Assessment of anthropogenic contributions, speciation, and pollution risk of heavy metals in soil within Ore city, Southwestern Nigeria

Abstract

This study assessed the levels, chemical speciation, and anthropogenic contributions of different heavy metals (HMs), i.e. Cadmium (Cd), Chromium (Cr), Nickel (Ni) and Lead (Pb) in soils within Ore city, Southwestern Nigeria. A total of 20 soil samples were collected from five sites (i.e. dumpsite, mechanic workshop, metal fabrication workshop, river sediment, and a control site), and composited into four representative samples (one for each anthropogenic site), and an additional composite sample was collected from a uncontaminated site, considered as a control. The quantity of HMs was assessed using Atomic Absorption Spectrophotometer (AAS) following sequential extraction. Pollution Load Index (PLI), Mobility Factor (MF), and Geo-accumulation Index (Igeo) were used to evaluate contamination intensity, metal mobility, and pollution status relative to background levels. According to the results, the range of different soil physicochemical properties were as follows: pH, 6.56–7.73; organic matter, 1.14–11.20%; cation exchange capacity, 8.75–36.20 cmol/kg; Pb, 10.44–61.40 mg/kg; Cd, 0.35–61.62 mg/kg; Ni, 1.72–124.37 mg/kg; and Cr, 12.80–99.68 mg/kg. Two HMs, i.e. Cd and Cr were mainly in the carbonated fraction, indicating high mobility, whereas Pb and Ni were largely bound to Fe–Mn oxide and organic fractions. The values of PLI were > 1 for all sites, with the mechanic workshop showing the highest contamination. The results of Igeo indicated contamination ranging from moderate to extremely contaminated, particularly for Ni and Cd at industrial sites. The study confirms notable anthropogenic enrichment of HMs and indicates potential environmental concern. Continuous monitoring, improved waste-management practices, and further risk assessment are recommended to support informed mitigation strategies.