Geoscience Reference
In-Depth Information
Table 6.3
Removal of lead load in road-deposited sediments by street sweepers in Palolo Valley, Oahu, Hawaii.
(From Sutherland 2003.)
Grain size fraction (
μ
m)
Mean Pb loading (%)
Mean street sweeper efficiency (%)
Pb load removal (%)
2000-1000
2.7
±
1.6
83.7
2.2
1000-500
10.8
±
6.2
81.7
8.8
500-250
13.4
±
6.2
79.3
10.6
250-125
12.8
±
6.3
75.0
9.6
125-63
9.7
±
2.4
66.7
6.5
<
63
50.6
±
14.9
48.7
24.7
Overall
62.4
6.6.3 Stormwater management ponds
moval, sediment quality guidelines come into
operation to determine its suitability for disposal.
With the exception of inland urban docks, most
sediment dredging from urban watercourses is
undertaken to remove contaminated sediments,
and to limit their impact upon water quality
(for details of dredging issues at coastal ports see
Chapter 1). Dredged material is either disposed
of through land application (e.g. Chen et al. 2002)
or, if contaminated, is disposed of to landfill.
An example of sediment dredging of urban
canals for remediation is that of Birmingham in
the UK (Bromhead & Beckwith 1994). Birming-
ham canals were built after 1770 and the banks
of the canals were heavily industrialized, by
for example metal manufacturing, chemical and
engineering works. Inputs and waste discharge
from these activities led to accumulation of
highly contaminated sediments, with sediments
having mean concentrations of 1.0% Cu, 0.7%
Zn, 0.3% Cr and 0.15% Pb (Bromhead &
Beckwith 1994). Dredging was undertaken to a
depth of 1.5 m below water level and resulted in
24,000 m
3
of sediment removal. Once removed
from the watercourses, such dredgings are gen-
erally treated as waste, and have to be disposed
of within strict guidelines.
Stormwater ponds are designed and engineered
to remove pollutants from stormwater runoff,
primarily through the settling out of sediments
from the water column. As the majority of the
pollutant loading is associated with the sediment,
this leads to an improvement in the quality
of the stormwater runoff, which can then be
discharged to natural water bodies. Pollutant
removal efficiencies of up to 90% have been
reported for such ponds (e.g. Wu et al. 1996).
Removal of the nutrient phosphorus has also
been documented, but the release of nitrogen
in the form of ammonia from the sediments to
the water column may also take place (Hvitved-
Jacobsen et al. 1984). The build-up of sediments
in these ponds leads to a reduction in volume and,
therefore, efficiency. Therefore, sediment needs
to be removed periodically (see section 6.6.4),
but these sediments are commonly contaminated.
For example, Marsalek & Marsalek (1997)
determined for a stormwater pond in Ontario,
Canada, that sediment metal levels were such
that the sediment could not be reused or placed
in residential landfill without treatment.
6.6.4 Sediment dredging
Dredging of sediment is a management tech-
nique used on urban aquatic sediments for two
purposes: to maintain draft in navigable canals
and docks, and to remove contaminated sediment
from waterways as part of pollution manage-
ment. Commonly the two become interrelated,
however, as once sediment is dredged for re-
6.7
FUTURE ISSUES
The major issue of concern in urban sedimentary
environments is that of increased urbanization
and industrialization. In the developed world
the input of contaminants into urban environ-
ments will at least stabilize, if not decrease,
