Water quality of the Modder River

[摘要] English: The Modder River is a relatively small river which drains an area of 7 960 km² , in the central region of the Free State Province, South Africa and has a mean annual runoff of 184 x 10�?m³ . Botshabelo is a city which was developed in the catchment area of the river and its sewage outflows are discharged into the Klein Modder River, a tributary of the Modder River. This study was conducted in order to determine seasonal and spatial patterns in the system, the influence of Botshabelo's sewage outflow on the water quality of the river, as well as the presence of any toxic compounds. It was determined that the Modder and Klein Modder Rivers do not follow distinctive seasonal patterns in terms of chemical parameters, however, N0�?N and P0�?P concentrations usually increased with increasing flow in the river. Physical parameters such as turbidity, flow and temperaturefollowed distinctive seasonal patterns. Turbidity and flow was high during the rainy season and temperature followed the air temperature. The Modder River is a very turbid system, influenced by physical (flow, turbidity and temperature) as well as chemical factors (high nutrient availability). The Modder River showed similarities with other South African rivers, such as the Vaal and Orange Rivers, in terms of turbidity/conductivity relationships and the range of physical and chemical parameters. Phytoplankton growth also showed distinctive seasonal patterns, with low chlorophylla concentrations in the winter and higher chlorophyll-a concentrations when temperatures became more favourable. Diatoms (especially Cyclotella sp., Stephanodiscus sp. and Nitzchia sp.) dominated the algal community in both the Klein Modder and Modder Rivers for most of the time, with the euglenophyte, Trachelomonas dominating occasionally. In the Klein Modder River, algal blooms occurred more frequently, as well as with higher concentrations than in the Modder River. This could be ascribed to the higher nutrient concentrations in the Klein Modder River, which, together with low flow conditions, provide favourable conditions for algal growth. There were periods when the nutrient concentrations in the waters of the Modder and Klein Modder Rivers were low, however, Botshabelo has an enrichment effect on the water quality, in terms of the nutrient concentrations. The inflow of the Klein Modder River into the Modder River caused on average, a 112 % increase in P0�?P�? a 171 % increase in N0-N nitrates and a 50 % increase in chlorophyll-a concentration. However,the Modder River showed a self-purification capacity and nutrient concentrations decreased significantly downstream to Mazelspoort, restoring the water to almost the quality of the unpolluted reference point. Based on toxicity tests performed with Selenasfrum capricornufum and Daphnia pulex, no high concentrations of potentially toxic compounds were found in either the Klein Modder or Modder River. However, the occasional presence of heavy metals can not be excluded. Bacteria concentrations were high in both rivers and may pose a threat to human and animal health. The use of a water quality model (PC-QUASAR) on the Modder River system, showed no results to predict the conditions in the rivers and for planning and management purposes, since the model could not be calibrated with the available data. The parameters of the model also showed great insensitivities regarding manipulation of important parameters. Because the Modder River is a very turbid system, it is also important that light availability be taken into account in any forecasting procedures m³ . Botshabelo is a city which was developed in the catchment area of the river and its sewage outflows are discharged into the Klein Modder River, a tributary of the Modder River. This study was conducted in order to determine seasonal and spatial patterns in the system, the influence of Botshabelo's sewage outflow on the water quality of the river, as well as the presence of any toxic compounds. It was determined that the Madder and Klein Modder Rivers do not follow distinctive seasonal patterns in terms of chemical parameters, however, N0�?N and P0�?-P concentrations usually increased with increasing flow in the river. Physical parameters such as turbidity, flow and temperaturefollowed distinctive seasonal patterns. Turbidity and flow was high during the rainy season and temperature followed the air temperature. The Modder River is a very turbid system, influenced by physical (flow, turbidity and temperature) as well as chemical factors (high nutrient availability). The Modder River showed similarities with other South African rivers, such as the Vaal and Orange Rivers, in terms of turbidity/conductivity relationships and the range of physical and chemical parameters. Phytoplankton growth also showed distinctive seasonal patterns, with low chlorophylla concentrations in the winter and higher chlorophyll-a concentrations when temperatures became more favourable. Diatoms (especially Cyclotella sp., Stephanodiscus sp. and Nitzchia sp.) dominated the algal community in both the Klein Modder and Modder Rivers for most of the time, with the euglenophyte, Trachelomonas dominating occasionally. In the Klein Modder River, algal blooms occurred more frequently, as well as with higher concentrations than in the Modder River. This could be ascribed to the higher nutrient concentrations in the Klein Modder River, which, together with low flow conditions, provide favourable conditions for algal growth. There were periods when the nutrient concentrations in the waters of the Madder and Klein Modder Rivers were low, however, Botshabelo has an enrichment effect on the water quality, in terms of the nutrient concentrations. The inflow of the Klein Modder River into the Modder River caused on average, a 112 % increase in P04-P, a 171 % increase in N0�?-N nitrates and a 50 % increase in chlorophyll-a concentration. However,the Modder River showed a self-purification capacity and nutrient concentrations decreased significantly downstream to Mazelspoort, restoring the water to almost the quality of the unpolluted reference point. Based on toxicity tests performed with Selenasfrum capricornufum and Daphnia pulex, no high concentrations of potentially toxic compounds were found in either the Klein Madder or Modder River. However, the occasional presence of heavy metals can not be excluded. Bacteria concentrations were high in both rivers and may pose a threat to human and animal health. The use of a water quality model (PC-QUASAR) on the Modder River system, showed no results to predict the conditions in the rivers and for planning and management purposes, since the model could not be calibrated with the available data. The parameters of the model also showed great insensitivities regarding manipulation of important parameters. Because the Modder River is a very turbid system, it is also important that light availability be taken into account in any forecasting procedures

[发布机构] University of the Free State