increases in nitrate inputs) has resulted in large-scale changes in phytoplankton
ecology in the Black Sea itself. Large numbers of the world’s rivers are now exten-
sively dammed for flood control and to provide hydroelectric power. These
numbers are expected to increase in the near future and the resulting effects on
riverine nutrient fluxes will also grow.5.6 Heavy metal contamination
Heavy metals such as mercury (Hg) and lead (Pb) are so called because of their
very high densities (Hg=13.5 g cm-^3 , Pb=11.3 g cm-^3 ) when compared to other
common metals, for example Mg=1.7 g cm-^3. Heavy metals are of concern
because of their toxicity to humans and other animals. However, other elements,
for example the semimetal arsenic (As=5.7 g cm-^3 ), are also toxic. It can be argued
that the term heavy metals should be more encompassing to include the toxicity
of the element. Toxicity depends on an element’s chemistry, the mode of contact
with the host organism, the concentration of the element and the host organism’s
biochemistry. Indeed some substances that are toxic at high concentrations may
be essential to life at low concentration (Box 5.5). Heavy metals are of concern
because of their toxicity to humans and other animals, but also because they are
non-biodegradable. All heavy metals occur naturally, but industrial activity can
markedly increase their concentrations in natural waters. These contamination
maxima tend to be localized rather than widespread. In Section 5.6.1 we high-
light the specific case of mercury associated with gold mining. Mercury is
particularly interesting as its toxicity is intimately related to biogeochemical
processes and redox conditions. In Section 5.7.2 we discuss the case of natural
rather than anthropogenic arsenic contamination of groundwater.5.6.1 Mercury contamination from gold mining
Mercury has had a host of industrial and commercial uses, ranging from use in
batteries, for the production of commercial chlorine, as a fungicide on seeds and
in the mining of precious metals such as silver (Ag) and gold (Au). In the latter
context the unusual property of metallic mercury (Hg^0 )—being liquid at ‘room
temperature’—is exploited to help isolate particulate gold from mined gravel slur-
ries. When mercury is added to the slurry it readily forms an amalgam with the
gold:
eqn. 5.21
This amalgam or alloy has a higher density than the surrounding gravel slurry
allowing easy separation. While this method of separation has long been used by
miners, for example in the Californian gold rush of the late 19th century, it is
currently an environmental concern in many developing countries. In the early
1980s, for example, the Amazon Basin experienced its own ‘gold rush’—some
claim the largest single gold rush in history—and mercury is still used extensively
as an amalgam in the myriad small-scale gold mines (garimpos). The environ-Au()metal+ÆHg^0 Au Hg- ()amalgam170 Chapter Five