Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 45

wells as a means for abstracting water. The two models proposed to explain the release of
arsenic from the arsenic-bearing materials are shown in Figure 2.5. These include:
1. Reduction mechanisms : Reductive dissolution of arseniferrous iron oxyhydroxides
releases the arsenic responsible for contamination of the groundwater.
2. Oxidation processes : Oxygen invades the groundwater because of the lowering of
the groundwater from the abstracting tubewells resulting thereby in the oxidation
of the arsenopyrite (FeAsS).
Based on these two possible mechanisms for arsenic release into the aquifer, and based
on detailed ield studies to determine the presence and distribution of As(III) and As(V)
and other reaction products, there are some serious questions as to whether the arsenic
releases processes are totally man-induced or a case of natural processes hastened and
aggravated by man-made events, i.e., abstraction of water from the tube wells. The almost
equal proportions of As(III) and As(V) in the aquifer testify to almost equal sets of activity
from both arsenic-release mechanisms.
2.4 Wastes, Contaminants, and Threats
The discharge of liquid and solid wastes from industrial and energy-producing plants and
facilities, together with extraction operations on site and inadvertent spills and deliberate
dumping of waste materials, combines to introduce contaminants, toxicants, and other
kinds of hazardous substances into the land environment—including the receiving waters
contained therein. The term contaminants used in this section includes those contaminants
that have are classiied as pollutants (priority listing) by various regulatory agencies. The
term pollutant will be used when necessary, to place emphasis on the hazardous nature of
the contaminant in question. Pollutants pose signiicant threats not only to human health
and other biotic receptors, but also to the health of the environment and the various eco-
systems in the geoenvironment. In the treatment of threats to the environment and public
health, there are several ways in which wastes and contaminants can be classiied or cat-
egorized. One could categorize the wastes and contaminants in terms of source produc-
tion, i.e., “where they come from.” Alternatively, one could categorize them in respect to:
• Level of toxicity, e.g., highly toxic, carcinogenic, priority listing.
• Inorganic or organic substances and chemicals, e.g., heavy metals, PAHs, PCE,
styrene.
• Type of industry (source industry or source activity), e.g., pulp and paper, forest,
electronic, pharmaceutical.
• Class of contaminants, e.g., pesticides, solvents.
• Nature of impact or threat, e.g., physical, chemical, biological.
• Type of receptor, e.g., land, water, human, other biota.
Experience has shown that although categorization of contaminants according to any
single method as described above is not practical or feasible, it is always necessary to obtain