Environmental Engineering Reference
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the solid phase of aquifers generally does not have as much capacity to adsorb con-
taminants as the organic-rich upper soil layers do. As a consequence, the majority
of the contaminants in the water-saturated groundwater zone usually resides in the
water phase, that is, in the groundwater. For the same reasons, and to standardise
the terminology, the term 'groundwater' is often used throughout this topic to mean
the whole water-saturated soil layer.
There is also a practical reason for focusing on the groundwater and not on the
solid phase of the water-saturated soil. Sampling of groundwater is relatively easy,
since groundwater can be sucked up via water-sampling tubes. Taking aquifer sam-
ples, that is, soil samples in the deeper soil layers that include solid phase material
and the groundwater, is a more difficult, time-consuming and costly activity, espe-
cially in the case of deep groundwater tables. For this reason, groundwater samples
are taken in routine investigations and aquifer samples are only taken in exceptional
circumstances.
17.1.3 Groundwater Quality
17.1.3.1 Natural Impact on Groundwater
Contaminants coming from various natural and anthropogenic sources may affect
groundwater bodies. Therefore, in all countries of the world, contaminants are found
in the groundwater. In fact, pure water only exists in the laboratory in the form of
demineralised water, especially as distilled water.
Water travels much more slowly in aquifers than in surface water systems, and
the time for infiltrating water and contaminants to migrate from the land surface to
the point of discharge is measured in years, decades or centuries, depending on the
type of contaminant, the aquifer characteristics and the length of the flow path. As
a consequence, there is a great deal of time for intimate contact between the water
and soil and rock material, which results in changes of water quality with regard
to mineral composition. As a result, metals are commonly found in groundwater in
practically every country in the world (e.g., US Environmental Protection Agency
2008). Given the wide variety in the geometry of hydrological systems, in soil and
rock materials, and, hence, travel times, the naturally impacted groundwater quality
( natural background concentration ) is highly variable.
Naturally impacted groundwater may sometimes result in unacceptable risks to
human health or the ecosystem. An example of this is the arsenic poisoning of
large numbers of people drinking shallow groundwater in Bangladesh and West
Bengal, India, which arose from a natural phenomenon, not industrial contamination
(Rahman et al. 2001 ).
17.1.3.2 Anthropogenic Impact on Groundwater
In addition to contaminants from natural sources, a wide variety of the contaminants
that originate from point sources, that is, small-scale sources that often originated in
leakage or dumping, are frequently found in the more densely populated areas of the
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