Natural Resources Extraction: Stressors and Impact Management - Sustainable Practices in Geoenvironmental Engineering - page 147

At pH values well above 3, Manahan (1990) reports that Fe(III) precipitates as hydrated
Fe(III) oxide Fe(OH) 3 (s).

Fe 3+ + 3H 2 O ↔ Fe(OH) 3 (s) + 3H + (5.4)

Release of the acid luid into the land environment within the mining site will allow
the luid to come in further contact with other exposed pyrites. The generation of acidic
leachate rich in iron and sulfate (known as yellowboy Fe(OH) 3 (s)) is characteristic of the
outcome of the various processes that accompany oxidation of the sulfur and iron in the
pyrite. So long as there is a source of these in the host rock and ores and so long as these
continue to be exposed to water, generation of yellowboy will continue unabated. The pres-
ence of sulfate-reducing bacteria in soil and water will exacerbate the problem. These bac-
teria are anaerobes that use sulfate as electron acceptors.
Although the generation of the acidic leachate, commonly known as AMD or acid rock
drainage (ARD), constitutes a major negative impact from mining operations, the cascad-
ing or domino effects that accrue from AMD can be severe. The domino effects arising
from discharge of the leachate into the environment include (a) severe health threats to
aquatic species, native habitat, and plant life, (b) pollution of groundwater and drinking
water, (c) deterioration of soil quality, and (d) release of trace metals and heavy metals
previously retained by the soil solids in the ground. Information reported in the Interstate
Mining Compact Commission study (IMCC, 1992) showed that for ive western states in
the United States, there were (in 1992) about 130,000 inactive and abandoned mine sites.
In a later study, Skousen et al. (2000) report that approximately 20,000 km of streams and
rivers in the United States have been degraded by AMD and that 90% of AMD reaching
streams originate from abandoned mines. This makes the problem of attaching ownership
of the problem and for cleanup of the sites and streams and rivers somewhat dificult.
The extent of acid generation at mine sites (underground mines, openings, leach ores,
spent ores, etc.) is a function of several factors. These include (a) type and concentration
of sulide minerals in the host ore and in the spent ores and leach piles, (b) the host rock,
(c) availability of oxygen, (d) site hydrogeology, (e) pH of the water in the system, and
(f) presence or absence of bacteria, e.g., Thiobacillus ferrooxidans.

5.2.4.2 Arsenic Release

Oxidation of the mineral sulides may result in the solubilization of trace metals and heavy
metals, effectively releasing them and allowing them to be mobile in the liquid phase. It is not
uncommon to ind evidence of arsenic, cadmium, cobalt, copper, lead, manganese, nickel, and
zinc as released metals. Arsenic poisoning of groundwater and aquifers has been reported
in many parts of the world. This problem has gained considerable publicity and has been
reported as “the largest mass poisoning of a population in history” (Smith et al., 2000) in rela-
tion the poisoning of the tubewells in Bangladesh and West Bengal, as previously mentioned
in Chapters 1 and 3. In this particular case, information available points to the presence of
naturally occurring arsenopyrites (FeAsS) and arseniferrous iron oxyhydroxides in the sub-
strate material as being the immediate source materials for the arsenic. If oxygen is available
in the groundwater, oxidation of the arsenopyrites will release the arsenic. Some reports have
speculated on the use of tubewells as a means for introduction of oxygen into the subsoil
strata. In the absence of oxygen, the processes associated with reductive dissolution of the
arseniferrous iron oxyhydroxides will release arsenic while increasing the bicarbonate con-
centrations. This will result in arsenic pollution of the groundwater. The reports given by