Page 19 - Soil and water contamination, 2nd edition

6 Soil and Water Contamination

This book follows a common subdivision based on multiple criteria, which distinguishes
the following pollutant groups: solid phase constituents, major dissolved phase constituents ,

nutrients, heavy metals , radionuclides , and organic pollutants. The major dissolved
constituents are comprised of inorganic substances that are abundant as dissolved ions in soil
moisture, groundwater, and surface water, and which make up the major part of the total
dissolved solids . Nutrients are those compounds that are essential for plant and animal life.
Heavy metals include metals and metalloids with a high atomic mass, which are associated
with contamination and potential toxicity. Radionuclides are elements having an unstable
nucleus which spontaneously disintegrates, thereby emitting ionising radiation . Organic
pollutants are substances made up of carbon (C), hydrogen (H), and oxygen (O), and a few
other elements. Except for man-made radionuclides and synthetic organic compounds, all
these substances occur naturally in the environment, but they are considered to be major
pollutants because they are highly concentrated and widely dispersed in the environment as
a result of human activities. The natural occurrence of substances in the environment will be
discussed in the next sections. The properties, role, and effects of the above pollutant groups
will be dealt with further in Part II of this book.

1.3.2 Background concentrations

Substances present in the environment are of either natural or anthropogenic (i.e. man-
made) origin. The presence of detectable concentrations of chemical substances in the
environment does not necessarily indicate the existence of pollution. Many substances, such
as nutrients and heavy metals, occur naturally in soils, groundwater, and surface waters as an
inevitable consequence of their natural occurrence in the Earth’s crust. A substance’s natural
(i.e. without human interference) concentration in soil or water is called the background
concentration . Background concentrations may vary in both space and time. It is therefore
important to distinguish between the natural occurrence of substances and the extent to
which this has been augmented by human activities. Although often difficult to make, such
distinctions are essential to ensure that informed decisions are made about the management
of soil and water.
Geochemists have long studied the relative abundance of elements in the Earth’s crust.
The average composition of continental crustal rocks is given in Table 1.1. The atomic
number, atomic weight s and full names of the elements are given in Appendix I.
The composition of natural soils and water is, however, only indirectly related to the
average concentrations listed in Table 1.1. Below a depth of 16 km the Earth’s crust consists
predominantly of igneous rocks , so therefore the average composition of the crust approaches
that of igneous rocks. In the crust near the Earth’s surface, however, sedimentary rocks
predominate over igneous rocks. Sedimentary rocks can be subdivided geochemically into
resistates , hydrolysates , precipitates , and evaporites . Resistates are rocks composed largely
of residual minerals not chemically altered by the weathering of the parent rock (mainly
quartz –SiO ), such as sandstone , conglomerate, and graywacke. Hydrolysates are rocks
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composed mainly of relatively insoluble minerals produced during weathering of the parent
rock (mainly clay minerals ), such as shales. Precipitates are rocks produced by chemical
precipitation of dissolved minerals from aqueous solution, such as limestone (CaCO )
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or dolomite (Mg,CaCO ). Evaporites are formed when soluble minerals are deposited
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because the water in which they were dissolved has evaporated, for example halite (NaCl) or
gypsum(CaSO ∙2H O). Because chemical precipitation is the basic process of the formation
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of both precipitates and evaporites, the distinction between precipitate rock and evaporite
rock is rather arbitrary and evaporites often occur interbedded with precipitates.
The composition of both igneous rocks and sedimentary rocks may vary greatly from
place to place. The variation in the composition of igneous rocks is the result of geological

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