Geology Reference
In-Depth Information
Dry
transformations
Wet
transformations
Air
concentrations
Initial
mixing
Transport
and diffusion
Scavenging
Total emissions
Scavenging
Dry
deposition
Wet
deposition
Manmade
Natural
Figure 1 Schematic diagram of the atmospheric cycle of a pollutant
1
(Reprinted from Environmental Science and Technology by permission of the
American Chemical Society.)
carefully designed to avoid leaching by rain and dissemination of
pollutants into groundwaters, which might subsequently be used for
potable supply. In the past, however, instances have come to light where
insufficient attention was paid to the potential for groundwater con-
tamination, and serious pollution has arisen as a result.
Another important consideration regarding pollutant cycling is that of
degradability, be it chemical or biological. Chemical elements (other than
radioisotopic forms) are, of course, non-degradable and hence once dis-
persed in the environment will always be there, although they may move
between compartments. Thus, lead, for example, after emission from
industry or motor vehicles, has a rather short lifetime in the atmosphere,
but upon deposition causes pollution of vegetation, soils, and waters.
2
On a
very long timescale, lead in these compartments will leach out from soils
and transfer to oceans, where it will concentrate in bottom sediments.
Some chemical elements undergo chemical changes during environmental
cycling, which completely alter their properties. For example, nitrate added
to soil as fertilizer can be converted to gaseous nitrous oxide by biological
denitrification processes. Nitrous oxide is an unreactive gas with a long
atmosphere lifetime, which is destroyed only by breakdown in the strato-
sphere. As will be seen later, nitrogen in the environment may be present in
a wide range of valence states, each conferring different properties.
Some chemical compounds are degradable in the environment. For
example, methane (an important greenhouse gas) is oxidized via carbon
