Soil and vegetable compositional relationships of eight potentially toxic metals in urban garden fields from northern Nigeria

BACKGROUND: Human exposure to heavy metals is attributed to consumption of vegetables raised in polluted soil environment. We examined the concentrations of As, Cd, Co, Cr, Cu, Ni, Pb and Zn in soils and lettuce (Latuca sativa) and amaranthus (Amaranthus caudatus) in 15 garden fields under long-term wastewater irrigation.RESULTS: The concentrations of As (0.3-2.1 mg kg⁻¹), Cd (0.07-0.3 mg kg⁻¹), Co (4.6-9.1 mg kg⁻¹) and Cr (21.6-36.2 mg kg⁻¹) in the gardens were consistent with background concentrations in soils, but Ni (12.6-25.7 mg kg⁻¹), Cu (12.5-24.6 mg kg⁻¹), Pb (25.7-71.6 mg kg⁻¹) and Zn (52.3-158 mg kg⁻¹) concentrations were double the concentrations normally encountered in arable fields in the region. The concentrations of Cd and As in the vegetables were within safe limits, but Co (0.14-0.67 mg kg⁻¹ fresh weight (fw)) and Ni (1.0-2.7 mg kg⁻¹ fw) concentrations in lettuce were relatively high for leaf vegetables. The concentration of Pb (0.65-4.80 mg kg⁻¹ fw) was above safe limit of 0.3 mg kg⁻¹ fw, while the concentrations of Cu (1.3-2.7 mg kg⁻¹ fw and Zn (10.2-23.6 mg kg⁻¹ fw) were close to the unsafe limits for leaf vegetables. There were no strong linear correlations between soil metal concentrations and bio-concentrations in the vegetable crops.CONCLUSION: We surmise that leaf vegetables can accumulate metals in their tissues to unsafe levels even when total metal concentrations in these soils are below the allowable concentrations in agricultural soils.