Seasonal Variation of Heavy Metal Concentrations in Soils and Cocoyam Tubers Around Selected Municipal Dumpsites: Assessment Using Translocation Factor and Contamination Indices

Abstract

Municipal open dumpsites are major sources of heavy metal contamination, posing significant risks to agricultural soils and food safety in developing countries. This study evaluated the seasonal distribution, soil to tuber translocation, contamination indices and human health risks of Pb, Zn, Ni, Cu, Cr and Cd in soils and cocoyam (Colocasia esculenta) cultivated around three municipal dumpsites in Akwa Ibom State, Nigeria. Composite soil samples were collected at three depths (0–10, 10–20, 20–30 cm) from each dumpsite and a control site, in both dry and wet seasons, to assess vertical and seasonal metal variation. Cocoyam tubers were also harvested at each location. Metals were determined by Atomic Absorption Spectrophotometry after digestion. Transfer factor (TF), contamination factor (CF), degree of contamination (Cdeg), hazard quotient (HQ) and hazard index (HI) were computed to assess metal mobility and health risk. Metal concentrations generally decreased with soil depth, showing surface accumulation from waste deposition. The highest concentrations occurred at Ikot Ekpene (13.50 and 88.50 mg/kg in Pb and Zn, dry season), while Pb increased to 37.50 mg/kg in wet season. Transfer factors exceeded unity for Pb at Eket (TF = 1.22, dry season), and for Pb at Uyo (TF = 1.27) and Cr at Eket (TF = 1.43) in the wet season, indicating high translocation into tubers. Soil contamination was highest at Ikot Ekpene (Cdeg = 18.36 dry season; 37.77 wet season), while tubers recorded Cdeg values of 22.40–24.67 (dry season). Hazard indices exceeded the US EPA threshold (HI > 1.0) at all dumpsites, peaking at 11.29 (Ikot Ekpene, wet season), with Pb the major contributor to risk. Cocoyam cultivated around municipal dumpsites carries a high heavy metal exposure risk. Improved waste management and restrictions on food crop cultivation in contaminated areas are recommended.