Environmental Engineering Reference
In-Depth Information
requires one to provide protective covers, isolation barriers, and pump-discharge opera-
tions. Implementation of these techniques is dependent on site- and operation-speciic
conditions. Adherence to the
keep it dry
principle is one of the basic requirements for mini-
mization of impacts to land use, and to the 4Rs technique (Figure 5.12).
For conditions where acid generation has occurred and acid leachate has found its way
to the land environment and its receiving waters, the two courses of required action are
(1) protection of the affected land receptors and water bodies from accepting further acid
leachates and (2) treatment of the affected land and water bodies. It has been suggested that
once pollution of the receiving waters such as those described in Section 5.2 has occurred,
destruction of aquatic habitat will render these waters to be bereft of aquatic life for a very
long time. Nevertheless, treatment of these waters is necessary. Methods for treating pol-
luted water were discussed in Chapter 3.
5.5.2.1 Acid Mine Drainage Management
AMD can also be managed. Active treatments that use wastewater treatment techniques
are possible. They are expensive and must be maintained regularly such as the use of
chemical or electrolytic treatments. The essential element here is the capture of leachate for
treatment—an expense that will be ongoing for an interminable period since the source of
acid generation will most likely be almost inexhaustible. Prevention of AMD is possible by
addition of limestone or by sealing the area with ly ash grouts. For example, Bulusu et al.
(2005) used a grout of coal combustion by-product to reduce AMD. The grout was durable
and did not exhibit signs of weathering.
Alkaline agents (calcium oxide, calcium hydroxide) and wastes (cement kiln dust, acety-
lene gas sludge) can be added to pools of AMD for neutralization. Alkaline trenches with
limestone or soda ash are used to neutralize runoff. Bacteriocides have to be used to inhibit
bacterial growth and hence pyrite oxidation. Natural treatment through carbonate forma-
tions may also be possible as in the case of Cretaceous chalk underlying the coalield slag
heaps in the north of France (Chamley, 2003). PRBs may also be a solution. Constructed
wetlands are a low-cost alternative for passive treatment as described in the next section.
Anoxic limestone drains are being evaluated for use because of their ease in maintenance
and operation. However, they require large areas for effective application.
The essential procedures for mitigating AMD stressors impacts include (a) diversion of
waters around the mine sites as part of the
keep it dry
(
keep it as dry as possible?
) strategy,
(b) channeling the generated leachates through constructed aerobic and anaerobic wet-
lands as a neutralization procedure and/or through permeable treatment walls (to be
discussed in Chapter 10), (c) complete inundation of mined-out sites—to deny access to
oxygen, and (d) capturing and channeling the generated leachates for active treatment
before discharge (Figure 5.13).
There is also a signiicant capacity for natural attenuation at mining sites. A particular
example of this is the Falun Copper Mine in Sweden. During its operation of more than
a hundred odd years, it is estimated that a half to one megatonne of copper, lead, zinc,
and cadmium were emitted into forest soils and streams in the area (Lindeström, 2003).
Concentrations of 70 and 2000 μg/L were found in streams in the city of Falun after treat-
ment of the mine water was initiated but decreased thereafter. Soils in the area, however,
were able to recover substantially faster. Most of the metals concentrated in the sediments
in two lakes. Copper was 120 to 130 times normal background levels, whereas lead and zinc
levels were 30 to 40 times. Cadmium accumulation was much less. The aquatic ecosystem
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