Geoscience Reference
In-Depth Information
tightly coupled to the hydrocycle. This section
attempts to give a general introduction of the basic
principles and processes regulating the migration
of chemical pollutants. Possible anthropogenic
sources of pollutants and types of deposition
to the ground will be mentioned only briefly.
The sources generally can be divided into activ-
ities related to: (i) combustion (e.g. for energy
production or waste incineration), (ii) technical
processes (e.g. metal and chemical industries)
and (iii) other anthropogenic additions (e.g.
distribution of pesticides and fertilizers) (e.g.
Baudo et al. 1990; Butcher et al. 1992).
Different pollutants have different ranges
of atmospheric distribution, owing to variable
atmospheric residence times (e.g. mercury has
a relatively long residence time in the atmo-
sphere compared with many other metals). The
spatial scales may be local, regional or global.
After transport in the atmosphere pollutants
are deposited either together with rain (wet
deposition) or without (dry deposition). The rate
of atmospheric deposition depends on the type
of pollutant and soil cover. A coniferous forest,
for example, has a large active surface area per
square metre forest floor and generally catches
pollutants effectively. Pollutants are transferred
to the soil as direct deposition, throughfall or
litterfall. After deposition to the ground, the
pollutants mainly migrate through the environ-
ment with water. It is, therefore, of interest to
study processes influencing the transport from
the watershed divide to the lake.
Some elements (such as copper) are micro-
nutrients and essential for life. Such elements
are needed within a given concentration range
and are toxic outside the range. The effect is,
thus, a function of concentration or load. Differ-
ent ecosystems do not, however, necessarily
show the same response to the same given load.
Sensitivity (or vulnerability) parameters include
water pH, water hardness or lake colour, which
will influence the effect of a given pollutant con-
centration (see section 4.6.2). Many pollutants
such as cadmium, mercury and most synthet-
ically manufactured organic pollutants do not
have any known physiological functions. They
are non-essential to life.
During migration through ecosystems, pol-
lutants might be physically or chemically trans-
formed and more or less toxic forms may appear.
There are three main pathways of transformation.
1
Degradation including (i) photolysis, mediated
by sunlight and (ii) biologically mediated mineral-
ization of organic matter into its inorganic basic
components (energy locked within more com-
plex chemical structures is used as a source of
energy by degrading microorganisms).
2
Radioactive decay of radionuclides at the rate
of their physical half-lifes. The decay produces
daughter nuclides, which in turn decay. This
continues until a stable element is reached.
3
Chemical speciation (e.g. oxidation/reduction,
metal methylation) due to changed environ-
mental conditions.
For mass transport of chemical pollutants there
are two basic physical processes, diffusion and
advection. Nature has an inherent reluctance
against structured orders, such as differences in
concentrations, for example across different inter-
faces such as air-water and water-sediment.
This is one interpretation of the second law of
thermodynamics and the force of diffusion. The
diffusive flux of pollutants is, thus, proportional
to the difference (gradient) of concentrations.
Volatilization/evaporation is one example of
a diffusive flux. Advection, on the other hand,
is the mixing by stirring. The force might be
external, for example wind-driven waves in lakes,
or internal, for example density differences in
stratified waters, which causes mixing. Advective
transport can be laminar or turbulent.
Retention is the opposite to migration, a term
expressing the storage of a pollutant within the
system in relation to the load. Pollutants with
long retention times (a high retention) are poten-
tially more hazardous because it means longer
exposure times within the ecosystem. The reten-
tion is affected by several factors. Some of the
most important are the chemical characteristics
determining the distribution coefficient,
K
d
, and
the physical factors influencing the flushing of
pollutants through the system. Influences of the
physical environment on the retention of pollut-
ants mainly include the rate of flushing though
the system. This is expressed by the theoretical
