Purpose: The present study investigated lake and river sediments affected by metals from an acid mine drainage (AMD) from a former uranium mine. The role of bacterial sulphate reduction in the immobilization of contaminants was evaluated and analyses of acid volatile sulphide (AVS) and sequential extraction were performed. Consequently, the potential mobility and bioavailability of contaminants were established. Materials and methods: Sulphur isotopic fractionation (δ³⁴S), AVS and sequential extraction procedure were used to assess the sulphate bacterial reduction and the availability of contaminants in the environment at six sampling stations. Results and discussion: The δ³⁴S indicated that bacterial reduction is a key process in the natural attenuation of contamination in the Águas Claras reservoir, precipitating metal sulphides. According to the USEPA criteria, adverse biological effects are expected for sample S1 (inside the reservoir) which is likely to be toxic, whilst for sediment S4 (in the river) the toxicity is uncertain. The other samples were classified as non-toxic, likely because of the decreased solubility of zinc sulphide. A decrease in the concentration of the contaminants downstream of the reservoir was observed. The predominance of U (0.4 %) in the labile fraction and the elevated concentrations of Zn (0.5 %) and Mn (0.7 %) in the sediments inside the reservoir raise concerns regarding the availability of these contaminants in the environment. Conclusions: The main environmental impact appears to be concentrated in the Águas Claras reservoir whereas the Antas creek does not seems to be affected by the AMD process. Although the bacterial sulphate reduction is effective in its production of sulphides capable of immobilizing the contaminants, the presence of Zn and U in the labile and reducible fraction is a matter of concern due to its long term bioavailability. Thus, continuous monitoring of the redox potential of the waters and sediments, mainly in the reservoir, is recommended in order to assess and possibly prevent later dissolution of sequestered contaminants.