Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 46

a proper identiication of the types or species of contaminants. Considerable signiicance
is placed on the potential health threat of contaminants. A popular approach that has
gained the attention of many regulatory agencies is the SPR (source-pathway-receptor)
method for determination of health threats and impacts created by the presence of con-
taminants in the geoenvironment and also by events or projects as source contaminants.
There have been questions raised as to whether the SPR approach discriminates between
levels of treatment or protection from health-threat events depending on the importance of
the receptors . A school of thought suggests very strongly that risk management should be
directly linked to receptor importance and also to certainty of pathways. It is not always
clear that pathways to potential receptors are well deined. Nevertheless, one must include
knowledge of the degree of certainty of pathways as an integral factor in the level of risk
determination.
2.4.1 Inorganic Contaminants
Evidence of the presence of inorganic contaminants classifying as pollutants in the geoen-
vironment (land and water) show that these are mainly heavy metals such as Pb, Cr, Cu,
etc. Yong (2001) has indicated that although those elements with atomic numbers higher
than Sr (atomic number 38) are classiied as heavy metals (HMs), it is not uncommon to
include elements with atomic numbers greater than 20 as heavy metals. The 38 elements
commonly considered as HMs fall into three groups of atomic numbers as follows:
1. From atomic numbers 22 to 34: Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, and Se.
2. From 40 to 52: Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, and Te.
3. From 72 to 83: Hf, Ta, W, Re, Os, Ir, Pt, Gu, Hg, Tl, Pb, and Bi.
The more common heavy metals (HMs) found in the geoenvironment come as a result of
anthropogenic activities such as management and disposal of wastes in landills, genera-
tion and storage of chemical waste leachates and sludges, extraction of metals in metallif-
erous industries, metal plating works, and even in MSWs. The more notable HMs include
lead (Pb), cadmium (Cd), copper (Cu), chromium, (Cr), nickel (Ni), iron (Fe), mercury (Hg),
and zinc (Zn).
2.4.1.1 Arsenic (As)
Strictly speaking, arsenic is a nonmetal—although it is often classiied as a metal. It is
a metalloid (semi-metal) with atomic number 33 and is in group 5 of the periodic table.
Arsenic is found naturally in rocks, most often in iron ores and in sulide form as magmatic
sulide minerals. The more common ones are arsenopyrite (FeAsS), realgar (AsS), nicolite
(NiAsS), and orpiment (As 2 S 3 ). Arsenic is also found naturally in soils in association with
hydrous oxides, and sometimes in elemental form in association with silver ores. Arsenic
found in the geoenvironment can come directly from weathering of the arsenic-containing
rocks and also from industrial sources such as manufacturing, processing, pharmaceuti-
cal, agriculture, and mining industries. Products such as paints, dyes, preservatives, herbi-
cides, and semiconductors are some of the more common contributors to the arsenic found
in the ground and in receiving waters. Extensive use of arsenic-containing (lead arsenate)
pesticides, herbicides, and insecticides in agricultural and farm practices can contribute
some considerable amounts of arsenic to the subsurface and the receiving waters.