Environmental Engineering Reference
In-Depth Information
Table 14.1. Maximum concentrations of selected metals recorded in sediments within
Metal
Maximum concentration
(mg kg
1
)
Target value
(mg kg
1
)
Intervention value
(mg kg
1
)
Copper
981
28
150
Zinc
1698
89
459
Lead
1574
72
451
Chromium
1963
58
219
Cadmium
20.5
0.8
12.5
Arsenic
82
24
45
Mercury
6.3
0.2
8
Nickel
81.3
14
83
Note:
a
The intervention values indicate the concentration of sediment contaminants above
which its use for human, plant or animal life is defined as being seriously impaired. Target
values indicate the sediment quality required for the full restoration of the sediment's use for
human, animal and plant life.
In summary, the overriding issues driving the poor water quality in the MSC
are three-fold: first, that the Mersey catchment is highly populated, creating a
high sewage derived load to the canal. Second, the industrial legacy of the
MSC means that the sediments contain high levels of persistent contaminants.
Third, the change in hydraulic regime arising from the structure of the MSC,
which was created for navigation, exacerbates the situation. It is unsurprising,
therefore, that the MSC has experienced poor water quality since its construc-
tion. Harper (
2000
) recognised the MSC as the 'sump' of the catchment; thus, it
would not be expected to behave as a 'normal' river. In fact the opposite was
anticipated and serious future problems were predicted, as indeed has been
the case.
Water quality and ecology: the industrial legacy
A summary of key water quality parameters in the MSC (April 1989 to May
1990) is given in
Table 14.2
, highlighting the condition of the water prior to
remediation. The highly variable water quality in the early 1990s restricted the
ecology to only the most pollution-tolerant organisms.
For many years, the macroinvertebrate diversity of the upper MSC was
characterised by four or five pollution-tolerant detritivorous taxa. typical of
poor water environments. The invertebrate community was dominated by a
limited number of detritivorous, pollution-tolerant species, including worms
(Oligochaeta), leeches (Erpobdella spp.), midge larvae (Chironomidae) and the water
hog-louse (Asellus aquaticus) (White et al.
1993
).
