Cécile Le Guern

Spatial variability of trace elements in allotment gardens of four European cities: assessments at city, garden, and plot scale

PurposeUrban allotment gardens (UAGs) are expanding worldwide, especially in large cities. Environmental pressures (direct and diffuse pollution, gardener practice, geogenic contamination) often result in the accumulation of potentially harmful trace elements in garden soils. The objectives of this study were to assess the spatial variability of trace element distribution in UAGs from city, garden, and plot scale in four European cities; to provide a baseline understanding and identify abnormal values under environmental pressures; and to evaluate the potential of portable X-ray fluorescence screening as a useful tool in soil management.Materials and methodsThe four cities (Ayr and Greenock (Scotland), Lisbon (Portugal), Nantes (France)) provided a wide range of environmental pressures on soils. The locations of the 14 allotment gardens were identified in consultation with the local municipality in each city to reflect various land uses or according to previous evaluation of soil quality. Soil sampling was carried out in 66 plots in total, from which 3 datasets were produced: (i) basic soil properties and trace element concentrations from a composite sample of topsoil for each plot (trace elements quantified by inductively coupled plasma–optical emission spectrometry/mass spectrometry (ICP-OES/MS) or using in-lab portable X-ray fluorescence (PXRF); (ii) in situ PXRF measurement on composite samples (263 plots in Nantes); and (iii) composite samples from 32 small areas within 4 plots in one garden of Nantes.Results and discussionThe results were analyzed to assess the spatial variability of soil properties. At city and garden scale, the variability observed for basic soil properties and major elements is dominated by local geology/parent material (pH, CaCO3, Fe) and gardening practice (OM, CaCO3), which vary between each country. The range of trace element concentrations is similar between each city except for Greenock. Extreme values are observed for Cu, Pb, and Zn reflecting human disruption. In most situations, the trace element contamination was explained through the historical and environmental situations of the site. The PXRF screening method proved useful in providing detailed mapping for hot spot detection or delineation, providing support for soil management at plot and garden scale.ConclusionsAs anticipated, basic soil properties appear to be controlled by the parent material. At plot and garden scale, the trace element variability shows the influence of land use history and background and strong inputs from external factors (e.g., by industrial activity or traffic emission). The PXRF screening method appears to be an efficient solution for soil management as it can be used to discriminate zones which may require restriction on cultivation.

A typology of anthropogenic deposits as a tool for modeling urban subsoil geochemistry: example of the Ile de Nantes (France)

PurposeUrban soils may be contaminated not only by industrial activities but also by the materials contained in the anthropogenic deposits. In order to anticipate the management of excavated soils linked to urban redevelopment, their quality and quantities need to be assessed. Urban soils and subsoils contain nevertheless a large variety of anthropogenic deposits and present important geochemical heterogeneities. The paper aims to develop various geochemical types of made grounds relevant for a 3D urban subsurface model.Materials and methodsTo build a 3D urban subsurface model, using geological tools, the challenge is to define a stratigraphic pile both geologically and geochemically relevant. In this frame, we defined a typology of made grounds based on an iterative approach. We used (a) published classifications, (b) a dataset of the study area (Ile de Nantes) comprising the borehole descriptions (2400), and (c) results of analyzed samples (more than 1800), gathered in a georeferenced database. The layers (8400) found in the borehole descriptions were interpreted according to the a priori made grounds typology and to the underneath natural layers. The relevance of the made ground typology proposed was verified by a statistical approach on the representative samples and analyses selected in the database.Results and discussionThree types of made grounds have been retained. They show contrasted geochemistry. The made grounds that are comparable to natural subsoils correspond in the case study to alluvial type materials. They show similarity with a typology and geochemistry of alluvial materials. The questionable man-made grounds, containing potentially contaminated anthropogenic materials such as bottom ash or slag, are characterized statistically by higher contaminant levels than other types of made grounds (e.g., PAH, Zn, Pb, Cu,...). The various man-made grounds, corresponding to the remaining materials and containing for instance demolition materials, are characterized statistically by higher levels of PCBs. The typology has been used in a 3D representation of the Ile de Nantes subsoils, which served as a decision aid tool for the developer.ConclusionsThe knowledge on urban subsoil geochemistry may help defining redevelopment projects, by adapting soil use to subsoil quality. In this frame, the development of a geochemically relevant made ground typology taking into account their intrinsic potential of contamination appears useful. A proper description of the intrinsic components of the made grounds is essential. It is indispensable to use some rigorously defined and internationally agreed terms.