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Water quality of the Modder River, South Africa

N Koning
JC Roos
JU Grobbelaar


The Modder River is a relatively small river in the central region of the Free State Province, South Africa and has a mean annual runoff of 184 x 106m3. Botshabelo is a city, which has developed in the catchment area of the river, and its sewage outflows are discharged into the Klein Modder, a tributary of the Modder River. This study was conducted in order to determine the influence of Botshabelo's sewage outflow on the water quality of the river. It was determined that the Modder and Klein Modder Rivers do not generally follow distinctive seasonal patterns in terms of chemical parameters, although NO3-N and PO4-P concentrations usually increase with increasing flow and conductivity decreases with increasing flow. Physical parameters such as turbidity, flow and temperature did however follow distinctive seasonal patterns from February 1996 to December 1997, as did phytoplankton growth. Low chlorophyll-a concentrations were exhibited in the winter and higher concentrations during spring. In the Klein Modder River, algal blooms occurred more frequently, and the algal biomass was higher than in the Modder River. This could be ascribed to the higher nutrient concentrations and lower flow velocities in the former. The inflow of the Klein Modder River into the Modder River caused on average, 112% increase in PO4-P, 171% increase in NO3-N, 50% increase in chlorophyll-a concentration, and 230% increase in E. coli counts.

(Afr J Aqua Sci: 2000 25: 202-210)

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eISSN: 1727-9364
print ISSN: 1608-5914