Sources of heavy metals in vegetables in Cape Town, and possible methods of remediation

Abstract

Cape Town includes two vegetable farming areas within the city limits, the Joostenbergvlakte/Kraaifontein area and the Philippi area. Both areas supply produce to local markets and further afield. Sporadically, high levels of cadmium, copper, lead and zinc have been found to occur in some of the soils, irrigation water resources and crops. To find the sources of specifically Cd, Pb and Zn to these agricultural systems, extensive analysis of several heavy metals in inputs such as fertilizers, agrochemicals and supplementary water resources to these farming areas was undertaken. Heavy metal concentrations in soils, irrigation water resources and crops were also determined. Two mitigation techniques that could be used to remediate Cd, Pb and Zn contamination were investigated. The first mitigation method included immobilization of heavy metals as phosphate complexes by using a triple super phosphate fertilizer, while the second method involved mobilisation and thus leaching of heavy metals away from plant roots using EDTA. These mitigation methods were tested in a pot experiment using cabbage as the experimental crop and soil from these areas as growth medium. A survey of common farming practices in these two areas and farmers‟ willingness to use remediation methods was conducted. The results in general indicated that crops from these two areas were fit for human consumption and that raw (unprocessed) cattle manure and chicken manure were the greatest sources of heavy metals in both farming areas. It was found that the use of EDTA led to elevated levels of Cd, Pb and Zn in cabbage, while the use of triple super phosphate at a low concentration contributed to limiting the uptake of Cd, Pb and Zn, but only minimally. Most farmers are willing to apply remediation methods but only when they have been proven necessary. In general, the same farming practices occurred in both areas. Farmers from the Philippi area tended to rely more heavily on subterranean water resources. It became clear that unprocessed manures should be used with caution and that more appropriate heavy metal remediation methods should be sought.