The partitioning and mobility of metals in the Blesbokspruit
Master Thesis
2004
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University of Cape Town
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Abstract
The Blesbokspruit is situated south-east of Johannesburg on South Africa's East Rand, famous for its prolific gold and uranium deposits. With the onset of mining in the region around 1930, the Blesbokspruit was altered from a non-perennial to a perennial stream and a wetland due to the construction of roads and embankments as well as the discharge of mine effluent. Various studies have highlighted the detrimental impact South African gold mines and their slimes dams have on the environment especially the release of metals into aquatic ecosystems. Unable to degrade naturally, metal pollution accumulates within aquatic ecosystems with the potential to move up the biologic chain. Water and sediment samples were taken from throughout the Blesbokspruit in an attempt to characterise the aqueous and sediment geochemistry of the stream. Filtered and acidified water samples were analysed for major cations and anions using Atomic Absorption Spectrophotometry and Ion Chroma topography respectively. The waters were found to have elevated levels of Cl⁻, SO₄²⁻, Na⁺ and Ca²⁺. These elevated salt concentrations are derived from the discharge of saline effluent into the Blesbokspruit by the Grootvlei Proprie ary Mines Ltd and are responsible for the increase in electrical conductivity downstream of the mine. The generally mildly alkaline nature of the Blesbokspruit is the result of the underlying dolomites of the Chuniespoort Group . ICP-MS was used to quantify aqueous and sediment metal enrichment relative to natural concentrations reported in the literature. The results of the ICP-MS indicate aqueous and sediment enrichment in chalcophile metals derived from the oxidation of sulphide minerals. U is also enrichment within the water and is derived from the ore minerals mined from the underlying Witwatersrand Supergroup. However, total metal concentrations are not indicative of bioavailable metal concentrations. For this reason it is important to determine how metals are partitioned within the sediment. A sequential extraction was performed on sediment samples from the Blesbokspruit as a method of determining metal partitioning and mobility under various possible environmental conditions. The sequential extraction extracted metals from the following five operationally defined fractions: the exchangeable fraction, sensitive to changes in ionic concentration; the pH- sensitive bound to carbonates fraction; the redox-sensitive bound to Fe and Mn oxides and bound to organic matter fractions and essentially inert and unavailable residual fraction. Metal extracts from the five fractions were analysed by ICP-MS. The results of the sequential extraction indicate that only minor proportions of metals are exchangeable, while a significant proportion of many of the metals analysed for were hosted in the residual phase and are therefore not bioavailable under most natural conditions. Varying amounts of the meta ls analysed were partitioned within the carbonates, Fe and Mn oxides and organic matter.
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Starke, M. 2004. The partitioning and mobility of metals in the Blesbokspruit. University of Cape Town.