Urban aquatic sediments - Sedimentology of Aqueous Systems - page 139

In a study of mercury cycling in eight streams in
three states in the USA, Marvin-DiPasquale et al.
(2009) found that sediments in the three urban
streams contained an average of three times as much
total mercury as that found in non-urban stream
sediments. However, it appeared that the capacity to
convert to methylmercury in the urban streams was
less than that of non-urban ones. Because the pro-
duction of methylmercury is controlled by methylat-
ing bacteria present in the sediments, it would seem
in this case that the sedimentary environment in
urban areas was not conducive to their presence and
hence less methylmercury was produced. Mercury is
the same as other metals in its sorption to particu-
lates and hence its concentration increases with
decreasing particle size (Hunerlach et al . 2004). It
preferentially binds to organic matter (Mason &
Sullivan 1998; Machado et al . 2008) and sulfur
(Marins et al . 1998) and hence its concentration
increases with rising amounts of these elements in
the sediment.
This chapter has given evidence of the pollution of
the urban aquatic environment caused by anthropo-
genic activities. In particular, this section has high-
lighted mercury, and lead and zinc, as elements of
particular concern. It has been shown that tradi-
tional drainage techniques, which are designed to
transport water and its associated contaminants out
of the urban area as quickly as possible, do a dis-
service to the receiving environment, providing as
they do a signifi cant transport mechanism for PAPs.
The following section presents a means of applica-
tion of some of the physico-chemical studies dis-
cussed in this chapter to provide one of the most
promising techniques for effi cient and sustainable
remediation of pollution in urban areas.

nature, whereby water is encouraged to infi ltrate
through a permeable surface that provides a means
of “cleaning” the water of its contaminants. These
surfaces will be incorporated in such individual
devices (or best management practices (BMPs) in the
USA) as porous paving (PPS), constructed wetlands
(see section 5.3), ponds, or swales, or the devices can
be deployed as a SUDS “train” in which several are
used together (for further details see Charlesworth
et al. 2003b).
Hence, the contaminated sediments generated in
an urban area can become treated such that, in some
cases, a signifi cant amount of the pollutants fi nding
their way into the SUDS devices can be removed
(Pratt 2004) by such processes as physical entrap-
ment in vegetated devices or the structure of a PPS,
systemic take-up by vegetation, or incorporation in
the micro-ecosystem that develops on the geotextile
associated with some PPSs (Newman et al. 2004).
Urban impacts are unsustainable should the status
quo prevail. Approaches such as SUDS provide a
means of sustainable urban living in the long term
by managing the behavior of human beings to take
account of water, rather than trying to modify the
behavior of water to suit the activities of society.

5.7 Conclusions

Weathering and erosion of material in aquatic envi-
ronments is a natural process leading to deposition
of sediment in rivers, lakes, and wetlands. However,
anthropogenic activities in urbanized catchments
pollute these environments, resulting in deterioration
of water and sediment quality in urban rivers and
lakes. This environmental degradation has become a
serious problem around the world owing to acceler-
ated urbanization and industrialization. Water in
cities is perceived as a nuisance at best, but at times
it can serve as a main water resource for surrounding
areas and thus water quality improvement becomes
important. It can also provide a means of enhance-
ment of urban areas, providing amenity and aesthet-
ics as well as being part of a sustainable drainage
approach, mitigating quality degradation as well as
fl ooding hazard.
This chapter has presented some of the character-
istics and mechanisms whereby sediment and its
associated pollutants are produced, transported, and
deposited in the city, and the way in which these are

5.6 Sustainable drainage systems

Although a detailed consideration of alternative
drainage techniques is beyond the scope of this
chapter, an introduction to sustainable drainage
systems (SUDS) will be given here. Also sometimes
called low-impact development in the USA (Dietz
2007), the functions of SUDS are threefold, as exem-
plifi ed by the SUDS triangle (Woods-Ballard et al .
2007; Charlesworth et al. 2003b) in which there is
an equal balance between water quality, quantity,
and biodiversity or amenity. This approach mimics