Gold tailings as a source of water-borne uranium contamination of streams - The Koekemoerspruit# (South Africa) as a case study Part III of III: Fluctuations of stream chemistry and their impacts on uranium mobility

Once dissolved uranium (U) from tailings deposits enters adjacent streams, subsequent downstream transport is affected by the rate at which U is immobilised in sediments, thereby lowering its concentration in stream water. For aqueous phases immobilisation includes adsorption onto sediments and suspended solids, as well as precipitation and co-precipitation as insoluble compounds, all mechanisms being largely controlled by pH and redox potential (Eh) of the stream water. Using real-time in situ measurements pronounced diurnal oscillations of both parameters were found at the site described here. The influence of the position of probes in detecting such short-term variations of stream-water quality are analysed by comparing in-house with in-stream measurements. Diurnal pH-oscillations are mainly caused by photosynthesis-based shifts of the calcium carbonate – carbon dioxide equilibrium in the stream that, in turn, is influenced by geological features of the catchment, biological activity as well as meteorological factors. These oscillations are likely to affect the mobility of dissolved U as well as the rate and distance of the downstream transport of U and other heavy metals significantly. Apart from this, event-related changes of stream chemistry, such as a sudden fall in pH in response to acid rain, were also observed. The dynamics and possible impacts on re-mobilisation of U from contaminated sediments back into the stream water are explored.