BEN SCHOEMAN DOCK BERTH DEEPENING Specialist ... - Transnet

Figure 21a: Predicted number of days during which copper concentrations (a sensitive
proxy for all trace metals) in the surface layer do not comply with RSA water quality
guidelines for the maintenance of ecosystems. (Winter, Cutter Suction Dredger; copied
from Figure B.7.12, Van Ballegooyen et al., 2006)
Particulate trace metal concentrations above the threshold effect level (TEL) may exert
toxic effects on the biota of the harbour in the water column, on the hard structures or in
the sediments. Tables 9 and 11 show that a large proportion of the sediments exceed
the TEL level for more than one of the trace metals. Consequently over and above the
possible effects of inundation the dredging may redistribute sediments that may be toxic
at the TEL level and above. Direct toxicity effects of contaminated sediments on bivalve
embryos have been measured by Geffard et al. (2002). The sediments tested covered a
wide range from heavily contaminated (24.6mg/kg PAH, 136.8mg/kg copper, 568mg/kg
zinc) to lower contamination levels at 2mg/kg PAH, 7.6mg/kg copper and 73mg/kg zinc.
Note that the defined toxicity effect was chronic deformations in 20% of oyster embryos
after 24 hour exposures (=24hr EC 20 ); this can be considered to be a low toxicity effect
level. For sediments comparable to those in the Ben Schoeman Dock Basin Geffard et
al.'s (2002) data indicate that such toxicity effects may be generated at 2000mg/l
suspended sediment. This is a high concentration and is not predicted to occur
anywhere outside of the immediate dredge area (Figure 20).
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