	Department of Water Affairs and Forestry
	Institute for Water Quality Studies

Last updated : 10 July 2001

EXECUTIVE SUMMARY

This report was written as a result of a pollution incident in the Pienaars River, flowing into the Roodeplaat Dam that was reported to the IWQS by Prof. Cock from Wits University. This initiated, in addition to the regular monitoring, an ad hoc investigation to determine the cause and extent of the incident.

The Roodeplaat Dam is situated north-east of Pretoria at the confluence of the Pienaars River, the Moreleta/Hartbees- and the Edendalespruit. The Roodeplaat Dam is hyper-eutrophic due to various nutrient inputs from the catchment, especially two Water Care Works (servicing formal residential areas and industries), diffuse sources (including extensive agricultural feedlots and informal settlements) and the re-circulation of nutrients from the bottom sediments in the impoundment.

The report discusses the historical trends of eutrophication related variables in the impoundment and highlights the increasing deterioration of the water quality in the Roodeplaat Dam that led to the incident.

Conclusions

The deteriorating water quality (the Roodeplaat Dam is a hyper-eutrophic impoundment) led to the eutrophication symptoms, in the form of a large toxic cyanobacterial bloom in the south-western inlets, and led to the reported pollution incident investigation. The results indicated that there was slight microbiological pollution present in the Pienaars River and the Moreleta/Hartbees Spruit tributaries that flow into the Roodeplaat Dam. The origin of this pollution is probably human wastes in the Pienaars River and animal wastes in the Moreleta/Hartbees Spruit and is usually associated with nutrient influxes.

To manage the eutrophication related water quality problems in the Roodeplaat Dam, the actual sources of the high nutrient concentrations and the microbiological pollution need to be determined and controlled.

The incidence of cyanobacterial toxin production in the Roodeplaat Dam should also be determined.

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