Environmental Engineering Reference
In-Depth Information
indoor exposure to dust particles at home and school,
etc.) would add to the exposure to playground soil
and that the overall risk to children in urban environ-
ments has not yet been reliably calculated. The quan-
titative estimates of risk from exposure to urban
particulate materials are also affected by a high
degree of uncertainty arising from the estimates of
exposure rates and from the toxicity data used in the
risk assessment. Despite the numerous studies
attempting to quantify exposure factors relevant to
a risk assessment for children during playing activi-
ties, there is a signifi cant variability in their numeri-
cal results (Evans
et al
. 1992; Buchardt-Boyd
et al.
1999; USEPA 1997, 2002 and references therein;
Hemond & Solo-Gabriele 2004), which refl ects the
diffi culties involved. Besides, some of these factors,
like exposure frequency, cannot be directly extrapo-
lated from one survey to another because playing
habits and time spent outdoors may differ substan-
tially from one region to another. Additionally,
quantitative estimates of the toxic potency of ele-
ments and compounds found in urban matrices are
being reviewed permanently with considerable
changes in their values and sometimes even in the
threshold or non-threshold behavior of the
toxicant.
Although these considerations suggest that the
numerical results of risk assessments in urban envi-
ronments should be interpreted with caution, they
do not invalidate the potential of risk assessment to
identify the contaminants of most concern and the
most relevant routes of exposure.
solution, and as particulate solids. Figure 4.2 shows
a simplifi ed model of the sources, pathways, and
sinks that constitute the urban geochemical cycle.
The most important mode of transport for trace
elements within the urban environment is probably
as particulate materials. The fi ne fraction of these
solid particles is especially relevant for two reasons.
Firstly because, as previously discussed, particles
with a diameter less than 100
m can be resuspended
and are easily transferred between soil, street dust,
and atmospheric aerosol. Secondly, it is generally
agreed that particles in the silt-clay size range have
the highest capacity to bind, and therefore transport,
trace elements.
Along with trace elements supplied by urban and
industrial sources, urban particulate materials always
include an underlying component of natural mate-
rial, which is associated with particles of natural soil
or with airborne particles whose origin is to be found
outside the city limits. Although the exact chemical
makeup of this component is strongly related to the
type of geological material in and around a particu-
lar city, it is probably the major source of the Ce,
Ga, La, Th, and Y found in urban environments.
This association of “natural” trace elements is sur-
prisingly stable in that it is found in cities of different
urban characteristics and has been found preserved
all along the urban cycle: in the atmospheric aerosol,
in street dust, in the urban soil, and in urban sedi-
ments. Furthermore, the same combination of ele-
ments has been discovered to mark the natural
component of urban particulate materials in cities of
such different characteristics as Madrid, Oslo, and
Ostrava (De Miguel
et al
. 1999).
Incomplete descriptions of the urban cycle of some
elements have already been reported, as in De Miguel
et al.
(1998), who followed the fate of silver in the
city of Madrid. Silver can be introduced as a com-
ponent of medical (X-ray plates, dental alloys), com-
mercial (photographic fi lm) or industrial materials
(high capacity Ag-Zn and Ag-Cd batteries). Disposal
of these materials ultimately results in the release and
transport of Ag in urban waters. As these wastewa-
ters are treated in urban wastewater treatment plants,
Ag becomes concentrated in the sludge produced
during the treatment process, where it reaches values
close to 45
μ
4.4 Urban geochemical cycles
Having established the importance of studying the
geochemistry of urban environments in terms of the
risk imposed by potentially increasing concentra-
tions of hazardous elements in concert with popula-
tion increase (cf Charlesworth
et al.
2003) the
question of managing the risk arises. To apply man-
agement strategies, it is important to be able to
predict where “hot spots” of contamination are
likely to occur. Hence there have been many attempts
to model urban geochemical cycles, with varying
degrees of success.
Trace elements circulate between different urban
media (i.e. atmospheric aerosol, street dust, urban
soil, urban sediment) in the gas phase, in aqueous
1
. This sludge is in turn processed
into a compost that is widely used by municipalities
as soil amendment in parks and gardens. Silver is
−
μ
g g
