Modelling the dispersion of point and non-point source pollutants in River Ndarugu in Kenya

Ndarugu River, during its course through the different agricultural and industrial areas of Gatundu, Gachororo and Juja farms, receives untreated industrial, domestic and agricultural waste of point source discharges from coffee and tea factories. During wet season the water is also polluted by non-point (diffuse) sources created by runoff carrying soil, fertilizer and pesticide residues from the catchment area. This study involved the calibration of water quality model QUAL2K to predict the water quality of this segment of the river. The model was calibrated and validated for flow (Q), velocity (V), biochemical oxygen demand (BOD), dissolved oxygen (DO), nitrate (NO3-), and electrical conductivity (EC) using data collected and analysed during field and laboratory measurements done in July, November and December 2013. The model was then used in simulation and its performance was evaluated using statistical criteria based on correlation coefficient (R2) between the observed and simulated data. The field data showed that BOD was higher downstream of point source pollutants. The model reflected the field data quite well with minor exceptions. In spite of these minor differences between the measured and simulated data sets at some points, the calibration and validation results are acceptable especially for developing countries where the financial resources for frequent monitoring works and higher accuracy data analysis are very limited.