Page 185 - Soil Degradation, Conservation and Remediation

174 6 Soil Pollution

and can pose serious environmental risks from acid rock drainage and the release of
toxic metals, and toxic reagents used in processing. The combination of liquids and

fine-grained solids make many tailings physically unstable. If left exposed to the air
and dried, tailings can also be blown on the wind causing air pollution and washed
into waterways and spread on land. Toxic pollutants that are commonly found
in tailings include cyanide, mercury, copper, lead, arsenic, cadmium, selenium,
zinc, and nickel.

6.2.8.3 Acid Mine Drainage

Acid mine drainage occurs when rocks with sulfur bearing minerals are exposed to
air, water, and microorganisms. The sulfur in the minerals turns into sulfuric acid
and moves in drainage water. Acid mine drainage is highly acidic and reacts with
rocks and minerals of the dump. The chemical changes within the minerals also

release significant amounts of toxic heavy metals like nickel, copper, and cadmium.

These metals can contaminate surface soils and waters and find their way into food
webs resulting in both acute and chronic impacts on wildlife and people.

6.2.9 Traffi c

Pollution in soils from traffic sources occurs along major highways due to emission of

particulate matter (de Kok et al. 2006 ), as well as gaseous pollutants, which can be
transported to soil by both wet and dry deposition. Many of the substances released
from traffic are insoluble in water, have high adsorption ability, and tend to bind to

mineral and organic particle in soil. The pollutants can be retained or transformed in

the soils or subsequently modified by soil microorganisms (Wesp et al. 2000 ).
Roadside soils often contain high concentrations of metallic contamination. The
bioavailability and environmental mobility of the metals are dependent upon the
form in which the metal is associated with the soil. Lead street dust and roadside
soil has been extensively studied and found to be present at elevated levels (Goldsmith
et al. 1976 ; Harrison et al. 1980 ). The lead in roadside soil is mainly found in the
form of lead sulfate (Harrison et al. 1980 ). Metals such as Cu, Fe, Zn, and Cd are
essential components of many alloys, wires, tires, and many industrial processes and
could be released into the roadside soil and plants as a result of mechanical abrasion
and normal wear. Analyses of roadside soil (Hewitt and Candy 1990 ) and plants
(Otte et al. 1991 ) revealed that they contain elevated levels of these heavy metals.
A number of studies have investigated the release of pollutants from traffi c into
air (Klein et al. 2006 ). Combustion of fossil fuels in vehicle engines is an important
source of a group of highly abundant pollutants called polycyclic aromatic hydrocar-

bons (PAHs). In addition, traffic can be a source of their derivatives, including some
persistent organic pollutants (POPs). Some of these contaminants, such as polychlori-
nated biphenyls (PCBs) and polychlorinated dibenzo- p -dioxins and dibenzofurans