State of the Bay Report 2010-Final - Anchor Environmental

o Decrease in oxygen concentrations in the water column
o Bioaccumulation of toxic pollutants
Anchor Environmental
o Transport of fine sediments to adjacent areas, and hence transport of pollutants
o Decreased primary production due to decreased light penetration to water column
(Erftemeijer and Lewis 2006, Bonvicini Pagliai et al. 1985, OSPAR Commission 2004, National
Ports Authority 2007).
Aside from dredging itself, dredged material may be suspended during transport to the
surface, overflow from barges or leaking pipelines, during transport to dump sites and during
disposal of dredged material (Jensen and Mogensen 2000 in Erftemeijer and Lewis 2006).
Saldanha Bay is South Africa’s largest and deepest natural port and as a result has
undergone extensive harbour development and has been subjected to several bouts of dredging and
marine blasting. Saldanha is perfectly situated for the shipment of large quantities of iron ore from
the Sishen mines in the Northern Cape. However, before the first shipment could be loaded the port
had to be protected from strong wave activity. To remedy this, the first major development
occurred in 1973 whereby Marcus Island was joined to the mainland via the construction of a
causeway. Further development involved the construction of the General Maintenance Quay and
the Rock Quay over the period 1974 to 1976. During this process 25 million cubic meters of
sediment were dredged from the Bay to facilitate the entrance of large ore carriers, and the
resulting dredged material was used to construct the harbour wall (Moldan 1978). A Multi-Purpose
Terminal was added to the iron ore jetty in 1980 and the Small Craft Harbour was built in 1984.
These developments all required extensive dredging and submarine blasting which significantly
impacted sediment composition (§6.1) and benthic community structure (see §0). Since this time
three further dredging operations have been implemented in Saldanha Bay.
The first of these was associated with the expansion of the Multi-Purpose Terminal in
1996/7 when 2 million m 3 of material was removed from an area approximately 500 000 m 2 in
extent on the Small Bay side of the ore terminal. The dredge spoil was disposed of on land in a
retention pond on the eastern side of the causeway. The bottom material in Saldanha Bay consists
mainly of sand interspersed with thin layers of calcrete, some silt/clay and shelly material. Early
borehole samples collected in 1995 from proposed dredging areas revealed that the substrate
contained an average of 33% silt/clay of which ~73% of the silt/clay fraction had a grain size of less
than 5 microns. It is thus apparent that a significant proportion of the substrate that was dredged in
1997 comprised very fine particles such as clay and calcrete (chalk is simply pulverized calcrete).
When calcrete is dredged white plumes of fine particles are released into the water column
(Schoonees et al. 1995), which occurred during the 1997 Saldanha Bay dredge event.
Maintenance dredging was required at the Mossgas quay and the Multi Purpose Terminal in
order to deepen the berth. Maintenance dredging took place at these locations from the end of
2007 to March/April 2008 with an estimated 50 000 m 3 of seabed material being removed from both
terminals. The Mossgas terminal was constructed in the 80’s and the depth has reduced from
approximately 9m to 6m over the last 20 years due to sediment build-up. A similar reduction in
depth has also occurred at the Multi Purpose Terminal. The sediment that was to be dredged was
mainly fine silt, fine to coarse sand, shelly fragments and seaweed. At the Multipurpose berth 201 it
was also expected that lead and copper would occur in elevated concentrations in the dredged
sediments. The concentrations of lead (Pb) at several sites within the proposed dredge area fall in
the range of special care requirements in terms of the London Convention for off-shore disposal of
sediments. It has been calculated that of the 3 000m 3 of sediments to be dredged at berth 201,
approximately 300m 3 would be Pb product that had accumulated over two decades of loading
operations (National Ports Authority 2007). This material was not dumped offshore but was mixed
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 51