Pollution as a threat factor to urban food security in metropolitan Kano, Nigeria

This study evaluated the potentials of pollution due to urban agriculture to pose risk to food security in Kano, Nigeria. Two sites irrigated with industrial and domestic wastewaters at Challawa and Jakara, respectively, were sampled for soil, water, and lettuce (Lactuca sativa L.). The samples were analyzed for Pb, Cd, Cr, and Cu being metals which when present in food may pose human health hazard. The values were compared with samples from a control site not associated with wastewater at Watari. The sites were each segmented into up, mid, and downstream sectors. Results showed that the two sites and the control were contaminated with Cu (pollution indexes [PI] = 0.14, 0.16, and 0.14, respectively). Domestic wastewater site was slightly polluted (PI = 1.09) while industrial wastewater site and the control were contaminated (PI = 0.8 and 0.54, respectively) with Pb. The two sites were excessively polluted with Cd and slightly polluted with Cr. The control was contaminated with both (PI = 0.74 and 0.06 for Cd and Cr, respectively). Metal levels in the waters of the sites and the control were higher than the recommended threshold for irrigation water. There was significant positive correlation between Pb, Cr, and Cd in water and in soil; while Cu in soil and in water negatively correlated. There was high metal transfer from soil to plants at the domestic wastewater site (Metal transfer factor Pb = 1.602, Cr = 1.126, and Cu = 1.834). Plants' accumulated concentrations were also high at the domestic wastewater site (Pb = 26.21 mg/kg, Cd = 1.03 mg/kg, Cr = 28.63, and Cu = 2.66 mg/kg). The plants' metal concentrations at the domestic wastewater site exceeded the allowable limits in vegetables. The human ingestion risk was in the order of Jakara > Challawa > Watari.