THE IMPACT OF THE RIVER NTAHANGWA, THE MOST POLLUTED BURUNDIAN AFFLUENT OF LAKE TANGANYIKA, ON THE WATER-QUALITY OF THE LAKE

Sources of pollution at the north end of Lake Tanganyika are concentra ted around Bujumbura, Burundi. The impact of River Ntahangwa on the wa ter quality of this part of Lake Tanganyika was compared with that of an unpolluted river, the Mugere. A shallow bay, not influenced by a ri ver, was used as a reference station. Chemical analyses were carried o ut at four week intervals during nearly a year. Samples were taken at different distances from the shore and at different depths. River impa ct was detected up to 100 m from both river mouths. Even at 50 m from the mouth of the polluted River Ntahangwa, no important decrease of di ssolved oxygen was detected. Chlorophyll-a maxima were found 100 m fro m the Ntahangwa mouth, but this was also the case 100 m from shore in the unpolluted bay. Lake water at 60 m depth was saturated with oxygen during the dry season, as it used to be in 1956. The rivers studied d escent initially within the lake due to greater sediment load and lowe r water temperature. Nevertheless, it appears that these rivers mix co mpletely in the lake within 100-300 m from the river mouths. The direc t effects from organic pollution of the Ntahangwa on the lake seem to be rather negligible. Appropriate anti-erosion measures should reduce massive sediment discharge into the lake.