Fate of metallic elements and their migration mechanisms in a waste mud impoundment and affected downstream were assessed. Physicochemical and mineralogical methods combined with PHREEQC calculation, statistical analysis and review of relevant literatures were employed. Results showed that the waste in mud impoundment had been severely weathered and acidized. Metallic elements exhibited high mobility and activity, with a mobility ranking order of Cd > Zn > Mn > Cu ≈ Cr > As ≈ Pb. Hydraulic transportation originating from elevation variation was the most important driving force for metallic elements migration. Although damming standstill was considered as an effective strategy for controlling coarse suspended particulate pollutants, metallic elements were still transported to the Hengshi River in both dissolved phase and fine suspended particle phase accompanied by the overflow of acid mine drainage. The concentrations of dissolved metallic elements were attenuated significantly along the Hengshi River within 41 km stretch. Precipitation/ co-precipitation of iron oxyhydroxides, especially schwertmannite, ferrihydrite and goethite minerals, were established as the most critical processes for metallic elements attenuation in river water. Accompanied by metals migration in the river, two pollution sensitive sites with notably high content of metals in the stretch of S6-S8 and S10, were identified in gently sloping river stretch.
Keywords: Acid mine drainage; Fate of metallic elements; Migration; Waste mud impoundment.
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