Geoscience Reference
In-Depth Information
leaded petrol (Nageotte & Day 1998; Massadeh & Snook 2002). Clear spatial distributions
of metal concentrations are present. For example, lead concentrations are higher in sediments
in inner city sites, than from those in outer city sites (Case Fig. 6.1a), reflecting the contrasting
levels of vehicular activity. Analysis of the spatial distribution of RDS composition shows that
metal levels are heterogeneous across the city, with different hotspot locations for different
metals. This illustrates the localized nature of contaminant inputs into RDS. Mineral magnetic
analysis of Manchester RDS (Robertson et al. 2003) reveals that the magnetic fraction is com-
posed predominantly of ferromagnetic multidomain particles, indicating inputs of predominantly
anthropogenic origin, derived primarily from automobiles.
Petrographic and mineralogical analysis shows that the sediments, in addition to quartz and
clay grains, comprise a significant component of anthropogenic grains (Case Fig. 6.1b). These are
either iron-rich combustion products (mostly iron oxides) or iron-rich glasses, probably also
derived from vehicular combustion. These grains contain high levels of heavy metals (e.g. Pb and
Cu up to 1500 ppm, and Zn up to 10,000 ppm) and are probably the major hosts for metals in
the Manchester RDS. Sequential chemical extraction schemes (see Chapter 1) show that most
metals are bound up with the reducible fraction (probably iron oxides and glasses), but in the
case of Zn a significant fraction is bound in the much more environmentally available exchange-
able fraction (Case Fig. 6.1c). This suggests that Zn is likely to be the most mobile metal in
the RDS in Manchester and release into road runoff may take place; an observation consistent
with storm runoff analysis. The RDS also contains significant levels of soluble nitrate, chloride,
sulphate and phosphate. These anions are key components in river water, of which nitrate and
phosphate are key nutrients. It is, therefore, likely that these sediments represent an input of
dissolved species into urban rivers, with subsequent impacts upon water quality.
Relevant reading
Massadeh, A.M. & Snook, R.D. (2002) Determination of Pb and Cd in road dusts over the period in which Pb
was removed from petrol in the UK.
Journal of Environmental Monitoring
4
, 567-72.
Nageotte, S.M. & Day, J.P. (1998) Lead concentrations and isotope ratios in street dust determined by
electrothermal atomic absorption spectrometry and inductively coupled plasma mass spectrometry.
Analyst
123
, 59 - 62.
Robertson, D.J., Taylor, K.G. & Hoon, S.R. (2003) Geochemical and mineral characterisation of urban
sediment particulates, Manchester. UK.
Applied Geochemistry
18
, 269 - 82.
sources of iron oxides in RDS it has also been
recognized that the complex and heterogeneous
nature of urban sediments limits the extent to
which these measurements can be used as sedi-
ment source tracers. For example, Charlesworth
& Lees (2001) consider that given the large
number of sources of magnetic material in urban
environments, and the processes that change
magnetic signatures, unmixing individual com-
ponents of the signal becomes very difficult.
The majority of contaminants to RDS are
derived from intrinsic sources, and the major
sources recognized are shown in Table 6.1. As
stated previously, Pb is predominantly derived
from leaded fuel (where tetraethyl-lead is used
as an additive). Lead levels in sediment have
declined, however, with the widespread reduc-
tion in use of leaded fuel (e.g. Nageotte & Day
1998). Copper and zinc have both been sourced
to vehicle activity, with Cu coming from cor-
roded car bodywork (Beckwith et al. 1986)
and Zn and Cd being derived from tyre wear
(Hopke et al. 1980). Chromium, bromine and
manganese are also present in tyres and brake
