BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

6.2.3.2 Alteration of benthic biological communities through toxins associated with
dredge spoil
The dredge spoil is characterised by trace metal concentrations within the LC special
care (action level) range and higher than threshold effect levels (TEL). It therefore has
the potential to alter benthos community structure on the site used for spoil dumping and
immediately adjacent areas. Because of differing benthos community structure in the two
candidate sites identified for spoil discharge there are varying implications of this.
Therefore, as for the effects of inundation by dredge spoil, the two sites are assessed
separately.
SITE #1:
Nature of impact – Trace metal contaminated dredge spoil exerts toxic effects on
benthos in the dredge spoil dump area and the immediately adjacent seabed and alters
benthic communities.
Extent – Local; restricted to the immediate area of the spoil dump area
Duration – Short: The toxicity potential of the dumped sediments depends on the
bioavailability of trace metals which should be limited due to probable adsorption to iron
(ferric) and manganese (manganous) hydroxides present in the now oxic sediments.
Trace metals should thus remain in the particulate phase. This is borne out by simulation
modelling results showing that RSA water quality guideline concentrations for copper are
not transgressed at any stage for any of the spoil dumping scenarios at either of the
spoil dump sites (Van Ballegooyen et al., 2006).
Tests with sediments comparable to those targeted for dumping indicate generally low
toxicity effects (Geffard et al. 2002). Further, if the sediment is sufficiently toxic to cause
deleterious effects the toxicity should be relatively rapidly dissipated by wave generated
resuspension and advection of silt and clay sized particles away from the dump site
area. Re-deposition of these sediments away from the dump site should be sufficiently
small to mitigate any latent toxicity (above). Therefore any toxic effects on biota in and
adjacent to the dump site should be worked out of the community within a few
generations.
Intensity –Low; toxicity effects should be minimal due to retention of trace metals in the
inorganic particulate phase.
Probability – Improbable; there is only one instance of a trace metal exceeding the PEL
level and this should be reduced below this threshold through dilution with less
contaminated sediments in the dredging and spoil dumping process
Status of impact – Negative
Degree of confidence – High
Significance – Insignificant; the spatial and temporal extent of any effect should be
small relative to the amount of similar habitat in Table Bay.
Mitigation – None
Impact Assessment Table
No
Mitigation
Extent Intensity Duration Consequence Probability Significance Status Confidence
Local Low Shortterm
Very Low Improbable INSIGNIFICANT -ve High
1 1
3
1
Mitigation n/a n/a n/a n/a n/a n/a n/a n/a
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