The significance of groundwater-stream interactions and fluctuating stream chemistry on waterborne uranium contamination of streams - a case study from a gold mining site in South Africa

Through seepage, dissolved uranium and other heavy metals migrate from tailings deposits of gold mines via groundwater into adjacent fluvial systems. The extent of associated stream contamination is determined, inter alia, by the retardation of dissolved contaminants along the pathway and the rate in which polluted groundwater enters the stream channel. Comparing several sediment-water systems of the aqueous pathway significantly higher immobilisation of U was found in (fast-flowing) surface water systems such as the stream than in (slow moving) alluvial groundwater of the floodplain. Mainly triggered by redox-initiated co-precipitation bottom sediments in streams act as geochemical barrier and long-term sink for U and other heavy metals from polluted groundwater. Real-time in situ measurements of hydraulic interactions between contaminated groundwater and streamwater suggest a highly dynamic water exchange between both water bodies, including daily inversions of the direction of flow in certain times of the year. This results in distinct diurnal differences of the associated stream contamination. The extent of subsequent downstream transport of U within the fluvial system is largely determined by pronounced diurnal oscillations of pH and redox potential in the stream, affecting U-speciation as well as adsorption and precipitation rates. In. addition event-triggered fluctuations of both parameter impact on the fluvial transport of U. (C) 2004 Elsevier B.V. All rights reserved.