Environmental Engineering Reference
In-Depth Information
(personal communication) a phosphorus concentration of less than 35
m
gL
1
is
required to limit the algal population and less than 15
m
gL
1
is required to
exert real control. Indeed phosphorus concentrations over 35
m
gL
1
are indica-
tive of eutrophic water (OECD
1982
). Presently, however, shading resulting
from the turbid conditions prevailing in the MSC severely limit photosynthesis
and therefore algal growth is currently constrained.
Sediment rafting and noxious gas generation was a major problem within
the MSC in the late 1980s and was investigated by APEM between 1989 and
1990 (Webb
1993
). The majority of sewage-derived organic sediments were
thought to be deposited in the Turning Basin over the winter. As the water
warmed in spring, microbial and fungal fibres grew through the newly
deposited sediment layer binding it together (Boult & Hendry
1995
). Due to
the highly labile, organic nature of the sediments, microbial activity gener-
ated gases such as methane and hydrogen sulphide. These gases would
accumulate within the cohesive sediment layer, making it buoyant and
eventually causing it to slough off and erupt at the water surface. During
the late 1980s, sediment rafts were so prolific that on occasion they could
cover the entire surface of the 28-ha Turning Basin, giving the impression of
solid ground. Anecdotal evidence suggests that during 1 week alone, three
dogs drowned after jumping onto the surface of the rafts assuming they
were solid.
Metal levels were found to be high throughout the sediment column in the
upper MSC (APEM
1996
) and exceeded the target environmental guidelines
(Dutch Guidelines
1994
). Concentrations of many metals increased with depth,
reflecting a decrease in contaminating industrial discharges. Although the
high concentrations of metals at the surface imply that the pollution entering
the MSC is still relatively highly contaminated, alternative explanations
include movement and reworking by resident organisms (bioturbation) and
dredging which exposes old sediments. Certain metals including the so-called
EC 'Black List' substances mercury and cadmium (EC Dangerous Substances
Directive 76/464/EEC) were present at extremely high levels; maximum concen-
trations are given in
Table 14.1
.
The level of contamination can be put into context by comparing with
contaminated sediments throughout the UK using data generated from the
National Dock Survey of 1987 (Hendry et al.
1988
). Lead, chromium and nickel
were present in the MSC at higher concentrations than those recorded in other
sediments, whilst concentrations of copper, cadmium and zinc were exceeded
by those at only a single other site (Newcastle, Glasgow and London, respec-
tively). Other industrial- and agriculturally derived pollutants such as pesti-
cides and organics were generally found at comparably low concentrations,
though occasionally elevated concentrations were recorded. These included
PCBs and cyanide; the latter increased with sediment depth.
