Geochemical Mapping and Source Attribution of Heavy Metal Contamination in Stream Sediments of the Ajaokuta Steel Industrial Zone, Kogi State

Industrial steel production generates substantial quantities of dust, slag, and process effluents that contaminate surrounding stream sediments with heavy metals over time. This study conducts geochemical mapping and source attribution of heavy metal contamination in stream sediments around the Ajaokuta Steel Complex in Kogi State, North Central Nigeria, a facility with a long history of partial operations and associated waste accumulation. Sediment samples from sixteen stations in five tributary streams were collected at two sampling rounds representing wet and dry seasons. Samples were digested by aqua regia and analysed for iron, manganese, chromium, nickel, vanadium, cobalt, lead, cadmium, zinc, and copper by inductively coupled plasma mass spectrometry. Enrichment factors, geoaccumulation indices, and contamination factor indices were calculated using local geochemical background values derived from uncontaminated reference sediments upstream of the industrial zone. Positive matrix factorisation was applied for source apportionment using metal profiles and co-occurrence patterns. Results reveal severe enrichment of chromium, vanadium, and manganese in sediments adjacent to the steel plant, with geoaccumulation indices indicating heavy contamination for these metals. Positive matrix factorisation identified three source profiles: ferrochrome process dust, slag leachate, and natural geogenic background. Wet season sampling showed higher metal concentrations, consistent with mobilisation of deposited dust during rainfall events. Ecological risk indices at downstream stations indicate very high risk for chromium and vanadium. The study provides a geochemical baseline for regulatory action and remediation planning at the Ajaokuta industrial zone. Keywords: geochemical mapping, heavy metals, Ajaokuta, stream sediments, source apportionment