Evaluation of Metal Mobility and Bioaccessibility in Soils of Urban Vegetable Gardens Using Sequential Extraction

Urban soils are recipients of a variety of pollutants, including the toxic heavy metals, which can accumulate in these soils. The consumption of vegetables and fruits grown in urban gardens with elevated heavy metal content may pose a health risk to residents, because the plants can absorb the heavy metals to different degrees depending on a number of factors (soil properties, forms of metals, plant specific factors, etc.). Total metal content and heavy metal fractions were determined using a sequential extraction procedure in the soils of urban vegetable gardens in order to assess both mobile and potentially mobile amounts of heavy metals in these soils. The heavy metal content of different vegetable types grown in these gardens and those soil properties relevant to controlling metal mobility were also investigated. Soil properties of the studied vegetable gardens influenced metal retention positively, but these soils were not heavily contaminated with metals. The high content of total Pb, Cu, Zn and Cd in the mobile and potentially mobile fractions did, however, indicate a possible anthropogenic enrichment of these metals. The vegetables grown in these gardens accumulated no toxic metal content but the concentration of Cu in the studied vegetables was generally higher than the mean Cu content of different plant foodstuffs, clearly indicating an anthropogenic impact on the Cu status of these soils. Based on the calculated bioaccumulation indices and in accordance with the result of the sequential extraction, Zn and Cd were the most mobile elements in the present soil–plant system.