During more than a century of gold mining in South Africa large amounts of tailings were produced, which now cover vast areas in densely populated regions. These dumps contain elevated levels of uranium and other toxic heavy metals associated with gold in the mined ore. Large-scale extraction of uranium from auriferous ore only took place during the cold war, leaving tailings with high uranium concentrations that were deposited before and after this period. Recent studies found elevated levels of the radioactive heavy metal in groundwater and streams, mainly attributed to the discharge of contaminated water from mines. In this paper the contribution of seepage from slimes dams to the uranium pollution of adjacent streams is analysed. Based on geochemical analyses of samples, field observations and long-term in situ measurements of hydraulic and hydrochemical parameters at selected mining sites across the Witwatersrand goldfields, the extent, mechanisms and dynamics of diffuse stream contamination by tailings seepage is characterised. Temporal and spatial variations of the process and the associated hazard potential are discussed. Keywords: gold mining, mine tailings, sediments, seepage, uranium contamination, uranium mobility, uranium pollution.