Effect of Wastewater Irrigation on Heavy Metal Accumulation in Lettuce (Lactuca sativa) Grown in Lagos, South West Nigeria

Urban vegetable farming in Lagos, Nigeria increasingly relies on wastewater from drainage channels and sewage outlets due to the scarcity of clean irrigation water. While this practice sustains production, it poses significant risks of heavy metal contamination in edible crops, particularly leafy vegetables such as lettuce (Lactuca sativa). This study examines heavy metal (cadmium, lead, chromium, and zinc) accumulation in lettuce irrigated with wastewater collected from three industrial zones in Lagos State. Lettuce seedlings were grown under greenhouse conditions using collected wastewater, borehole water (control), and a 50:50 mixture of both. Plant samples were harvested at maturity and analyzed using atomic absorption spectrophotometry. Results indicate that cadmium and lead concentrations in wastewater-irrigated lettuce exceeded the FAO and WHO maximum allowable limits for food crops, while zinc remained within acceptable ranges. The mixed irrigation treatment showed intermediate accumulation levels. The study concludes that unrestricted wastewater irrigation poses a measurable food safety risk for Lagos urban vegetable producers and consumers. Recommendations include water quality testing before irrigation use, adoption of phytoremediation strategies, and stronger regulatory frameworks for urban agriculture. These findings contribute to emerging literature on food safety in Nigerian urban horticulture systems. Keywords: wastewater irrigation, heavy metals, lettuce, Lactuca sativa, Lagos