Unveiling the Hidden Risks: Heavy Metal Concentrations in Soil and Vegetables Irrigated with Kalobe Wastewater Stabilization Ponds, Mbeya, Tanzania

This study evaluated the concentrations of heavy metals in wastewater, soil, and tomatoes and Napa cabbage irrigated with effluent from the Kalobe Wastewater Stabilization Pond (KWWSP) in Mbeya, Tanzania. Human health risks were assessed using Chronic Daily Intake (CDI), Target Hazard Quotient (THQ), Hazard Index (HI), and Target Cancer Risk (TCR) indices. The results showed that cadmium (Cd) in all ponds was below the FAO/WHO permissible limits, while lead (Pb) and chromium (Cr) were below detection levels. Heavy metals in soil were found in the order of Pb (5.95 mg/kg) > Cr (0.63 mg/kg) > Cd (0.25 mg/kg), all within FAO/WHO acceptable limits, indicating suitability for agricultural use. Cd levels in Tomatoes (0.14 mg/kg) and Napa cabbage (0.40 mg/kg) exceeded permissible limits. Cr levels in the Tomato and Napa Cabbage were 1.87 and 2.10 mg/kg, respectively, and were close to the safety threshold, suggesting health concerns with long-term consumption. Cd exposure through vegetable intake was within but near acceptable limits, while Cr exposure, particularly for Napa cabbage, exceeded recommended safety thresholds. This resulted in elevated noncarcinogenic risks (THQ and HI > 1) and carcinogenic risks (TCR above the USEPA's acceptable range). These findings suggest that consuming wastewater-irrigated Tomatoes and Napa cabbage may pose human health risks. Continuous monitoring of heavy metals, safe irrigation alternatives, and cropping restrictions using inadequately treated wastewater is essential to safeguard public health and long-term environmental sustainability.