Geology Reference
In-Depth Information
The effects of contaminants or pollutants on freshwater depend upon
their chemical, physical, and biological properties, their concentrations
and duration of exposure. Aquatic life may be affected indirectly (e.g.
through depletion of oxygen caused by biodegradation of organic matter)
or directly, through exposure to toxic or carcinogenic chemicals, some of
which may accumulate in organisms. Such toxicity may, however, be
modified by the presence of other substances and the characteristics of the
particular water body. For example, metal toxicity is affected by pH,
which influences speciation; dissolved organic carbon has been shown to
reduce bioavailability by forming metal complexes and by subsequent
adsorption to particulate matter in freshwater.
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The toxicity of many
heavy metals to fish is also inversely related to water hardness, with Ca
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competing with free metal ions for binding sites in biological systems.
In biologically productive lakes which also develop a thermocline in
summer, the bottom water may become depleted in oxygen, leading to a
change from oxidizing to reducing conditions. As a result, the potential
exists for remobilization of nutrients and metals from bottom sediments
to the water column, one more example of how alterations in master
variables in the hydrological cycle, in this case the redox status, can
affect the fate and influence of pollutants.
A further demonstration of the importance of fundamental properties of
both pollutants and water bodies is provided by the behaviour of chemicals
upon reaching a groundwater aquifer. Soluble chemicals, such as nitrate,
move in the same direction as groundwater flow. A poorly soluble liquid
which is less dense than water, such as petrol, spreads out over the surface
of the water table and flows in the direction of the groundwater. Poorly
soluble liquids which are denser than water, such as various chlorinated
solvents, sink below the water table and may flow separately along low
permeability layers encountered at depth in the aquifer and not necessarily
in the same direction as that of the overlying groundwater.
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The rest of this chapter therefore consists of two major sections, first
on fundamental aquatic chemistry of relevance to the understanding of
pollutant behaviour in the aquatic environment and second on asso-
ciated case studies and examples drawn from around the world, includ-
ing reference to water treatment methods where appropriate.
3.2 FUNDAMENTALS OF AQUATIC CHEMISTRY
3.2.1
Introduction
3.2.1.1 Concentration and Activity. All natural waters contain dis-
solved solutes. In order to understand their chemical behaviour in
