Chemistry Reference
In-Depth Information
in lakes in which the contamination of the fi sh with
methylmercury is high (Chan
et al
., 2003; Scheuhammer
and Blancher, 1994). The adverse effects are related to
effects on the nervous system, and the breeding behav-
ior is affected. The effects begin to appear when the
mercury concentration in the food of the birds exceeds
0.3
8.12 Nickel
Nickel concentrations in the ocean vary from
2-3 nmol/L in the surface waters to 5-10 nmol/L in the
deeper waters (Bruland, 1980; Yeats
et al
., 1995). NiCO
3
0
is the dominating species in seawater, followed by Ni
++
and NiCl
+
(Mantoura
et al
., 1978).
Nickel is emitted into the atmosphere, and a mobi-
lization factor of 3.5 has been estimated (Lantzy and
Mackenzie, 1979). Ninety-two percent of the nickel
transported in the rivers of the world is precipitated
in the estuaries, and an average sediment enrichment
factor in lakes and coastal areas of 1.3 has been found
(Salomons and Förstner, 1984).
g Hg/g wet weight (Scheuhammer, 1991); more
than this value is widely occurring in Canada, as well
as in other countries.
Exposure to methylmercury in the laboratory is
capable of reducing shell thickness in the eggs of
domestic fowls (Lundholm, 1987; 1995) by affecting
the overall calcium metabolism of the bird (a mecha-
nism somewhat different from the one exerted by
DDE). Earlier studies, however, showed no such
effect of organic mercury in mallards (Haegele
et al
.,
1974) and methylmercury in ring doves and Ameri-
can kestrels (Peakall and Lincer, 1972), and it is ques-
tionable whether exposure to methylmercury was
a participating agent in the widespread eggshell thin-
ning observed among predatory and fi sh-eating birds
during the 1960s.
µ
8.13 Selenium
Concentrations of total dissolved selenium in ocean
water typically vary from 0.5 nmol/L in the surface
layers to 1.0-1.5 nmol/L in the deeper layers (Cutter
and Cutter, 1995). In the photic zone of the upper few
hundred meters, organic selenium compounds (Se(-II)
dominate, whereas selenite and selenate exist in equi-
librium at depths below approximately 300 m at a ratio
of approximately 0.65 (Se(IV)/Se(VI) ) (Cutter and
Cutter, 1995).
Selenium concentrations in soils show large vari-
ability, leading to defi ciency symptoms in terrestrial
organisms in some parts of the world and toxic symp-
toms in others.
Selenium is emitted to the environment especially
through combustion of fossil fuels, and the anthropo-
genic emission to the atmosphere was estimated to
14,000 tons (Lantzy and Mackenzie, 1979). The natural
emission was estimated to approximately 3400 tons,
with only 410 tons bound in particles (Lantzy and
Mackenzie, 1979).
Mobilization of naturally occurring selenium as a
result of human activity may be seen under special
circumstances. In an interaction between fairly com-
plicated geological, chemical, and hydrographical
conditions, intensive irrigation in California led to
mobilization of soil selenium that ended up in surface
waters in the Kesterson Reservoir (Hamilton, 2004);
teratogenic effects were seen among the water birds
of the reservoir, and reproduction among the fi sh was
affected.
8.10.6 Implications for Human Health
Predatory fi sh—both in the marine and freshwater
environment—accumulate methylmercury to fairly
high levels and mammals—such as seals, dolphins, and
some whales—that feed on predatory fi sh accumulate
methylmercury to even higher levels. Polar bears that
feed on seals again attain even higher levels.
Humans who rely on fi sh or predatory mammals at
the top of aquatic food chains as their main food source
may be exposed to high levels of methylmercury in
their diet—levels that easily exceed the limits recom-
mended by WHO.
It is beyond the scope of this chapter to cover the
details of human exposure to and the effects of mer-
cury that are described in Chapter 33.
8.11 Molybdenum
Molybdenum is an essential element with special
importance for plants. It exists in seawater in the hex-
avalent state (as molybdate); surface concentrations of
0.1
mol/L (Collier, 1985).
Molybdenum is emitted to the environment through
combustion of fossil fuels (up to 300 mg/kg in lignite),
and fl y ash may contain up to 60 mg/kg (Davis, 1991).
The mobilization factor for molybdenum is as high as
45 (Lantzy and Mackenzie, 1979).
Ecotoxicological problems with emitted molybde-
num have especially been associated with effects on
grazing livestock (Davis, 1991).
µ
8.14 Silver
In the oceans, the depth profi le for silver shows
typical nutrient-like characteristics (Flegal
et al
., 1995;
Ndung'u
et al
., 2001; Zhang
et al
., 2001), with low
concentrations (0.7 pmol/L) in the surface water and
