Pb and Zn contamination in Zoar Vlei, Cape Peninsula

Master Thesis


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University of Cape Town

The sediments in Zoar Vlei act as a considerable sink for Pb and Zn derived from the adjacent Paarden-Eiland semi-industrial area and the Milnenon-Rugby residential area. Through elemental analysis of 34 sediment samples by WDXRF spectroscopy, the sediments were shown to be significantly enriched in Pb and Zn.Lead and Zn concentrations are highest at the entrance to the southern pan, and decrease nonhwards through the vlei system. Kaolinite, quanz, illite and smectite are present in the clay fraction of the sediment, as was shown by analysis of a clay separate by XRD. The results of the sorption experiments revealed that both Pb and Zn are sorbed by the sediments. The selective affinity of the sediments for Pb exceeds that for Zn, as Pb sorption was shown to displace Zn from (he sediment while Zn sorption did not induce Pb release. The high degree of irreversibility of both Pb and Zn sorption indicates that sorption is specific in nature, and that under the present environmental conditions, the sediments act as a suitable storage facility for these metals. Acidification of the vlei water in equilibrium with the sediments caused Zn release through neutralisation reactions. Total water analyses were performed on 72 vlei water samples using high performance ion liquid chromatography and ICP-AES. Zinc concentrations in the vlei were elevated compared to natural background values, and decrease in a northerly direction through the vlei system, however remain elevated at the outflow into the Milnerton Lagoon. Zinc appears to be entering the vlei via storm water drains from both the industrial and residential areas.

Bibliography: pages 73-77.