Geochemical mapping and urban allotment garden soil management

International audienceUrban Allotment Gardens (UAG) are expanding worldwide, especially in large cities. A large program towards UAGs has been launched since a few years in the city of Nantes (France). A soil quality assessment was conducted in 26 gardens on the city-territory. As eight of the gardens showed soil contamination problems, detected by a light screening (4 or 5 samples per garden), a diagnostic on the entire in surface of these gardens was conducted to help stakeholders in decision-making on the soil management.Environmental pressures (pollution, gardener’s practices, geogenic contamination) result often in potentially harmful trace elements in garden soils. The objective of the study was to assess the spatial variability of trace element distribution in UAGs at garden scale, to evaluate abnormal values under environmental pressures, and to assess the portable X-ray fluorescence screening method as a useful tool in soil management. The sampling and measurement strategy was based on a preliminary study of the environmental and historical context (including geology and historical land use) and on in-situ heterogeneity tests. On site measurements of trace elements content in soils were carried out during dry periods using a portable X-ray fluorescence analyser. Additional ICP-MS analyses were used to check the accuracy of the results. Finally, the spatial distribution of trace elements in soils was mapped and used to discuss the management options with the stakeholders.Each garden showed a specific spatial distribution of trace elements, linked to its environmental context but mainly to the land-use evolution. The contamination zones account for 10 % to more than half of the surfaces of the gardens, Pb and As being mostly present. At first, the analysis of geological maps and land-use evolution of sites allowed a more efficient screening strategy (definition of zones which the same characteristics). Aerial photographs were useful to detect potential impact from the environment of the garden, refillings of the zones and the development of the UAGs. PXRF measurements of trace elements concentrations combined with description of soil profiles allowed a better interpretation in terms of land-use changes and impact of parent materials. The PXRF screening method and mapping were very helpful for hot spot detection or delineation. But even if the spatial distribution and origin of trace elements were well described at garden scale, the source of trace elements has to be validated through additional methods in three study-cases (former market gardening).The method used was proved to be a cost-effective solution for soil management at garden scale because it could also be used to discriminate between zones with and without cultivation restrictions. The soil management solutions were adapted to the contamination levels: excavation and refilling, addition of non-contaminated soil, change in use (orchard, collective green space). Moreover, an innovative phytoremediation solution is currently carried out in one of the gardens