Abstract

Acid mine drainage seeping from an abandoned mine in the Witbank Coalfield has flowed into a nearby stream, the Blesbokspruit, which is a tributary of the Olifants River. The amount of precipitation over the mine area which sinks into the mine has been greatly increased by associated subsidence features. There is a time lag between the maximum precipitation and the maximum flow of acid mine drainage from the mine of several months. The water issuing from the mine has a low pH, high electrical conductivity and high total dissolved solids. It has severely impaired the quality of the waters of the Blesbokspruit. The heavy metals concentrations of the stream water are well above the levels set by the South African standards. In addition, there is a high concentration of heavy metals in the stream sediments and in algal mats which occur at the seepage points. Salts have been precipitated from the acidified waters, one of the chief of which is jarosite. Formation of ferrihydrite is associated with the algal mats.

Four decantation ponds constructed in the headwaters of this stream have had no effect on the pollution. Indeed, the concentration of aluminium in the waters exiting from the ponds is higher.

The Blesbokspruit flows through a wetland and on leaving the concentration of some heavy metals has been reduced. This presumably is attributable to the sorption capacity of the high organic content of the sediments in the wetland. Nonetheless, the quality of the water is not improved sufficiently for it to be acceptable, even with the dilution effect of the entry of water from an uncontaminated stream downstream of the wetland.

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