobic conditions and immobilize at least
some of the metals (e.g. Cu, Pb, Cd, Zn) as sulfides. Similar passive
treatment of tailings-impacted groundwater has also employed precipi-
tation of metal sulfides as the key clean-up step.
65
Active chemical
treatment with lime, producing sludges, and biosorption by reeds,
wetlands, etc., are other methods which have been tried. Subsequent
to the flooding of the Wheal Jane mine, an active treatment (chemical
neutralization) plant and a passive treatment plant, the latter featuring
three schemes differing only in the pre-treatment method (lime dosing,
anoxic limestone drain, lime-free) used to modify the pH of the influent
mine water, were set up. All three systems at the Wheal Jane Passive
Treatment Plant included (i) constructed aerobic reed beds designed to
remove iron and arsenic, (ii) an anaerobic cell to encourage reduction of
sulfate and facilitate removal of zinc, copper, cadmium, and the
remaining iron as metal sulfides, and (iii) aerobic rock filters designed
to promote the growth of algae and facilitate the precipitation of
manganese.
66
The study showed considerable success in the removal
of key toxic metals and clearly demonstrated the potential for natural
attenuation of acid mine drainage, particularly iron oxidation, by
microbial populations.
