ANTHROPOGENIC POLLUTION IMPACT ON PHYSICO - CHEMICAL CHARACTERISTICS OF LAKE KIVU, RWANDA

Anthropogenic activities such as industrial effluent, domestic and agricultural waste disposal constitute major sources of pollution in water. The effects of these pollutants on the physico-chemical properties of Lake Kivu, Rwanda were investigated at three locations (Gisenyi, Kibuye and Cyangugu) between February 2005 and December 2006. Water samples used for physico-chemical analysis were collected from 12 stratified points on the lake. Samples collected were analysed and interpreted using appropriate international procedures. The physico-chemical properties measured were temperature, conductivity, pH, transparency, salinity, total alkalinity, dissolved organic matter (DOM), total dissolved solids (TDS), dissolved oxygen (DO), biological oxygen demand (BOD), nitrate – nitrogen, phosphate – phosphorus and sulphate. Mean water temperature measured (24.7 + 0.58°C) was highest in Kibuye and lowest (24.5 + 0.31°C) in Cyangugu. The lake water was moderately hard with mean total alkalinity values of 88.1 ± 1.63mgL^-1; 79.7 ± 2.36mgL^-1 and 81.0 ± 2.46mg L^-1 recorded in Gisenyi, Kibuye and Cyangugu respectively; salty and alkaline with mean pH values of 8.87 ± 0.07 in Gisenyi; 8.86 ± 0.06 in Kibuye and 8.91 ± 0.06 in Cyangugu. Total dissolved solids (TDS) level in the lake was above the 500mg L^-1 prescribed by WHO while the mean biochemical oxygen demand (BOD) reading was within the range for unpolluted or moderately polluted lake. Water from the lake showed mineralization with mean conductivity values ranging from 1058 - 1184μScm^-1 in Gisenyi; 1058 - 1147μScm^-1 in Kibuye and 1031 - 1116μScm^-1 in Cyangugu. The mean dissolved oxygen values were below the 5mgl^-1limit recommended by WHO for fresh water fish species. SO_4- ^-2 and NO₃ – N concentration in the lake water does not constitute any serious health risk to both man and the aquatic organisms it supports. However, there is the need to constantly monitor the lake water to mitigate any build up of the ions to a hazardous level.