Chemistry Reference
In-Depth Information
sources leading to metal contamination of the
atmosphere.
mercury that cause concern from an ecotoxicological
and toxicological point of view (Scheuhammer, 1991).
1.1.2 Emissions into Water
1.2.2 Oxygen Depletion
Mining activities and a number of industrial proc-
esses involving metals (e.g., catalysts in chloroalkali
processes, anticorrosive surface treatment) are some
of the major point sources from which metals are
released into the aquatic environment, but metals from
more diffuse sources such as atmospheric deposition,
storm water (erosion of gutters, wearing of tires, etc.),
and ordinary household waste water may end up in
the aquatic environment. Deliberate emission of met-
als into the aquatic environment has taken place when
copper has been used as an herbicide to prevent algal
growth or used in fi sh farms to treat parasite infections
in the gills of the fi sh.
Eutrophication of coastal marine areas has led to an
increasing number of incidences of oxygen depletion
in the bottom layers of the water column during recent
decades. In bottom waters with full oxygen saturation,
the upper few millimeters of the sediment are normally
oxidized, and manganese and iron are found in their
oxidized forms (MnO
2
and Fe
3+
). If the oxygen tension
decreases below approximately 16% of full satura-
tion, MnO
2
is reduced to Mn
2+
that may leak out of the
sediment into the water column. The manganese con-
centrations in anoxic or oxygen-depleted waters may
increase from approximately 1
g/L to up to 1 mg/L
(Kremling, 1983). The reduced manganese in the water
column is oxidized fairly slowly (Section 8) on reintro-
duction of conditions of full oxygen saturation, and
benthic organisms may be exposed to increased man-
ganese concentrations for extended periods of time.
This leads to accumulation of manganese in benthic
organisms, and the use of the manganese content of
lobsters has been suggested in monitoring programs
for oxygen depletion (Baden and Neil, 2003; Baden
et al
., 1994; Eriksson and Baden, 1998).
µ
1.1.3 Emissions to Soil
On a general scale, atmospheric deposition has
constituted one of the major inputs of metals into soil,
especially in areas affected by deposition from metal
smelting activities. In certain areas, specifi c sources
for metal contamination may, however, be important.
Phosphate fertilizers have been important sources for
contamination of agricultural soils with cadmium and
likewise sludge from sewage treatment plants has
added metals (especially cadmium has caused con-
cern) to farmland when used as soil amendment. The
addition of copper to, for example, the feed of pigs
may lead to copper concentrations in the pig manure
that are high enough to exceed the soil quality criterion
for copper if the manure is used as soil amendment
over several years.
Some metals and metalloids (e.g., copper and arsenic
compounds) have been used as pesticides and thus
spread deliberately in the terrestrial environment.
1.2.3 Pyrite Oxidation
If waterlogged, anaerobic terrestrial areas are
drained, oxygen typically penetrates deeper into the
soil, and this leads to oxidation of pyrite (FeS
2
) in the
soil, and the oxidized iron compounds (a complex mix-
ture of iron oxides and hydroxides) will leach toward
the streams together with the sulfuric acid from the oxi-
dation of the reduced sulfur. The iron contamination in
streams of such areas may have severe consequences
for both invertebrates and fi sh in the streams.
1.2 Indirect Mobilization of Metals
1.2.1 Acid Rain
2 THE BIOGEOCHEMICAL
TRANSPORT OF METALS
When acidic precipitation falls on soils in which the
buffer capacity has been exceeded, cations bound to
the negative charges on the surfaces of soil particles
are exchanged with hydrogen ions, and the concentra-
tion of metals in the pore water of the soil increases.
As rainwater percolates, the metal ions will be trans-
ported either toward the groundwater or toward
streams and lakes. The acidifi cation of lakes is often
associated with increases in metal (i.e., alumina, cad-
mium, lead, mercury) concentrations (Nelson and
Campbell, 1991) in both lake water and biota, and it is
especially the elevated concentrations of alumina and
2.1 Atmospheric Transport
Metals undergo—as the other elements in the Earth's
crust—natural, biogeochemical cycles in which they
are transported between lithosphere, hydrosphere,
and atmosphere. For a number of metals, human
activity has increased the mobilization from the litho-
sphere to a degree where the addition of metal to the
hydrosphere—especially via the atmosphere—from
anthropogenic sources far exceeds the addition from
natural sources.
