Characterization of non point source pollutants and their dispersion in Lake Victoria: A case study of Gaba landing site in Uganda

Abstract

The aim of this research is to characterize non point pollutants and their dispersion in Lake Victoria. Lake Victoria is one of the largest freshwater bodies in the world. The lake is bordered by Kenya, Tanzania, and Uganda, and has streams and rivers stretching as far as Burundi and Rwanda feeding into it. A number of studies have reported deteriorating water quality due to pollution. In this study, non point source pollution was the focus and such as 68 and 64 samples were collected over two rain seasons and dry season, respectively. All samples were analyzed for nutrients, namely, ammonia, nitrite, nitrate, and phosphate. Portable meters were used to measure pH, electrical conductivity (EC) total dissolved solids, temperature and dissolved oxygen instantaneously at point of sample collection while Wagtech methods was used to measure nutrients. Total suspended solids (TSS) were measured using gravimetric method. Within the lake, samples were taken at for horizontal transects of 10 m interval over a distance of 50 m from the shore where surface runoff was released. At each 10 m sampling point, three samples were drawn at vertical distances of 0.5, 1.0 and 1.5 m from water surface using a hand pump with graduated delivery pipe. The criterion to be maximized is the R-squared (R²) value, which is expressed in percent. The values obtained from R² in this study reflect the percentage of output (that is, concentrations of ammonia, phosphorus, nitrates and nitrites) variation explained by the model. Optimization of R² value is important to avoid model over fitting during model identification. Over fitted models produce poor fits during model validation. The results generally show runoff during wet season increases the concentrations of ammonia, phosphorus, nitrites and nitrates. Ammonia varied from 0.1 to 0.19 mg/L, phosphorus from 0.01 to 0.18 mg/L, nitrites from 0.01 to 0.05 mg/L and nitrates 0.02 to 0.36 mg/L. Field measurements confirmed that nutrient concentrations decrease as one move deeper from the shores into the lake due to dilution.

Key words: Control applications in environmental processes, batch process modeling and control, water quality, nutrients, Lake Victoria.