Geochemical properties of soils in urban allotment gardens from four European cities

Suitma 8, 8th International Conference of the Working Group on Soils in Urban, Industrial, Traffic and Mining Areas , MEXICO, MEXIQUE, 20-/09/2015 - 25/09/2015Urban allotment gardens (UAG) are expanding worldwide, especially in industrialized countries. However, gardens in the urban environment are often subject to environmental pressure, mainly as a result of pollution from urban and industrial activities as well as through gardener's practices (Hursthouse et al., 2004). Potential risks to human health can derive from the presence of harmful substances either by consumption of contaminated vegetables, breathing dust after resuspension or by direct contact with soil. Investigations on the quality of UAGs soils have been launched in 4 European cities: Greenock and Ayr (Scotland), Lisboa (Portugal), Nantes (France- ANR VBD Jassur program). Soils of the UAGs were sampled to characterize the physical properties (soil texture) and chemical properties (fertility (phosphorus, nitrogen, organic carbon), contamination by trace elements). Trace metals contents were determined in the laboratory by ICP OES/MS analysis after digestion with acidic solution. The results were analyzed to evaluate the quality of the UAG soils at city level taking into account the pedo-climatic conditions, the geochemical background and the anthropogenic activities (vs. geogenic anomalies). The heterogeneity of the soil quality within each UAG was also investigated to be able to implement the most appropriate management methods for soils in the case of excessive contamination. The relevance of in situ measurements by portable X-ray fluorescence spectrometry was also addressed as a technical means to help in decision-making. The results highlight the wide variation in contamination of soils (e.g background geochemistry, traffic, contaminated backfill materials...). In most situations, the historical and environmental study of the site has been helpful in understanding the contamination. High levels of phosphorus were detected due mainly to gardening practices. The spatial distribution of some trace elements (As, Pb, Zn) was very heterogeneous within each UAG. In case of vegetable contamination, an in situ and fast investigation method such as portable X ray fluorescence spectrometry was proposed to local authorities (in Nantes) to produce trace elements maps of UAGs. The maps help to discriminate the zones within the UAGs with the highest concentrations and to keep cultivation restriction only on relevant plots. This method was proved to be a possible solution to face with the heterogeneity of UAG soil quality