Analysis of Nonpoint Source Pollution Loading on Water Quality in an Urban-Rural River Catchment Using GIS-PLOAD Model: Case Study of Sosiani River Watershed

Abstract

Nonpoint sources of pollution (NPS) are inherently difficult to identify and estimate since they originate from diverse sources. This study presents a case study on the estimation of NPS loading using the GIS-based Pollutant Load (PLOAD) model in River Sosiani, Eldoret Town in Kenya, through the analysis of the transport and decay of pollutants in a catchment, in comparison with in situ measurements in order to validate the model results. Within a catchment area of 574.30 km², with four main land use classes, the developed areas contributed to the highest event mean concentrations. From the PLOAD simulation results, fecal coliforms (FCOL) was the most dominant NPS with a maximum concentration of up to 5g/m² and the least was phosphates (PO4) with a maximum concentration of 1.9 g/m². Cross-plot validation of the PLOAD results with the in situ event mean concentrations of PO4, NO3, BOD, TDS, TSS, and FCOL showed a good fit for all the four sampling sites along the River Sosiani. Some Best management practices (BMP) were introduced in the model in order to mitigate against the pollutants. Constructed wetlands showed significant reductions in the PO4, BOD and TDS loadings. The approach and results in this study can be used for the monitoring and mitigation against the impacts of urban effluent on a river.

Keywords: nonpoint source pollution (NPS); urban-rural river watershed; water quality; event mean concentrations (EMC); GIS-PLOAD model; best management practices (BMP)