Urban allotment gardens (AGs) provide a unique combination of productive and recreational spaces for the inhabitants of European cities. Although the reasons behind the decision to have a plot, as well as the mode of use and gardening practices, are well recognised in the literature, these issues are mainly considered in relation to particular case studies within a single country. The regional diversity of European allotment gardens is still poorly understood, however. This knowledge gap became an incentive for us to carry out the present study. The research was conducted in seven countries: Austria, Estonia, Germany, France, Portugal, Poland and the UK. Surveys were used to assess the motivations of users regarding plot uses and gardening practices. Information was also collected during desk research and study visits, making use of available statistical data. Allotment gardens in Europe are currently very diverse, and vary depending on the historical, legal, economic and social conditions of a given country, and also as determined by geographical location. Three main types of plots were distinguished, for: cultivation, recreation–cultivation, and cultivation–recreation. The recreational use of AGs has replaced their use for food production in countries with a long history of urban gardening. The only exception is the UK. In some countries, the production of food on an AG plot is still its main function; however, the motivations for this are related to better quality and taste (the UK), as well as the economic benefits of self-grown fruits and vegetables (Portugal, Estonia). Among the wide range of motivations for urban gardening in Europe, there is increasing emphasis on active recreation, contact with nature and quality food supply.

Purpose Urban 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 methods The 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 discussion The 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. Conclusions As 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.

To mitigate groundwater level decline, managed aquifer recharge (MAR) with secondary treated wastewater (STWW) is increasingly considered and implemented. However, the effectiveness and potential risks of such systems need evaluation prior to implementation. In this study, we present a large-scale sand tank experiment to analyse processes related to the infiltration of real STWW through the vadose zone and subsequent mixing with oxic native groundwater. The varying composition of STWW from 15 infiltration cycles over six months of operation and the retention times were the main drivers of the observed processes, which were characterized by a wide range of analytical techniques such as in situ high-resolution oxidation-reduction potential (ORP) measurements, closed mass balances of solutes, characterization of dissolved organic carbon (DOC), stable nitrate isotopes analysis, as well as numerical flow and transport modelling. Depending on the composition and infiltration rates of the STWW, both nitrification and denitrification could be observed, even simultaneously at different locations in the tank. Furthermore, due to the variability of the real STWW we observed enhanced arsenic mobilisation during times of elevated phosphate concentrations of the infiltrating STWW. Additionally, uranium was mobilised in our experimental system via carbonate mineral dissolution caused by the infiltrating STWW which was undersaturated of calcite for all infiltration cycles. Overall, our results showed the importance of conducting studies with waters of complex matrix, such as real STWW, and considering mixing with groundwater to assess the full range of possible processes encountered at MAR field sites.