Geoscience Reference
In-Depth Information
(a)
(b)
V
V
(d)
(c)
P
V
10
ยต
m
Fig. 6.12
Minerals precipitated within canal sediment after deposition (backscattered electron (images). (a- c) Vivianite
(Fe
3
(PO
4
)
2
.8H
2
O) taking the forms of radiating needles and laths. (d) Authigenic framboidal pyrite (FeS
2
; p). Scale bars are shown
for each micrograph.
pollution. Such sediments act as ideal archives
for environmental change (e.g. salt marshes,
floodplains). Many archives from fluvial and
estuarine sediments have shown the increasing
impact of urbanization on rivers and estuaries
(e.g. Cundy et al. 1997; Walling et al. 2003).
Many of these pollution changes are mixed in
with inputs from industrial and mining activities,
however, and thereby these studies do not give
good, clear information on past trends of urban
sediment quality and quantity. Although sedi-
ments do accumulate in urban environments, the
engineered and disturbed nature of the urban
environment has meant that continuous, undis-
turbed sediment records are uncommon. Many
receiving water bodies have undergone dredging
operations for shipping or remediation, or are
too shallow to allow the accumulation of fine-
grained sediments. There are, however, a limited
number of studies that have provided informa-
tion on short- to long-term changes in urban
sedimentary processes. Short-term records have
come from road-deposited sediment monitoring
programmes, whereas longer term records have
been provided by sediment profiles in urban
lakes and canals.
Although no long-term continuous monitoring
data sets exist for RDS composition, individual
studies over two decades on specific cities can be
combined to provide useful data on temporal
changes. One such example is Manchester, UK,
where Pb levels have been documented to have
fallen (see section 6.3.1). A more recent study
(Robertson et al. 2003) has shown a further reduc-
tion to an average of 265 ppm in 2000. Similar
results where found by Charlesworth et al.
(2003a) for the city of Birmingham, UK, where Pb
levels in RDS were documented to have dropped
by one-third from 1987 to 2002. Recent analyses
have also shown an increase in platinum group
elements in RDS since the early 1990s (e.g. Wei
& Morrison 1994a; Motelica-Heino et al. 2001;
