Determination and partitioning of heavy metals in sediments of the Vaal Dam System by sequential extraction

To assess the risk to human health posed by metals in sediments in the Vaal dam area, from which Rand Water supplied drinking water to approximately 9 million people, sequential extractions were carried out on 12 metals taken from 9 sampling points, with a view to determining whether they were of natural or anthropogenic origin, and whether resuspension from the sediments into the water body was probable. The samples were taken when the river was exceptionally low, and subjected to an extraction procedure which had previously been validated on artificial sediments whose metals content was known. The mineralogy of the Vaal catchment was also known and included clays which were likely hosts to many metals. The presence and concentration of detectable amounts of each metal after each extraction are given and the mechanism of adsorption of representative metals onto the sediment at water pH values of 5.2-7.3 were determined. Relatively low levels of metals proved to be extractable, even with them at higher than normal concentrations by virtue of the river's low flow, indicating only slight release from the sediments. Provided that the water pH did not drop below about 7, the sediment should continue to act as an effective sink for metals.