State of the Bay Report 2010-Final - Anchor Environmental

The concentrations of metals in sediments are affected by grain size, total organic content
and mineralogy. Since these factors vary in the environment, one cannot simply use high absolute
concentrations of metals as an indicator for anthropogenic metal contamination. Metal
concentrations are therefore commonly normalized to a grain-size parameter or a suitable
substitute for grain size, and only then can the correct interpretation of sediment metal
concentrations be made (Summers et al. 1996). A variety of sediment parameters can be used to
normalize metal concentrations, and these include Al, Fe and total organic carbon. Aluminium or
iron are commonly used as normalizes for trace metal content as they ubiquitously coat all
sediments and occur in proportion to the surface area of the sediment (Gibbs 1994); they are
abundant in the earth’s crust and are not likely to have a significant anthropogenic source (Gibbs
1994, Summers et al. 1996); and ratios of metal concentrations to Al or Fe concentrations are
relatively constant in the earth’s crust (Summers et al. 1996). Normalized metal/aluminium ratios
can be used to estimate the extent of metal contamination within the marine environment, and to
assess whether there has been enrichment of metals from anthropogenic activities. In this study
metal concentrations were normalized against (divided by) aluminium and not iron due to the
known anthropogenic input of iron from the iron ore terminal and industrial activity in Saldanha Bay.
Enrichment factors were also calculated for Saldanha Bay samples by dividing metal concentrations
obtained in 2009 by the average metal concentrations recorded for pre-development sediments in
1980.
Figure 6.16. Metal:Al ratios for Copper, Lead, Cadmium and Nickel for sediments sampled in 2010 from
Saldanha Bay-Small Bay (SB), Big Bay (BB) and Langebaan Lagoon (LL)
The concentration of cadmium and the normalized Cd:Al ratios were highest at site SB 1
(Yacht Club Basin) which is historically an area of high metal contamination (Figure 6.16 and Figure
6.18). This may be due to the discharge of a variety of anthropogenic contaminants related to
boating activity (fuel leakage, anti fouling paints etc). The normalized Cu:Al ratios show a
considerable amount of spatial variability. The sites in the southern region of the lagoon (LL33, LL37,
LL39, LL40 and LL41) and along the ore jetty (SB3, SB14 and SB15) and in the yacht club basin (SB1)
in Small Bay have high normalized copper concentrations. Sites SB1, SB14 and SB15 correspond to
areas of high boating activity, and also exhibited elevated Cu:Al ratios in 2008 and 2009. The input
of copper at these sites may thus be related to the use of antifouling paints which characteristically
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon