Geoscience Reference
In-Depth Information
(a)
(b)
(c)
(d)
Fig. 6.6
Sediment grains within road-deposited sediments (backscattered electron images). (a) Spherical Ca-Si-Al-Fe glass grain.
(b) Iron oxide fragment, probably derived through the oxidation of a metal Fe grain. (c) Metal-rich glass grain, probably derived from
smelting processes. (d) Aggregated grain of quartz and clay particles; probably building material. All scale bars
=
100
μ
m.
In addition, abundant anthropogenic grains are
present, including glass particles from industrial
processes and high temperature combustion,
metal slags, cement grains, metallic fragments
and iron oxide particles (Fig. 6.6).
Road-deposited sediments characteristically
exhibit high concentrations of metals, an aspect
of which most research has been directed towards
(e.g. Beckwith et al. 1986; Kim et al. 1998;
Charlesworth et al. 2003a; Robertson et al.
2003). Levels of metals in these sediments are
commonly an order of magnitude higher than
those of natural sediments. Table 6.2 shows
typical values of metals in RDS for a number
of urban environments. There is clearly a wide
range of values, and Charlesworth et al. (2003a)
analysed these data to show that a significant
positive relationship exists between population
size and metal levels in RDS. Particular study
has been made on the levels of Pb within urban
street sediments, largely based on concern over
human exposure to toxic levels of this contamin-
ant. A significant enrichment in Pb levels in inner
city centres has been documented (Duggan &
Williams 1977; Thornton et al. 1994; Robertson
et al. 2003), supporting other evidence, particu-
larly Pb isotopic data (Zhu et al. 2001), that Pb
is derived from petrol combustion in such sedi-
ments. More recent studies of RDS has shown a
decrease in the levels of Pb, consistent with the
reduced use of leaded petrol. In 1975 average lead
levels were found to be 941 ppm in Manchester
City, against a background level of 85 ppm
(Nageotte & Day 1998). By 1997 this had fallen
to 569 ppm (Nageotte & Day 1998). A more
recent study (Robertson et al. 2003) has shown
a further reduction to an average of 265 ppm
in 2000 (see Case Study 6.1). These relatively
