Sediment dispersion at the shore of Lake Victoria: A case study of Ggaba, Uganda.

Abstract

This study assessed the sediment and nutrient loads from Ggaba into Lake Victoria, and investigated the vertical dispersion of sediments at the shore, and seasonal distribution of sediments and selected NPS nutrients at the shore of Lake Victoria at Ggaba. Sampling was carried out in the dry and wet seasons between September 2010 and March 2011 during which, water samples were drawn at horizontal 0 metres (m) (the shore), 30 m and 50 m off shore and vertical distances; 0 m (at the surface), 1 m and 1.5 m from the surface using van dorn water bottle samplers. All points were simultaneously sampled at time intervals of three minutes for twelve minutes. The samples were preserved in water bottles for analysis in the Chemistry laboratory at Makerere University. The key elements investigated were: Total Dissolved Solids (TDS) and Total Suspended Solids (TSS). Others were phosphates (P04), nitrates (N03), nitrites (N02) and ammonium nitrate (NH4-N) and the physical parameters included pH, conductivity and temperature. The vertical dispersion of pollutants was investigated using Depth-Dependent Dispersion Coefficient for Modeling of Vertical Solute Exchange in a Lake Bed under Surface Waves. The results indicate that TDS values were high (62.6 to 343.13mg/l) relative to the high intensity of human activities in the watershed area. Sediment yield values were low (ranged between 21.25 t/km2/yr and 274.82 t/km2/yr) due to the small catchment used. The sediment contained coarse sand and gravel (0.039cm, 0.125cm and 0.395cm respectively). Generally horizontal and vertical distances had a significant effect on the concentration of physico-chemical parameters and nutrients with concentrations reducing with increase in horizontal distance from the shore and vertical depth ((P<0.001). Sediment and nutrient concentrations were highest in the wet than dry season (P<0.05). Water temperatures in the dry season were higher than in the wet season (P<0.05). Dispersion coefficient (DE) varied between 0.003cm2/s and 0.26 cm2/s insinuating that the sediment materials constituted sand, coarse sand and gravel. DE was strongly correlated to TDS concentration at the shore and rainfall amount (R2 =0.90). It thus important to take these measures: Reduce the transport of materials from the hotspot areas into the lake by creating buffer zones near farms, roads and beaches, further research to investigate processes beyond 50m from the shore and higher depth and long term monitoring of dispersion and sedimentation.