Urban aquatic sediments - Sedimentology of Aqueous Systems - page 134

Ellis et al. (1986) further suggest that these urban
fl ushing effects can be cyclical owing to local climate
leading to periods of dry and then wet conditions
where material tends to accumulate on surfaces and
is then washed off after rain. Pollutant concentra-
tions can therefore be dependent on storm duration
and frequency (Marsalek 1990). However, Ellis et al .
(1986) found that fi rst fl ush was not necessarily
present during all storm events, but when it was,
secondary peaks may accompany it as material was
fl ushed from contributing surfaces further from the
site of discharge. Brinkmann (1985) suggests that
both urban sediment and water quality are depend-
ent on site-specifi c characteristics, leading to results
obtained being applicable to the specifi c stormwater
catchment where the study was performed. The fol-
lowing section further explores the issue of site spe-
cifi city, which has important ramifi cations for
subsequent stormwater management strategies in
urban areas.

Pb ( μ g/g -1 )

180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0

5.5 Transport of particulate-associated
pollutants in urban aquatic
environments; partitioning and
speciation

Fig. 5.5 Distribution of lead in a Brazilian urban watershed.
From Poleto & Merten (2007).

the urban environment (Ellis et al . 1986; Morrison
et al . 1989), and can result in a peak in contaminant
loading before the peak in discharge on the urban
storm hydrograph. It is due to the structure of tradi-
tionally hard-engineered drainage systems in urban
areas, which encourages PAPs adhering to the road
or pavement surface or associated with road gutters
and gully pots (Charlesworth & Foster 2005) to be
removed in the fi rst 10% of rainfall during a storm.
Other factors involved in the production of a fi rst
fl ush and cited by Ashley et al. (1992) include:
￿ rainfall intensity;
￿ antecedent dry period;
￿ cleaning protocol;
￿ localization and type of drainage system;
￿ drainage system gradient.
Although there is some argument about whether
the fi rst fl ush is common to all urban environments
(e.g. Deletic 1998), or may be a site and storm spe-
cifi c phenomenon (Lawler et al . 2006), there is suf-
fi cient evidence for allowance to be made for it in
management strategies targeting water quality in
urban environments (Kayhanian & Stenstrom 2008).

Although section 4 stated that most pollutants are
transported in the environment attached to particu-
lates, the environmental conditions prevalent in
urban areas are subject to rapid and constant change.
With changing conditions, such as lowering of pH,
or changing redox (Brikker 1999), formally particu-
late-associated contaminants can be released into
solution. There have been few studies of changing
partitioning of contaminants between sediments and
water, but Morrison et al . (1984) were able to use a
storm hydrograph to show that the partition coeffi -
cient between dissolved and particulate-associated
heavy metals changed little as the storm progressed,
and that it remained similar for a storm following 2
days after the initial one. Many recent studies (e.g.
Glenn et al. 2001, 2004; Fan et al . 2004; Hallberg
et al . 2007; Sansalone & Ying 2008) have applied
the partition coeffi cient of these contaminants in the
design of urban water treatment processes. However,
pollutants are not just adsorbed to particulate sur-
faces, but they can be chemically linked by adsorp-
tion, co-precipitation, formation of organometallic
co-ordination complexes, and incorporation into the