Confluent Environmental was recently requested to investigate the cause of a fish kill in the Bronkhorstspruit and Wilge rivers in the upper Olifants catchment in Gauteng and Mpumalanga. The upper Olifants River is well known for the myriad of anthropgenic impacts it endures, with mining and associated acid-mine drainage often being highlighted as one of the main culprits. The Bronkhorstspruit River upstream of the Premier Mine Dam is heavily infested with the alien invase Water Hyacinth. It is also subjected to high inputs of partially treated sewage. A cable had been strung across the Premier Mine Dam to prevent the transport and spread of Water Hyacinth downstream of the dam into the Wilge River. A heavy rainfall event (80 mm) on the 9th of January led to a sudden increase in flows in the Bronkhorstspruit and Wilge rivers entering the dam. The cable containing Water Hyacinth snapped, most likely as a result of the increased water flows in combination with the influx of large quantities of Water Hyacinth. This led to large quanitities of the plant over-topping the dam wall flowing and entering the Wilge River. A few days after this event, residents noticed that large sections of both rivers appeared black in colour.
Fish died as a result of anoxic conditions in the river. The black colour in the river is often associated with high concentrations of dissolved organic carbon, the origin of which is most likely from a combination of the high sewage inputs and the large quantities of decomposing water hyacinth. DOC provides an excellent food source for micro-organisms in the river and increased rates of respiration lead to decreased oxygen levels.