Evaluation and Health Risk Assessment of Heavy Metals Contamination in Agricultural Soil of Onna River Basin, Nigeria

Abstract

Background: Heavy metal contamination of agricultural soils can impair soil quality, affect crop productivity, and pose ecological and human health risks through multiple exposure pathways. Assessing metal concentrations and associated contamination and risk indices is therefore essential for supporting evidence-based soil management and environmental protection. Aims: This study assessed the levels of contamination and associated health risks of selected heavy metals, including Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn, in agricultural soil from the Onna River Basin Development Authority, Onna, Akwa Ibom State, Nigeria. Place and Duration of Study: The study was conducted at the Onna River Basin Development Authority, Onna, Akwa Ibom State, Nigeria, in November 2024. Methodology: Soil samples were collected at depths of 0–15 cm and 15–30 cm using a 1 m auger and analysed in the laboratory using standard methods. Heavy metal contamination was assessed using the contamination factor (CF), degree of contamination (CD), pollution load index (PLI), and geo-accumulation index (I-geo). The assessment also included the potential ecological risk index and non-carcinogenic and carcinogenic health risk estimates. Results: Metal concentrations were higher at the 0–15 cm depth than at the 15–30 cm depth. Cu, Ni, Pb, and Zn were below the stated target values for uncontaminated soil. Cd concentrations at 0–15 cm and 15–30 cm were 1.063 and 0.847 mg/kg, respectively, both exceeding the stated target value of 0.8 mg/kg. The farmer reported the use of NPK+ 12 12 17 2 fertiliser, poultry manure, and herbicide on the field during the preceding 7 years. The CF values for Cu, Pb, Zn, and Ni indicated low contamination, whereas those for Cd indicated moderate contamination at both depths. The I-geo values for all assessed metals were below 1, corresponding to the uncontaminated-to-moderately-contaminated category. The hazard index (HI) values were below 1 for adults and children. Most lifetime cancer risk estimates were within the stated acceptable range of 1 × 10-6–1 × 10-4; however, the estimated ingestion risk for children at 0–15 cm was in the 10-2 range. Inhalation risks were in the 10-9–10-10 range and were regarded as negligible. Conclusion: Cd concentrations at both sampling depths exceeded the stated target value for uncontaminated soil and therefore require attention in the management of the agricultural site.