Ecological Risk and Bioaccumulation of Heavy Metals in Soil–Vegetable Systems under Wastewater Irrigation

Abstract

This study investigates the implications of wastewater irrigation on soil and vegetable safety in Tamburawa, Kano State, Nigeria, amid challenges of limited freshwater resources. It highlights the ecological risks and bioaccumulation of heavy metals; copper (Cu), chromium (Cr), iron (Fe), lead (Pb), and zinc (Zn) in the rhizosphere soil and edible parts of vegetables (cabbage, carrot, lettuce, onion, and spinach) multiple samples of which were analyzed using Atomic Absorption Spectrophotometry (AAS). Key physicochemical soil parameters were assessed to determine their effects on metal mobility. Pollution indices, including the contamination factor (CF), pollution load index (PLI), and geo-accumulation index (Igeo), were employed to gauge pollution severity. The findings indicated that lead (Pb) and iron (Fe) levels surpassed FAO/WHO permissible limits, with Pb concentrations reaching 23.0 mg/kg in carrot shoots and Fe at 1046.5 mg/kg in onion leaves, whereas Cu and Zn remained within safe limits. The soils were slightly alkaline with pH 7.76 and a sandy-loam texture which, combined with low organic carbon and cation exchange capacity, facilitated metal mobility. The high CF values for Pb and Fe suggest severe contamination, particularly in carrot and spinach rhizosphere soils. PLI values exceeding 1 corroborated the presence of cumulative pollution at several sites. Overall, the results underscore the significant ecological risks and heavy metal bioaccumulation associated with wastewater irrigation, necessitating immediate action in wastewater treatment, soil remediation, and ongoing monitoring to safeguard urban food systems.