Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 366

Acid mine drainage (AMD) and acid rock drainage (ARD) are major nonpoint
source pollutions in many former mining regions where the overburden is excavated
to expose the materials (i.e., coal, metal ores, nonmetallic minerals). Similar problems
originate from waste rock and tailings formed during the processing of valuable min-
erals containing sulfide minerals such as pyrite (FeS 2 ) when they were exposed to air
and water.
AMD and ARD are formed by the oxidizing action of air and water in the presence
of naturally occurring bacteria, which catalyze the effect of oxidizing iron sulfides
and essential sulfur in the exposed sulfidic strata releasing large quantities of iron
and sulfate into solution. In addition, H + ions are liberated during the oxidation
process producing an acidic solution that readily weathers and releases other metals
(i.e., copper and zinc) into solution.
AMD and ARD are main water deteriorating pollutants from mining. In addition,
waters with high iron and metal content contribute to diffuse water pollution causing
watershed-scale damage in mining areas. Accidents such as the breach of dams and
well eruptions may also cause serious damage to waters [e.g., cyanide spill, red mud
spill (Chapter 5), mineral oil spills in the seas and oceans].
Elevated iron concentrations of in-stream sediments and suspended solids are
major diffuse Fe sources in surface waters affected by ferruginous mine waters. Fer-
ruginous mine waters are considered deleterious to surface water quality primarily
due to the smothering of benthic habitats by Fe precipitates (Jarvis & Younger,
1997), resulting also in esthetic damage. When groundwater containing ferrous iron
(Fe 2 + ) is released from pyrite oxidation discharge at the surface, the resultant oxida-
tion to ferric iron (Fe 3 + ) often results in rapid precipitation of ferric oxyhydroxide
(ochre). In a watershed-scale study of mine water pollution, Mayes et al. (2005) have
highlighted the significant contribution of diffuse sources to in-stream Fe loadings.
According to Mayes et al. (2005), this diffuse contribution to surface water pollu-
tion typically equates to 50% of in-stream loadings in low flow and up to 98% of
in-stream loadings in high flow. They have confirmed that re-suspension of Fe-rich
sediments is the major source of elevated in-stream Fe concentrations and loadings in
high flow.
Uranium mining-related discharges of naturally occurring radionuclides results
in radioactive contamination of water bodies, sediment and soil throughout a water-
shed area with time-related and spatial variability of the contamination levels and
radionuclide activity levels. The mining-related source of radionuclide discharges caus-
ing diffuse pollution are discharge of radionuclides together with mine water and
process water, seepage of slimes from tailings ponds bearing dissolved radionuclides,
erosion of contaminated particles of slimes from tailings ponds and their dispersion
with runoff after heavy rainfall.
Special cases arise where excavation (open pit mining) emits dust, combustion gas
or where processing, beneficiation activities emit chemicals, solvents, sulfur dioxide
and metals into the air. Although the units are point sources, the deposition (fallout)
and hence, the impact will be over a wide area, as diffuse pollution.
An example of typical diffuse source pollution from hydrometallurgy and pyromet-
allurgy occurred at the Salsigne gold mine in France. To extract the gold from the mined
ore, arsenic was used, and approximately 10,000 tonnes of arsenic trioxide/year as
well as metals such as lead and copper were released into the environment. The main