Estimation of heavy metal contamination of sediments in the Ouaka River watershed in the Central African Republic

Abstract

The study, based on heavy metals such as cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), vanadium (V), zinc (Zn) and iron (Fe) in the sediments of the Ouaka river watershed, aimed to determine the concentration profiles of the various contaminated zones and to assess the sources of pollution of these sediments, which are of both human and natural origin. These profiles were established against a backdrop of heavy industrial activity (discharges from the Ngakobo sugar refinery and the Bambari cotton gear factory), agricultural activity (with the use of chemical fertilizers and pesticides) and mining activity (artisanal gold mining), all of which may be responsible for heavy metal contamination of the sediments in this watershed. A total of 34 sediment samples were collected and analyzed by inductively coupled plasma mass spectrometry. Results for heavy metal concentrations (mg/kg) ranged from 0.006 to 0.019 for Cd; 1.7 to 7.11 for Co; 11.5 to 30.8 for Cr; 2.3 to 7.4 for Cu; 2.7 to 13.4 Ni; 1.36 to 25.99 for Pb; 11 to 18 for V; 7.9 to 20 for Zn and 11356.2 to 17875.5 for Fe. These values are significantly lower in the sediments studied than the reference values used in the upper continental crust. To assess the degree of contamination in these sediments, the enrichment factor, contamination factor, geo-accumulation index and pollutant load index were calculated. The result is that, with the exception of Pb in sample B15, which is moderately enriched, the other heavy metals are only slightly enriched, and this is confirmed by the geo-accumulation index and pollutant load index, which show the absence of contamination and pollution in this catchment. The results of this work will enable the Central African Republic to control the intensive discharge of industrial and mining residues, and also to limit the use of chemical fertilizers in agriculture in favor of natural fertilizers.