Application of the QUAL2K Water Quality Model to Assess Pollutant Dispersion in River Sosiani in Western Kenya

This study applied the QUAL2K water quality model to investigate the pollutant dispersion dynamics in River Sosiani, a vital freshwater source in western Kenya. The river, which historically supported diverse domestic, agricultural and recreational uses for the Eldoret City residents, is currently facing severe degradation due to urbanization and inadequate waste management practices. The model was calibrated and validated using weekly field data collected over six months from designated sampling points. Model performance was evaluated using standard statistical measures, including the R-Squared correlation (R<sup>2</sup>), the Nash-Sutcliffe efficiency (NSE), and the ratio of the Root Mean Square Error to the observations’ standard deviation (RSR). The results demonstrated good to excellent performance, with R<sup>2 </sup>values ranging from 0.82 to 0.95, NSE value above 0.75, and RSR values below 0.5 confirming the model’s reliability in simulating the rivers pollutant dispersion dynamics. The simulation results revealed deterioration in water quality from upstream to downstream. Precisely, dissolved oxygen (DO) decreased significantly along the river course, while carbonaceous biochemical oxygen demand (CBODf), electrical conductivity (EC), temperature, total phosphate (TP), and nitrate-nitrogen (NO<sub>3</sub>-N) concentration all increased. pH remained within the neutral to slightly alkaline range, with some localized shifts downstream, while flow discharge (DS) increased progressively from upstream to downstream. These trends, revealing an increasing pollution load, mainly in urbanized areas, highlight the significant impact of anthropogenic activities on River Sosiani ecological health and underline the urgent need for targeted interventions to mitigate further degradation.