Urban aquatic sediments - Sedimentology of Aqueous Systems - page 138

would provide a means of tracing specifi cally in the
urban environment (Charlesworth et al. 2000).
Methods of source identifi cation have included the
use of natural tracers (Russell et al. 2001), mineral
magnetic studies (Charlesworth & Lees 1999), and
the generation of a sediment fi ngerprint (Peart &
Walling 1986; Carter et al. 2003). This last method
was based on comparison of the properties of the
unknown sediment with material from potential
sources; it was developed by identifying a small
group of geochemical variables that could explain
the variability of the sources. Following this, samples
of suspended sediment were classifi ed using multi-
variate statistics. The focus of the work was on dis-
tinguishing sources originating from rivers beds and
surface soils and although results were reasonably
good, there were limitations. Yu & Oldfi eld (1989)
then used artifi cial mixtures of sediments of diverse
sources to evaluate the capacity of the method in
correctly separating the different sources. The results
showed that the mathematical procedure that they
used was a practical and effi cient method of estab-
lishing the relation between sediments in suspension
and the multiple sources involved. This work shows
that quantitative calculations are more useful than
purely qualitative descriptions, allowing the identifi -
cation of sources contributing to river sediments
(Minella 2003). However, the validity of establishing
a geochemical fi ngerprint depends on whether the
average properties of sediments in suspension can be
compared directly with the same properties of poten-
tial sources, using conservative properties.
The value of the study of urban environmental
quality has enabled the impacts of sediment and
associated pollutants to be assessed. Point sources of
sediment can now be controlled, for example on
construction sites (see, for example, USEPA 2005)
and concern can now turn to the management of
diffuse sources. With the reduction in release of Pb
to the environment owing to the introduction of
unleaded petrol and the removal of Pb in paints,
sediment studies have shown that the concentration
of Pb in the environment has reduced. This has led
to the focus on other metals such as Zn and Cu in
urban environments. However, mercury is generally
considered the most toxic of metals (Jitaru & Adams
2004; Boszke & Kowalski 2006), and although this
is associated with release during specifi c industrial
processes and to a lesser extent waste disposal, in

some cities of the world it is one of the most concern-
ing of metal pollutants. It is one of the most likely
of metals to impact on human health because of its
propensity to bioaccumulate up the food chain and
to methylate in water or sediment into its most toxic
form (Hortellani et al. 2005). The following section
therefore provides a case study of mercury in urban
environments.

5.5.3 Mercury in urban areas

Many studies have shown that industrial and agri-
cultural activities, waste disposal, gold mining, and
the use of fossil fuels are sources of mercury to the
environment (Sanders et al. 2006). Annually, in
Brazil, more than 85 million light bulbs are thrown
away in sanitary landfi ll, totaling about 3.5 tonnes
of mercury. As well as this source of mercury, it is
estimated that, from 1983 to 1993 more than 900
tonnes of mercury contaminated the Amazon because
of prospecting activities. For each kilogram of gold,
1.3 kg of mercury is lost to the environment and of
that, between 55% and 65% is released to the
atmosphere with the rest fi nding its way into aquatic
ecosystems. Hence the sources of mercury can be
both point and diffuse. According to Boszke and
Kowalski (2006), coal and lignite combustion in
Poland are responsible for the release of 44% and
18.5% of atmospheric mercury respectively, with
cement production and the disposal of fl uorescent
light tubes emitting 16.6% and 6.4% each. Mercury
is thus mainly emitted into the atmosphere and water
where it has high mobility, organic matter affi nity,
and a biomagnifi cation capacity that makes this
element one of the most harmful metals to biota
Gorski et al. (2003). However, the main property of
concern is that the metal is capable of conversion to
methylmercury, which is highly toxic. This can accu-
mulate in the tissue of fi sh and mollusks in much
greater quantities to those found in the environment.
The World Health Organization has therefore estab-
lished a maximum limit of 0.5 g g −1 total mercury in

fi sh, and a recommended maximum consumption of
400 g per week of fi sh and/or fi sh products (USEPA
2000). These values are alarming compared with the
amounts of fi sh eaten by riverside populations of
some Amazonian regions, where their daily con-
sumption of fi sh is about 250 g per individual (Poleto
& Castilhos 2008).

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