Abstract

Mpenge stream is regarded as one of the most important domestic water resource in Musanze District; Rwanda. The chemical speciation study of selected heavy metals (Zn, Cu, Co, Cr, Ni, Pb, and Cd) in sediment samples collected from that stream was carried out by means of an analytical procedure involving sequential chemical extraction method and to determine the heavy metals content in different fractions of sediment. The heavy metals were found in five fractions: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter and residual fraction. Their levels were determined by flame atomic absorption spectrophotometer and the total mean values were in the order Cr> Zn>Cu > Ni=Pb > Co > Cd. In fact the total metal content in mg/kg dry matter (mean values) were for Cr=169.17±8.77; Zn=136.67±2.88; Cu=44.17±1.44; Ni=33.33±5.22; Pb=33.33±14.33; Co=25.00±0.00; and Cd=5.00±0.00. As per United States Environmental Protection Agency’s (US EPA) guidelines for sediments quality, sediments were moderately polluted for all analyzed elements apart from chromium that showed that the sediments were heavily polluted. Furthermore, the speciation results showed that high levels of these assessed metals (Zn, Cu, Co, Cr, Ni, Pb, and Cd) were associated with exchangeable and carbonate bound fractions for all heavy metals apart from copper, pointing out that they are in potentially available forms and may pose serious problems to water consumption.

Key words: Speciation, sequential chemical extraction, heavy metal, bottom sediment, stream.