Abstract:
Surface water pollution is a problem that is prominent all around the world, the amount of pollutants increases as the population increases and the land that was uninhabited is changed to provide resources for humans. Rivers, watersheds and ground water are all impacted by the pollutants that are generated from the point sources such as households and industrial discharges as well non- point or diffuse sources(FN & MF, 2017). Water has always been a subject of great interest to man since it is essential for human survival. It is for this reason that the study takes the case of river Ewaso Ng'iro because of the need to access to clean water because most people depend on water from this basin.
The study area was upper ewaso ngiro basin in an arid and semi-arid land where the area has various land uses like agriculture and pastoralism. The main objective of this study was to assess surface water pollution in upper ewaso ngiro basin using GIS and remote sensing and swat modelling. The datasets used included soil data, rainfall data phosphates nitrates data, 30M DEM and Landsat images for 1989, 1999, 2009 and 2019. Classification of the Landsat images was done using ENVI software and map generation using the ArcGIS software. The study involved use of the SWAT model in simulation of surface run off and sediments load per HRU. The inputs to the SWAT model was DEM (30 m), the soil, average rainfall per annum, and land use and land cover, the HRU (hydrological response units) were defined in accordance with uniqueness of the soil, land cover and the slope. Reclassification of the nitrates, phosphates, surface run off and sedimentation load per hydrologic response unit was performed using reclassification tool ArcGIS software, then weights were assigned through weighted overlay in the determination hotspot areas affecting surface water pollution in the basin. The factors affecting surface water pollution in the basin were found to be changes in land use and land cover especially increase in built up areas and agricultural activities, increase in phosphates and nitrates distributed in the basin. The sediments load increased in the basin over the years. A correlation of surface runoff and precipitation was found to be 0.81. From the results it is evident that factors affecting surface water pollution are changes in land use and land cover through various human activities such as increased in built up areas which causes leads to clearing of vegetation this has contributed to increased surface run off in the basin which surface water pollution in the basin. The areas that were vulnerable to pollution were those around bare land areas, areas where there were buildings and forested areas because of higher concentration of nitrates and phosphates around forested areas. Water resources like boreholes and dams should be constructed in order to supplement the basin because most people are pastoralists, this will minimize surface water pollution in the basin from human activities.