Environmental Engineering Reference
In-Depth Information
lutants in the environment; geographically they are
spread globally, with above background values
found in polar ice cores. Hence, this Chapter will
concentrate on the distribution of these elements in
the urban aquatic environment. Chemically, these
elements are characterized by their high densities
(Baird 2002), and in contrast with organic pollut-
ants, are not biologically or chemically degraded and
hence are conservative in nature. Heavy metals such
as cadmium (Cd), copper (Cu), chromium (Cr), lead
(Pb), mercury (Hg), nickel (Ni), zinc (An), arsenic
(As), cobalt (Co), and selenium (Se), make up the
group of chemical elements that appear frequently in
urbanized areas (Porto 1995; Gromaire
et al
. 2001;
Banerjee 2003; Poleto & Laurenti 2008). These ele-
ments can accumulate locally and be transported
long distances (Marchand
et al
. 2006). For example,
studies by Nichols
et al
. (1991) of the sediments
deposited in the upper connecting channels of the
Great Lakes found that although pollution was at its
heaviest closest to the industrial source areas, there
were still signifi cant concentrations up to 60 km
downstream. Decreasing concentrations may be
either associated with dilution effects as the sedi-
ments are mixed with less contaminated material as
they are transported further from the urban centre,
or may be due to the smaller volumes of fi ner, rela-
tively contaminated, particulates being preferentially
transported in the water body, whereas the relatively
less contaminated coarser material settles out earlier
in the journey. Patchy areas of higher concentration
may be associated with specifi c urban sources such
as storm sewer outfalls (see, for example, Foster
et al
. 1996; Rhoads & Cahill 1999). Table 5.1c sum-
marizes the concentrations of contaminants found in
some studies of urban rivers and streams. It includes
data from De Miguel
et al
. (2005) from rivers passing
through both Coventry, UK, and Madrid, Spain, and
highlights the variability in concentrations that have
been found in urban streams.
There is evidence, however, that most metals spe-
cifi cally generated in urban areas tend not to travel
far from their source areas (Foster & Charlesworth
1996). Foster & Charlesworth (1996) compared
sediments deposited in paired lake catchments in the
city of Coventry, rural Warwickshire, and the Scilly
Isles, all in the UK. It was found that there was an
order of magnitude difference in the concentration
of Pb and Zn in bottom sediments from city-centre
organic matter content. These variations are the
result of factors such as geology, relief, land use,
climate, and anthropogenic impacts in the river basin
itself. Sediments in rivers draining urban watersheds
tend to be more organic because of, for example,
domestic effl uents (Rocha & Martin 2005), which
are responsible for deleterious effects on water quality
(Gromaire
et al
. 2001; Chebbo & Gromaire 2004).
5.4 Urban sediment quality
Environmental quality is normally expressed in terms
of water chemistry, but some of the most toxic
elements, particularly metals, are not transported in
solution; rather, they form associations with particu-
lates by adsorption and precipitation (Sigg 1998).
Table 5.1a,b shows some of the limited data avail-
able on heavy metal concentrations in suspended
sediment loads of urban rivers and lakes. In fact, of
the 128 priority pollutants listed by the United States
Environment Protection Agency (Bartram & Ballance
1996), 65% are either only, or mostly, found in
association with biota and particulates.
Many studies have found that the concentration of
these contaminants increases with decreasing particle
size owing to the large surface area of all the particles
combined (Wilber & Hunter 1979). However, coarser
particles can also be associated with signifi cant con-
taminant loads and hence their larger mass compared
with fi ne particles makes coarser particles also an
important factor (Horowitz 1991). The usual empha-
sis is on the fi ner fraction such as silts and clays, which
have negative surface charges (Striegel 1987), because
these are perceived to be more signifi cant in terms of
their contaminant-carrying capacity. Transportation
attached to river sediments is considered the most
important way metals are circulated in diverse river
catchments (Castilhos 1999; Guerra 2000).
Three groups of major contaminants are frequently
identifi ed in aquatic urban sediments and because of
this they are used in sediment guidelines (see
MacDonald and Ingersoll, this volume), which
inform management strategies for protection of the
environment and biota:
trace elements;
hydrocarbons derived from petroleum; and
synthetic organochlorides.
Trace elements, especially those called “heavy
metals,” are among the most frequently found pol-
