Heavy Metal Speciation in Sediments of the Chari and Logone Rivers Near the City of N’Djamena, Chad

Abstract

Sediments are a major sink and potential secondary source of potentially toxic metals in freshwater ecosystems, and their environmental risk depends not only on total metal concentrations but also on chemical speciation and mobility. During this study, the pH ranged from 5.99±0,01a to 7.33±0,01c in the first sampling series in June, whilst it ranged from 6.92±0,01a to 6.94±0,01c in the second. Electrical conductivity ranged broadly from 11.16±0,01a to 191.10±0,01c over the two study seasons, with a maximum recorded in the second series in December. Residual moisture ranged from 0.50±0,01 to 5.50±0,01. This study investigated the speciation of Pb²⁺, Cr⁶⁺, Cd²⁺, Mn³⁺, Cu, and Fe²⁺ in sediments collected from the Chari and Logone Rivers near N'Djamena, Chad, at three sites (Logone, Chari, and their confluence) during two contrasting hydrological periods — June 2024 (flood) and December 2024 (recession). Sediment samples were characterised for pH, electrical conductivity, and moisture content, and metal fractionation was assessed using the modified BCR (Bureau Communautaire de Référence) (Community Office Reference) sequential extraction procedure, which partitions metals into exchangeable/acid-soluble, reducible, oxidisable, and residual fractions. Potential mobility and bioavailability were further evaluated using the Risk Assessment Code (RAC). Results showed that Pb was mainly associated with the reducible fraction across all sites, with a substantial labile fraction (up to ~60% in the Logone sediments), indicating high potential mobility. Cr and Cu distributions varied by site, being predominantly reducible or oxidisable in the Logone River but shifting towards oxidisable and residual fractions in the Chari River and at the confluence. Mn was overwhelmingly bound to the reducible fraction (>85% at all sites), reflecting a strong association with Fe–Mn oxyhydroxides, whereas Cd was almost exclusively associated with the oxidisable fraction, pointing to a strong affinity for organic matter and sulfides likely linked to anthropogenic inputs. RAC values confirmed that Pb, Cr, and Cu presented the highest potential ecological risk due to their significant labile fractions. These findings demonstrate that anthropogenic activities substantially influence heavy-metal partitioning in the Chari–Logone system and underscore the need for continuous monitoring and pollution-source management to safeguard this ecologically and socio-economically important watershed.