BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

CMS (1995b) employed bioassays to determine the toxicity of harbour water collected
from seven locations including sites in Victoria Basin, Duncan Dock and the entrance to
the Victoria Basin. These tests showed that water quality was highly variable, having
appreciable toxicity at some points but being 'biologically acceptable' at others, even
when these sites were close by. The toxicity arose from a number of pollution sources
including hydrocarbons leaking out of quay walls (ruptured or leaking fuel supply pipes),
fish factory effluents and sewage contamination. CMS (1995b) concluded that neither
the harbour water body as a whole or the wider Table Bay system was at serious risk
from pollution within the harbour. The authors noted, however, that this conclusion
needed to be checked by repeat surveys at regular intervals. Aside from the CSIR
sediment quality surveys discussed above this does not appear to have been done.
5.2.3 The proposed dredge spoil dump sites in Table Bay
The 'official' dumping site for the Port of Cape Town is located 150 nautical miles
offshore in 3 500m water depth (CSIR 1991) and is designated 'Dumping Ground
(Ammunition)' (SAN Chart Za55). Although defined as an official site for harbour dredge
spoil by CSIR (1991) the only reference given in support of this is a 'pers comm' by a
(then) DEA&T official; i.e. no formal documentary evidence was produced. For the
purposes of this assessment this specific site is deemed to be unsuitable for the
following reasons:
o Sailing distances are too long to make it an economically viable option for the
proposed dredging programme
o There is no available information on seafloor characteristics in the region of the
designated dump site that would allow any reliable prediction of the ecological
consequences of dumping dredge spoil there. Obtaining such information
through field surveys would be prohibitively expensive and logistically complex
as it would require sophisticated sampling gear and a large ship to deploy this.
Therefore, despite its designation, given the present state of knowledge on the
area and the low probability of radically improving this dumping dredge spoil at
the site would be counter to the precautionary principle
o The behaviour of sinking dredge spoil through the depth range at the designated
site is unknown therefore ecological effects in the water column or on the sea
floor can only be guessed at, and
o The (19)96 Protocol of the London Convention requires that dump sites be
monitored post-dumping to determine the behaviour of dredge spoil. For the
reasons advanced above this would not be easily achieved and may lead to RSA
being in contravention of its obligations under the protocol.
Therefore alternative sites were considered to be necessary and part of the research in
this assessment was identifying candidate areas and then conducting surveys to qualify
their suitability for receiving harbour dredge spoil. The broad criteria that were applied in
identifying the candidate sites included:
o
o
o
The sites should be within 'economical' sailing distance of the dredge area
The sea floor at the candidate sites should be sand or fine sand to allow
incorporation of the predominantly sand/silty sand dredge spoil into local
ecological processes
The sites should be sufficiently distant from environmentally sensitive areas,
such as the Tafelsig-Blouberg shoreline, the Diep River estuary, Robben Island
and the coastline of the Table Mountain marine protected area, to limit the
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