Modelling the Fate and Transport of Organic Micro-pollutants and Phosphates in the Simiyu River and Speke Gulf (Lake Victoria), Tanzania

Justus Rwetabula

Onderzoeksoutput: PhD Thesis


Lake Victoria is the largest freshwater lake in Africa, and one of the major sub-basins within the Nile basin sharing its resources with Tanzania, Kenya and Uganda. The water quality of Lake Victoria has been deteriorating due to point and non-point pollution sources from domestic, industrial and agricultural activities. Pollution from agricultural activities is mainly fertilizers and pesticides. The main processes affecting the fate of pollutants in runoff are water discharge, erosion and sediment transport, and chemical, biological and biochemical interactions within the soil-plant-water system. To avoid environmental problems, the riparian countries established the Lake Victoria Environmental Management Project (LVEMP), which became operational in 1997, aiming at the rehabilitation of the degraded lake ecosystem. In Tanzania there are many rivers polluting Lake Victoria. The Simiyu catchment, located in the southeast of Lake Victoria, is considered to be one of the main contributors to the deterioration of Lake Victoria, because it is relatively large (10,800km2), with many agricultural activities, using agrochemicals and generating high yields of sediments. The catchment is generally flat, dominated by wasteland, bushland, grassland and cultivated landuse types, and sandy loam soil type. The main objective of this Ph.D. thesis is to develop and apply a physically-based, distributed watershed model for prediction of river discharge/runoff and contaminant (phosphorous and pesticides) transport in the Simiyu catchment, Tanzania. To carry out this study, it was necessary to obtain baseline data investigating to what extent the Simiyu river contaminants are entering Lake Victoria. Field measurement campaigns were carried out between 2001 and 2004 in the Simiyu catchment and Speke gulf (Lake Victoria). Respectively twelve and eight monitoring stations were established in the Simiyu river and Speke gulf. Activities involved, measurements of discharge in the Simiyu river and of currents in the Speke gulf, water and sediment sampling in the Simiyu river and Speke gulf, and analysis of water quality parameters. In addition, meteorological data was collected, and DEM, landuse and soil maps were developed from satellite images, field investigation and information from literature. These data together with the field work were used for model calibration. The average discharge in the Simiyu river is about 30 m3/s during rainy season. Five samplings were carried out in the Simiyu river and Speke gulf. In the water samples observed total phosphorous concentrations were on average about 1000 mg/m3 in the Simiyu river, and about 770 mg/m3 in the Speke gulf. In the Simiyu river total pesticide concentrations were in order of 10000 µg/m3 for DDT, HCH, and Endosulfan, and in the Speke gulf 1000µg/m3 for DDT, and 10000 µg/m3 for HCH and Endosulfan. A grid-based distributed hydrological model WetSpa, is applied to fulfil the proposed objectivities. The model combines topography, landuse and soil maps in raster format, and meteorological data, and predicts discharge and contaminant hydrographs and spatial distribution of hydrologic characteristics in the catchment. Contaminant loading is estimated as a function of the runoff and contaminant release rates for different landuse types. A diffusive approximation method is used to trace runoff and contaminant transport to the basin outlet. The model is evaluated based on three year data (2001-2004) of daily river discharge and measured contaminant concentrations and loads at the catchment outlet of Simiyu river. The estimated annual flow and total phosphorous load are about 500x106 m3 and 709x
Originele taal-2English
Toekennende instantie
  • Vrije Universiteit Brussel
  • De Smedt, Florimond, Promotor
Plaats van publicatieBrussels
StatusPublished - 2007


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