Modeling non-point source pollution in Lake Victoria: A case of Gaba landing site

Abstract

Lake Victoria is one of the largest fresh water bodies in the world. Currently it is being threatened by various types of pollutants originating from different land uses-both point and nonpoint sources. The objectives of this study were; to identify and characterize different land-use activities, and locate NPS pollution hot spots, quantify pollutant and sediment loads, assess pollutant dispersion following discharges into the Lake, develop a one-dimensional mathematical model for nutrients discharged and recommend ways of managing such flows. Characterization of land use was achieved using GIS and remote sensing techniques. Within the lake, samples were taken both in the vertical and horizontal direction using hand driven pump and hand held GPS. Physico-chemical parameter concentration was determined using digital hand held meters and for nutrients, photometric method was used using Photometer 7100. TSS was analyzed using gravimetric method. The model was developed basing on the fundamental principle of conservation of mass for managing surface water quality. Gaba fish landing site was identified as a NPS pollution hotspot and runoff from this site conveys sediments and nutrients that are contributing significantly to lake Victoria pollution. Comparison between dry and rainy season pollution concentrations revealed that Ammonia-N, Nitrite-N and Ortho-Phosphate had higher pollution concentrations during the rainy season unlike Nitrate-N which had a higher concentration during dry season. The latter was attributed to sustained input of surface runoffs while the former was attributable to the nitrification processes These concentrations of Ammonia-N and Nitrite-N are similar to those that have been found by earlier studies to be toxic for the fish within the lake. Associated increase in physico-chemical parameter concentration was likely to affect fish availability and assemblage in areas close to shore settlements. The respective distances traversed by these nutrients were found to be 38meters for ammonia-N, 45meters for nitrite-N, and 34meters for both nitrate-N and ortho- hosphate. The respective model concentrations of these pollutants compared well with measured concentration at the traversed distances. A number of management measures were suggested to improve on water quality.