Environmental Engineering Reference
In-Depth Information
persistent organics may be released into the water column under anoxic
conditions and general dredging disturbance. Late spring and summer
dredging operations have the potential therefore to lead to serious deleterious
effects in the Turning Basin.
An additional consideration in the MSC is the potential for metals to be
mobilised from sediments under oxidising conditions (Forstner
1993
). In a
laboratory investigation to elucidate the dynamics of metal transport from a
metal-contaminated and eutrophic lake, Nguyen et al. (
2007
) found that zinc,
copper and cadmium were removed from solution during the anoxic period
and released to solution during oxic conditions. In contrast, iron was rapidly
released to the water under anoxic conditions, but taken up during oxic
conditions. Xue et al.(
1997
) found that hypolimnetic oxygenation in a eutro-
phic lake enhanced the release of copper from the sediment but also acceler-
ated the entrapment and deposition of copper and zinc by freshly formed
manganese and iron oxides. However the oxic surface layer was suggested as
an effective retainer or 'lid' for arsenic in a saline lake (Lyons & Lebo
1997
).
Riedel et al. (
1997
) undertook microcosm experiments to investigate metal
fluxes from sediment under variable water column oxygen concentrations
(saturated, 10% saturation and anaerobic). Under saturated or 10% oxygen
saturation, arsenic fluxes were negligible, whilst in contrast, copper fluxes
out of the sediment increased with increasing oxygen concentrations.
Within the MSC, the effect of the artificial oxygenation remediation on the
surface sediments is not fully understood at present, though the beneficial
impact is thought to be limited by the constant 'rain' of organic material
deposited to the sediments. Surface water arsenic, cadmium, chromium, lead
and mercury concentrations have been monitored biannually since 2002 in
Salford Quays in compliance with the EC Bathing Waters Directive (76/1260/
EEC). Most metals have remained at extremely low concentrations, being below
the limits of detection. Arsenic however is detectable, though it remains at very
low concentrations (maximum of 3.8
m
gL
1
) and below the 50
m
gL
1
stipulated
in the EC Surface Water Abstraction Directive (75/440/EEC). Moreover, as dis-
cussed earlier, there has been no evidence to suggest an adverse effect of metal
contamination on the biota within either the MSC or Salford Quays.
Currently, high turbidity in the MSC reduces the photic depth and hence,
algal photosynthesis. However with the anticipated reduction in suspended
solids in effluents following the AMP investment by United Utilities, together
with the currently high nutrient levels (augmented by dredging operations),
the situation is likely to be reversed in the future. Based on evidence from
Salford Quays and also Preston Riversway Dock (APEM
1991
), we suggest that
severe blooms of blue-green algae may occur and will represent a serious threat
to both the recreational and aesthetic value of riparian developments along
the canal. Higher organisms may also be affected due to changes in water
