Heavy Metal Presence in Roadside Soils along Major Highways in Igboora, Oyo State, Nigeria

Abstract

This study assessed the concentrations and spatial distribution of selected heavy metals in roadside soils along major highways in Igboora, Oyo State, Nigeria. Soil samples were collected from three highway locations: Abeokuta–Igboora–Iseyin Highway, Igboora–Idere–Igangan Road, and OYSCATECH main road. At each location, samples were obtained at 0, 20, and 40 m from the road edge and from two soil depths, 0–20 and 20–40 cm. The concentrations of lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn), chromium (Cr), and nickel (Ni) were determined using atomic absorption spectroscopy after acid digestion. Soil pH and organic carbon were also measured. Data were subjected to an analysis of variance, and treatment means were separated using Duncan’s Multiple Range Test at the 5% probability level. The results showed significant differences in soil properties and heavy metal concentrations among highway locations, sampling distances, and soil depths. Soil pH ranged from slightly acidic to near-neutral, while organic carbon varied significantly across locations, distances, and depths. Zinc was the most abundant metal in the roadside soils, with the highest concentration of 499.15 mg kg⁻¹ recorded at a distance of 0 m and a depth of 0–20 cm along the Abeokuta–Igboora–Iseyin Highway. In general, most metal concentrations were higher near the road edge and declined with increasing distance from the highway. Surface soils also contained higher concentrations of most metals than subsoil samples, except for Pb, which showed relatively higher accumulation at a depth of 20–40 cm. The findings indicate that roadside soils in the study area contain measurable levels of traffic-associated heavy metals, particularly close to highway margins. Based on international soil quality guidelines, the soils showed low to moderate contamination. Zinc was the principal contaminant; cadmium and copper showed moderate enrichment, while lead, chromium, and nickel remained within acceptable limits. Continued monitoring and a 40 m roadside cultivation buffer were recommended.