Geoscience Reference
In-Depth Information
be threatened, together with a variety of animal species and their habitats (e.g.,
Hannam et al.
2003
). Changes in vegetation often are the first signs of increasing
salinity. These may involve the appearance of a salt-tolerant plant species or the
reduced growth of existing species. The salinity rise of a lake may lead to a
dramatic change in the ecological system and species composition, which in turn
modifies the food chain and thus the bioaccumulation of toxic elements such as
selenium (e.g., Funakawa et al.
2000
; Spoor
1998
; Cramer and Hobbs
2002
).
Wetland salinization is a good example of such processes. More than one-fourth of
the world's wetlands have been lost due to salinization (Vengosh
2003
). The
destruction of wetlands has reduced the diverse assembly of millions of waterfowl
and shorebirds. Moreover, salinization is likely to harm the reproductive capacity
of birds and place further stress on their diversity. Wetland destruction has been
particularly devastating in arid zones. Coastal wetland salinization is caused by a
significant decrease in river discharge, diversion of freshwater for irrigation net-
works, and upstream seawater intrusion under the influence of tides.
3.4 Radionuclides
Radionuclides are present in the environment both as naturally occurring species
and as anthropogenic substances. Emission of ionizing radiation during the decay
of active atoms is the main contamination route for this group of pollutants: This
radiation can disrupt atoms, creating positive ions and negative electrons, and
cause biological harm. Exposure to large radiation doses can be fatal to humans,
while lower doses cause mainly elevated cancer risks (Stannard
1973
; Zhu and
Shaw
2000
; Shaw
2005
). Ionizing radiation may be emitted in the process of
natural decay of some unstable nuclei or following excitation of atoms and their
nuclei in nuclear reactors, cyclotrons, X-ray machines, or other instruments.
The exposure to ionizing radiation from natural sources is continuous and
unavoidable. For most individuals, this exposure exceeds that from all human-
made sources combined (UNSCEAR
2000
). The two main contributors to natural
radiation exposures are high-energy cosmic ray particles incident on the earth's
atmosphere and radioactive nuclides that originate in the earth's crust and are
present everywhere in the environment, including the human body itself.
Naturally occurring radionuclides of terrestrial origin (also called primordial
radionuclides) are present in various degrees in all media in the environment. Only
those radionuclides with half-lives comparable to the age of the earth and their
decay products exist in significant quantities in these materials. Irradiation of the
human body from external sources is mainly by gamma radiation from radio-
nuclides in the
238
U and
232
Th series and from
40
K. These radionuclides are
present in the body and irradiate the various organs with alpha and beta particles,
as well as gamma rays. Some other terrestrial radionuclides, including those of the
235
U series,
87
Rb,
138
La,
147
Sm, and
176
Lu, exist in nature but at such low levels
that their contributions to the dose in humans are small.
