Environmental Engineering Reference
In-Depth Information
5
Urban aquatic sediments
Cristiano Poleto
1
, Susanne Charlesworth
2
& Ariane Laurenti
3
1
Hydraulic Research Institute, Federal University of Rio Grande do Sul, Brazil
2
Department of Geography, Environment and Disaster Management, Coventry
University, UK
3
Department of Pathology, Federal University of Santa Catarina, Brazil
5.1 Introduction
As point source emissions of pollutants are increas-
ingly brought under control, diffuse sources are
emerging as a serious and continuing threat to the
aquatic environment, and this is refl ected in an
increase in legislation and initiatives designed to
reduce and control them.
The process of urbanization begins with the removal
of vegetation and exposure of bare soil. In the classic
study by Wolman & Schick (1967), in Maryland,
USA, the increase in sediment delivery after land
clearance was over 1000 times that of natural erosion
rates and illustrates the ongoing problems experi-
enced by urban areas the world over. In association
with changes to the terrestrial environment, the city's
aquatic systems are not only impacted by the greater
delivery of sediment, but also physical changes to
channel morphology such as straightening, channeli-
zation and canalization (Fig. 5.1). Such changes lead
to disconnection of the river continuum, increasing
stream discharge, fl ooding, and increasing sediment
erosion, which adds further layers of complexity to
what was already a complex natural ecosystem and
only a holistic approach to its study will enable
better understanding to be gained. Thus the urban
aquatic environment can in some ways be considered
unique in that water is transported in artifi cial con-
duits, enabling it to exit the city as quickly as pos-
sible. Owing this speed, it can carry relatively large
loads of particulate-associated pollutants (PAPs),
which eventually reach receiving watercourses.
Urban runoff is now one of the major sources of
pollutants to the aquatic environment (Jefferies
et al
.
2007), and Deletic
et al
. (2000) cite sediments as
“the most important potential pollutant” (p. 3386)
carried in association with that runoff. If this is not
acknowledged and is subsequently untreated it has
the potential to downgrade receiving water quality.
5.2 The urban aquatic environment
The urban hydrological cycle consists of modifi ed
natural features normally involved in processes gov-
erning the transport and deposition of fi ne sediment,
such as channelized river reaches, lakes, and ponds
with concrete banks and culverted infl ows, as well
as uniquely urban landforms such as storm drains
and gully pots. These features enable the rapid
removal of water from the urban area (Pearson
1990) and as such are designed with transport in
mind rather than deposition. The smooth profi les of
storm sewers for instance, coupled with high water
discharges, change the magnitude and frequency of
fl ooding (Douglas 1999) and therefore do not
encourage sediment deposition. However, it has been
found (Butler & Davies 2000) that up to “80% of
urban drainage systems in the UK have at least some
permanent sediment deposits” (p. 315). Sediments
are themselves considered to be contaminants
(Horowitz 1995), but their importance in terms of
urban geochemistry lies in the contaminants that can
become adsorbed and transported with them (Lick
1987; Striegel 1987). It is diffi cult to divorce water
and sediment quality because the fi rst is the transport
medium of the second.
The overall environmental quality of water bodies
located in urban areas is generally quite poor owing
to pollutants of domestic, industrial, hospital, or
agricultural origin, leading to anoxic processes,
