The risk associated with heavy metals contamination in vegetables growing in urban and peri-urban areas

Mexico City is among the largest and most polluted megacities in the world, and there is a general concern among residents to look for a healthier and more sustainable space. However, growing food in urban gardens relies on available space that usually is located alongside main roads or nearby industrial areas. So there is a risk that contaminants from soil, air and water affect the produce quality. This research examines the heavy metal risk for lettuce consumption produced in urban and peri-urban locations around Mexico City, through the measurement of heavy metals in soils, water and vegetative material. The concentrations of heavy metals for all cases were below the threshold limit values for soils, according to different regulations. Tlatelolco and Insurgentes gardens, close to main avenues, presented the highest lead pollution, reflected in a moderate to heavily contaminated geoaccumulation index by Pb. Heavy metals content in irrigation water for all gardens and the rain water from Tlatelolco were below toxicity threshold limit values for agricultural water. A high As concentration in lettuce samples was found at Tlatelolco and Insurgentes gardens, also, Cr, Cd are above the permissible limit values for the lettuce samples taken at the organic market in Chapingo, while high concentration of Cr was found from lettuce samples taken from the experiment in Chapingo. The lettuce grown in the urban gardens in Mexico City revealed a hazard index greater than 1, suggesting a potential health effects. The samples taken in this study are not enough to have a generalized conclusion, about the contamination of soil, water and vegetative material by heavy metals.