Geology Reference
In-Depth Information
reactions, which would be extremely slow under the prevailing condi-
tions of low pH (e.g.
o
4.5).
59
The combination of acid waters and
coatings can have devastating effects upon aquatic biota, with depletion
of free-swimming and bottom-dwelling organisms, the loss of spawning
gravel for fish and direct fish mortalities. Typical approaches to treat-
ment of acid mine drainage have included methods to remove the iron
floc by oxidation and to adjust the pH through the use of limestone filter
beds.
3.3.1.4 Acid Mine Drainage and Release of Heavy Metals. The pro-
duction of sulfuric acid from sulfide oxidation in mines can also lead to
the leaching of metals other than iron, with the result that the emerging
acidic waters may be laden with heavy metals. Thus in the former
metalliferous mining area of south-west England, where there are now
many abandoned mine workings, the closure and flooding of the famous
Wheal Jane tin mine in 1992 led to a highly acidic cocktail of dissolved
metals (Cu, Pb, Cd, Sn, As) entering the Carnon and Fal Rivers at 7-15
million litres per day and spreading throughout the surrounding estu-
aries and coastal waters.
60
In the USA, the EPA has identified over 31,000 hazardous waste sites,
with the largest complex of 'Superfund' sites to be remediated in western
Montana, in the Clark Fork River Basin where there have been more
than 125 years of copper and silver mining and smelting activities.
Moore and Luoma
61
have characterized three types of contamination
resulting from large-scale metal extraction: primary, consisting of wastes
produced during mining, milling, and smelting and deposited near their
source of origin; secondary, resulting from transport of contaminants
away from these sites by rivers or through the atmosphere to soils,
groundwaters, rivers, etc.; and tertiary, where contaminants may be
remobilized many kilometres away from their point of origin. More
generally, Furrer and co-workers,
62
in an examination of flocs from
streams polluted from acid mine drainage in California and Germany,
found that when aluminium-rich acid mine drainage mixes with near-
neutral surface waters, fluffy aluminium oxyhydroxide flocs precipitate
and move downstream as suspended solids, transporting adsorbed
pollutants, e.g. heavy metal cations such as Pb
21
,Cu
21
, and Zn
21
.
The flocs appear to be formed from aggregation of the aqueous
polyoxocation, AlO
4
Al
12
(OH)
24
(H
2
O)
12
71
[Al
13
], which is toxic and
possibly responsible for the declining fish populations in rivers polluted
by acid mine drainage.
Some of the largest waste deposits occur in tailing ponds containing
acid mine water. In the Clark Fork River Complex, it is estimated that
