BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

Figure 21b: Predicted number of days during which copper concentrations (a sensitive
proxy for all trace metals) in the surface layer do not comply with the ANZECC (2000)
guideline concentration for moderately polluted water. (Winter, Cutter Suction Dredger;
copied from Figure B.7.14, Van Ballegooyen et al., 2006)
Further to the above, most of the particulates, with the absorbed trace metals, will
sediment out in the immediate dredge area, as predicted by the water quality modelling,
and predictable by the characteristic low bed shear stress of harbour areas. Therefore
any impacts that may be generated by redistributed sediment contaminants would be
limited to communities already affected physically by dredging and, if not, through
previous exposure to the contaminated sediment. Therefore the probability of further
deleterious effects on biota appears to be remote.
The potential environmental impact of remobilised and/or redistributed trace metals in
the harbour area is assessed as:
Nature of impact – Remobilisation of contaminants in the dredge area over the
dredging period may have toxic effects on organisms outside of the dredge area in the
harbour
Extent – Local, transport out of the harbour area should be minimal or, when it does
occur plumes are predicted to be diluted to the point that any sedimentation from them
should not markedly increase background trace metal concentrations
Duration – Short-term exposure and short term effects given lifespans of the organisms
that may be affected.
Intensity – Low, biological communities in any of the possible impact areas are likely to
have already been damaged from the physical process of dredging
Probability – Improbable, biogeochemical considerations and simulation modelling of
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