Geoscience Reference
In-Depth Information
potential impact on human health, including
damage to the kidneys and nervous system.
In contrast, Serrano-Belles & Leharne (1997)
documented the enhanced release of Pb from
RDS upon the addition of chloride, in the form
of salt, probably as a result of the formation of
chloro-lead complexes. There is significant scope
for more detailed mineralogical analysis of urban
sediments, and the role individual mineral phases
play in contaminant uptake and mobility.
There have been only a few studies that have
looked in detail at the grain-size distributions
in RDS, and the distribution of contaminants
between grain-size fractions. In general, RDS
displays a wide range in grain sizes. Droppo
et al. (1998) reported the mean grain size of RDS
in Hamilton, Canada to be 227
et al. 1980). Studies on city-scale variability have
shown that Pb levels are lower in outer city loca-
tions compared with inner city sites, indicating
the role that traffic plays in the distribution of
this contaminant (Duggan & Williams 1977;
Massadeh & Snook 2002; Robertson et al. 2003).
Similar patterns have been observed for urban
soil samples. For example, Madrid et al. (2002)
showed that metal concentrations are higher in
soils in the old quarter of Seville, rather than its
outskirts. This was put down to vehicular-sourced
metals. There has been a paucity of systematic
spatial analysis of RDS composition over the scale
of a city. A spatial survey of Manchester, UK in
2004 (Fig. 6.7; unpublished data) documented
a large range in metal levels, with hotspots of
Pb, Cu and Zn. These distribution patterns were
related to both vehicle density and industry.
Another spatial survey (in Birmingham, UK) by
Charlesworth et al. (2003a) documented a large
range in metal levels, with hotspots of Pb, Cu and
Zn (Fig. 6.8). These distribution patterns were
related to both vehicle density and industry.
There has been very limited study into the
temporal variability of RDS. The limited studies
that have been published indicate that there
is temporal variability, especially in the input of
anthropogenic material. Sodium and chlorine
levels in RDS have been shown to fluctuate, with
high levels being present in the winter months
as a result of road salt application. Of particular
importance is the weather, with contaminant
levels (especially Pb) being highest following
a number of dry days. Massadeh & Snook
(2002), however, suggest that lower Pb levels
in Manchester RDS in the summer is a result
of lower traffic densities during the summer.
Long-term data sets are limited to those on Pb
that were highlighted in a previous section.
μ
m. Sutherland
(2003) found the
m fraction to dominate
the mass fraction of RDS in Hawaii, accounting
for 38% of the sample. Several studies have also
been carried out to determine the contaminant
loading on different grain-size fractions, with
differing results. Biggins & Harrison (1986)
showed that the mass-dominant fraction of Pb
was in the 250 -500
<
63
μ
m fraction, but that there
was a range from 2 to 30% of the mass load-
ing in the
μ
m fraction. Stone & Marsalek
(1996) found similar results for RDS in London,
UK. Sutherland (2003) found a much higher
Pb loading in the
<
63
μ
m fraction for RDS
in Hawaii, with this fraction accounting for
an average of 51% of the Pb mass. In a similar
manner to sediments from other environments,
the increase in contaminant loading in finer grain
sizes is generally believed to be a result of the
increased surface area with decreasing grain size,
providing greater surface area for metal sorption
to clay minerals or organic matter. The recogni-
tion that contaminant loading is heterogeneously
distributed is important when considering the
management and pollution abatement of RDS
(see section 6.6.1).
The spatial variability of RDS composition has
been studied at a range of scales. Studies of RDS
have shown that variability exists across the
street environment, with different levels of con-
taminants being present in gutter samples from
those in street centres and pavements (Linton
<
63
μ
6.3.2 Gully pots and sewer systems
Gully pots and sewers are the key elements
of subsurface urban drainage systems and the
movement and storage of sediments in these has
a marked impact upon both the physical and
chemical aspects of urban drainage. The study
of sediment build-up and pollutant loading in
