State of the Bay Report 2011-Final.pdf - Anchor Environmental

State of Saldanha Bay & Langebaan Lagoon 2011
Sediments
5.4.3 Summary
A multivariate analysis was conducted using PRIMER to determine the similarity (Euclidean distance)
between sites based on the concentrations of the different trace metals in the sediments. The
results are presented graphically in Figure 5.29 with different colour symbols indicating significant
groupings of sites. These are organized based on the general relative concentrations of trace metals,
with the red (A) indicating the highest concentrations and the blue (E) the lowest. The highest
concentrations of all the trace metals measured were recorded at the Yacht Club Basin, the only
exception being lead which was greater at the multi-purpose quay. This result correlates well with
that of the particle size composition results which indicated that the Yacht Club Basin harboured the
highest mud fraction. This suggests that a primary factor responsible for the high trace metal
concentration is the poor flushing of fine sediments from this area. This is consistent with results
observed since 2008. The grouping of most of the Small Bay sites into the category with the second
highest concentrations of trace metals indicated that Small Bay had been subjected to a greater
extent of contamination compared to Big Bay and Langebaan Lagoon. This is attributable to the
poor circulation and flushing in Small Bay in combination with trace metal contamination by the
surrounding industries and activities. The groups of sites with moderate to low concentrations of
trace metals were not grouped spatially, but rather spread throughout Big Bay and Langebaan
Lagoon. This indicates that the retention of trace metals varies spatially, most likely due to
variations in depth and current strengths at different localities. Interestingly, Sites S1 and D1,
positioned in Salamander Bay and Donkergat, respectively, grouped with the Yacht Club Basin and
multi-purpose quay indicating that they are most similar based on the concentrations of trace
metals. This indicates that these areas have been subject to high levels of trace metal
contamination, most likely attributable to recent dredging activities in this part of the Bay. The low
mud content at Donkergat and Salamander indicates that these areas are well flushed and it is
probable, in the absence of further dredging, that trace metal concentrations will decline again over
time.
Elevated trace metal concentrations recorded in Saldanha Bay in 1999 were ascribed to an
accumulation of these metals in the sediments of the Bay over the preceding 20 years, much of
which was re-suspended as a result of dredging operations and had settled in the surface layers.
Construction of the Marcus Island causeway and the ore terminal had contributed to this process by
reducing wave action and modifying circulation patterns prevailing in the Bay. Subsequent
monitoring has revealed a substantial overall decrease in the concentrations of metals in the Bay,
suggesting that a disturbance, like dredging which remobilises the fine sediments and re-suspends
metals, can severely affect the health of the Bay and that it takes between three to six years before
the contaminated sediments are removed from the Bay by natural processes. It was also shown that
metal concentrations were elevated near the Multi-purpose Quay as a result of lead and copper ore
dust entering the environment during export activities. In addition, metal concentrations were high
(often exceeding ERL values) in the Yacht Club Basin and this may be due to the fact that this area is
a depositional zone for fine grained sediments and organic matter onto which metals adsorb.
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