Environmental Engineering Reference
In-Depth Information
can be modifi ed to take account of whether pollut-
ants are bioavailable or bioaccessible, and the ways
in which potential risk can be estimated.
pearing totally beyond that distance (Llamas
et al.
1993). Charlesworth
et al
. (2007) plotted the distri-
bution of Zn, Ni, Cu, Cd, and Pb across the city of
Coventry, UK, and found “hot spots” associated
with the heavily-traffi cked main roads and industrial
areas, but these elevated concentrations were similar
to trends found in Madrid (as explained above) in
that they had reduced considerably at the city limits.
Unlike the infl uence of atmospheric deposition, the
disposal of urban and commercial wastes, and the
addition of fertilizers and composted sewage sludge,
have a very localized effect on the trace element
content of urban soils. If present, however, these
sources can contribute a larger amount of several
trace elements to the urban soil than atmospheric
deposition. De Miguel
et al.
(1998) found that urban
soils amended with composted sewage sludge pre-
sented levels of Cu, Ni, Pb, and Zn that were two to
three times higher than those in urban soils that did
not receive compost additions. Consequently, the
highest levels of metals in soil were detected in some
of the best-kept parks and gardens in the city, where
fertilizing takes place on a regular basis. Concern
over the potential implications of sewage sludge
application, in terms of increased trace element load
in soil, has fuelled research and legislative actions in
this fi eld (Giusquiani
et al.
1992; Tiller 1992; Chaney
& Ryan 1994; Gies 1997; Berti & Jacobs 1998).
Urban soil not only acts as a net accumulator of
trace elements but also provides a signifi cant amount
of them to the atmospheric aerosol and, particularly,
to street dust. An example of this role of the urban
soil is provided by De Miguel
et al.
(1997), who
found that some of the highest concentrations of lead
in the street dust of Oslo, Norway, were not associ-
ated with dense traffi c but with nearby soils where
lead had accumulated over long periods of time from
a lead smelter that was shut down several years
before the street dust sampling campaign took place.
Some of the sources of street dust have been estab-
lished in this section, and the fact that they can
become entrained and transported in the atmos-
phere. Previously, it has been established that there
are hazardous contaminants stored in various urban
environmental compartments. Section 4.3 considers
whether their presence constitutes a risk to the envi-
ronment as a whole, or arguably more importantly,
whether they constitute a risk to human health. This
involves a consideration of how urban geochemistry
4.3 Risk and health implications
Urban geochemistry has an obvious focus on the
environmental aspects of life in the city. It is not
surprising, therefore, that one of the major research
interests in this fi eld concerns the potential adverse
health effects of exposure to urban pollutants. Until
recently, with few exceptions, most studies had
either established an inferred link between elevated
concentrations of toxic elements in street dust and
soil and the observed incidence of a given effect in a
population, or had directly equated risk with pre-
dominance of mobile chemical species, as determined
in sequential or selective extraction protocols
(Banerjee 2003; Robertson
et al.
2003). The ecotoxi-
cological signifi cance of trace elements in street dust
has also been directly evaluated by means of bio-
assays (Wang
et al.
1998), instead of being indirectly
inferred from the results of a sequential extraction
procedure as was introduced by Tessier
et al.
(1979).
In the past few years, risk assessment strategies -
extensively employed by regulatory authorities to
defi ne soil screening levels or soil guideline values -
have increasingly been adopted and, when necessary,
adapted to the peculiarities of urban environments
to appraise the relevance of toxic elements and com-
pounds in urban matrices (Boyd
et al.
1999; Granero
& Domingo 2002; Korre
et al
. 2002; Wcislo
et al.
2002; Hemond & Solo-Gabriele 2004; Nadal
et al.
2004; Ferreira-Baptista & De Miguel 2005; Kim
et al.
2005; Lee
et al.
2005; De Miguel
et al.
2007).
Strategies such as those outlined above are based
on the separate assessment of (a) the toxicity of the
chemicals included in the analysis by exposure route
(i.e., inhalation, ingestion, and dermal contact), and
(b) the levels of exposure to those chemicals for the
potential receptors. For non-carcinogenic toxicants,
a range of exposures from zero to some fi nite value
(reference dose or acceptable/tolerable daily intake)
are assumed to be tolerated by the organism with
essentially no expression of the toxic effect. If the
daily dose to which a receptor is exposed exceeds
the corresponding reference dose, the receptor is
considered to be potentially at risk. On the other
hand, there is no level of exposure to a genotoxic
