Sources
a substance which is located in, on or under the land and has 
the potential to cause harm to human health, water resources
or the wider environment. The contamination derives from
point or diffuse sources.
Considering pollution of soil and water resources, a distinction 
can be made of two types of sources, i.e., point sources for
which the position of the release point is can be identified,
and diffuse sources. Diffuse sources are those for which
the local effect can not be well tracked back to the source,
or for which the source is characterised by a large geographical
spread. However, these definitions do not suffice.
To enable the distinction of point and diffuse sources,
it is necessary to provide a clear definition of the system
under consideration. This can be made clear by considering
the emission of contaminants by exhaust pipes to the atmosphere.
Emitted pollutants can be well tracked back to their source,
in its immediate vicinity, but if an industrial plant has
many of such sources, such tracking back becomes difficult
at some distance. Hence, in practice, at regional or larger
scales, air pollution becomes difficult. For soil and water
resources, the upper boundary consists of the soil/water
surfaces. Immission of atmospheric pollutants (source
being the atmosphere) is then diffuse, though often not
uniform.
In the following, the discussion is focussed to diffuse
pollution of the soil and water resources.
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