Natural Resources Extraction: Stressors and Impact Management - Sustainable Practices in Geoenvironmental Engineering - page 166

Source control—active mines
Deny exposure to O 2 and H 2 O to debris
piles and exposed pyrite in mine site

Pyrite,
FeS 2

Exposure to oxygen
and water

Release of acidic iron and sulfate-rich
fluid into the geoenvironment

Source controls

Surface hydrological and
drainage controls
Drainage basin management
Water barrier systems

Receiving waters
Flora and habitat
Surficial soils

Management
Drainage basin management
Fluid flow channelization

Mitigation and remediation

Water treatment facilities
Constructed wetlands
Permeable reactive barriers

FIGURE 5.13
Basic elements of source control and passive treatment procedures for AMD stressor mitigation.

was able to return due to the low bioavailability of the metals and possible interactions
between the metals.

5.5.2.2 Wetlands

Natural wetlands are areas of land with the water level close to the land surface, thus
maintaining saturated soil conditions and vegetation that includes plants, peat, wildlife,
microbial cultures, cattails ( Ty p h a spp.), reeds ( Phragmites spp.), sedges ( Carex spp.), bul-
rushes ( Scirpus spp.), rushes ( Juncus, spp.), water hyacinthe ( Eichhornia crassipes ), duckweeds
( Lemna spp.), grasses, and others (Mulligan, 2002). Algae and mosses, together with the wet
areas can trap the heavy metals. Constructed wetlands have been speciically designed to
include these species for the removal of BOD, suspended solids, nutrients, and heavy met-
als for optimal performance. It has been reported by Reed et al. (1995) that 1000 managed
wetlands are in operation throughout the world. In 1988, 142 North American wetland
systems were used for acid-mine drainage (Wieder, 1989).
Iron and manganese removal are often the key objectives in treatment of mine drainage.
Because of possible clogging of subsurface systems due to precipitation of iron and man-
ganese in subsurface systems, preference is usually given to surface systems since they
can be aerated more eficiently. Since the pH typically decreases from 6 to 3 in AMD phe-
nomena, the Tennessee Valley Authority (TVA) has developed an anoxic limestone drain
(ALD) for use as a treatment tool. This consists of a high calcium limestone aggregate