Origin of trace elements in an urban garden in Nantes, France

Due to their location on old urban sites, impacted by human activities or road traffic, soils in urban gardens are often contaminated with a range of contaminants that could pose health risks. The aim of this study is to determine the origin of high trace element concentrations (arsenic and lead) in an urban community garden. Trace elements were quantified in situ in the topsoil (0-20 cm) of the 95 plots of the garden and in four soil profiles, using a portable X-ray fluorescence spectrometer. The accuracy of the spectrometer results was checked by measuring trace element concentrations by inductively coupled plasma mass spectrometry (ICP-MS) after an acid digestion (HF and HClO4). Leafy and root vegetables were sampled to assess lead transfer in vegetables. The accumulation of lead in vegetables was measured by ICP-MS after an aqua regia digestion. The bioaccessibility of lead was estimated by a calcium chloride extraction. Three anomaly levels could be defined from the mapping of arsenic and lead on the whole garden. The increase of trace elements content with depth, in correlation with pedological/geological characteristics, supports the hypothesis of a geogenic origin of these anomalies. The enrichment of the topsoil is related to the pedogenesis of soil, from micaschists as parent material. The geogenic origin of lead does not prevent its accumulation by vegetables. Although trace element anomalies have generally an anthropogenic origin, some anomalies may also have a natural (geogenic) origin, as shown in this study. The comprehension of lead origin, the mapping of its spatial distribution in the garden and the characterisation of its accumulation in vegetables were used as a basis for operational decisions including soil management. X-ray fluorescence spectrometry is a well-adapted method for this purpose. It allows an in situ and fast semiquantitative investigation on a lot of sampling points, saving time and laboratory analysis cost.