Field assessment of cadmium, lead and zinc contamination of soils and leaf vegetables under urban and peri-urban agriculture in northern Nigeria

Urban and peri-urban agriculture in dry semi-arid northern Nigeria relies on untreated wastewater for all-year irrigation and the production of vegetables for urban markets. Human and animal exposure to potentially toxic metals is attributed to the consumption of vegetables raised in metal-polluted soils. The objective of this study was to determine the bioavailability and soil-plant transfer of Cd, Pb and Zn to amaranthus (Amaranthus caudatus) and lettuce (Latuca sativa) raised in the garden fields and to assess their safety for human consumption. Ten farmers' fields were selected per location for analysis of Cd, Pb and Zn in soils and vegetables. Whereas total concentrations of Cd and Zn were greater than the safe or permissible limits for agricultural soils, the Pb concentration was less than its maximum allowable concentration. However, the concentration of Pb and Cd in edible portions of amaranthus exceeded the safe limit for human consumption by 7-13 times, while lettuce exceeded the limit by 11-17 times. Cadmium was more rapidly transferred from soil through root to shoot than Zn>Pb. The plant tissue concentrations of the metals were not significantly correlated with the Diethylene triamine pentaacetic acid (DTPA) and dilute CaCl2-extractable concentrations of the metals in the soils. Furthermore, permissible limit of Pb established as standards for agricultural lands may not be suitable to ensure produce safety in Urban and peri-urban agriculture (UPA) in the city of Kano.