Comparative Analysis of Heavy Metal Bioavailability in Irrigated and Rain-Fed Vegetable Production Zones in Kano State, North West Nigeria

Urban and peri-urban vegetable farming using untreated wastewater for irrigation is a widespread practice across major Nigerian cities, including Kano, creating potential public health risks through heavy metal accumulation in food crops. This study compared heavy metal concentrations and estimated bioavailability in vegetables produced under irrigated (using urban drainage water) and rain-fed cultivation systems across three sites in Kano State. Paired soil and vegetable samples of Spinacea oleracea, Lactuca sativa, and Lycopersicum esculentum were collected from six irrigated and six rain-fed plots. Lead, cadmium, chromium, and arsenic concentrations were determined by atomic absorption spectrophotometry. Bioavailability was estimated using the bioaccessible fraction method involving simulated gastrointestinal digestion and sequential extraction. Target hazard quotient and carcinogenic risk calculations were performed. Results showed significantly higher lead (0.48 vs 0.12 mg per kg) and cadmium (0.18 vs 0.04 mg per kg) concentrations in irrigated vegetables. Bioaccessible fractions exceeded safe thresholds for lead in Spinacea oleracea from irrigated sites. Target hazard quotient for cadmium exceeded 1.0 in high-consumption scenarios for irrigated vegetable consumers. Rain-fed vegetables presented lower risk across all metals. The study provides empirical risk characterisation data that substantiates the need for regulatory controls on wastewater irrigation practices in Kano peri-urban vegetable zones. Keywords: heavy metals, bioavailability, irrigation, vegetables, Kano.