Nitrogen Assimilation Capacity of the Vaal River, South Africa

J.C. Roos (South Africa)


Denitrification, alkalinity, eutrophication, and assimilation capacity


Water pollution in the Vaal River resulted in a significant change in physico-chemical conditions in the water. Eutrophication, caused by excessive inputs of phosphorus (P) and nitrogen (N), in the Vaal River was found to be an important threat to the water quality of the river system. The dissolved phosphate concentrations were high and showed an increasing trend. However, the nitrate concentration in the Vaal River showed a decreasing trend during the past ten years, suggesting that the river has a high nitrogen assimilation capacity. Denitrification is apparently a major sink for nitrogen in the Vaal River, thus suggesting that the river would be able to process relatively high inputs of nitrogen. Hence, the N:P ratios (inorganic) in the Vaal River are decreasing, which could favour the development of cyanobacterial growth. The alkalinity in the Vaal River is gradually increasing, which is probably linked to the denitrification process in the river. High flow and flood conditions reduce alkalinity levels significantly in the Vaal River. Improved quality of the sewage effluent and upgrading of informal, unsewered human settlements will contribute to the environmental sustainability of the Vaal River ecosystem.

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