Mercury in topsoils
Mercury (Hg, atomic number 80) is recognised as a dangerous pollutant worldwide. Mapping of surface soil Hg concentrations, a priority pollutant, at continental scale is important in order to identify hotspots of soil Hg distribution (e.g. mining or industrial pollution) and identify factors that influence soil Hg concentrations (e.g. climate, soil properties, vegetation).
Here we present soil Hg concentrations from the LUCAS topsoil (0–20 cm) survey including 21,591 samples from 26 European Union countries (one sample every
~200 km2). LUCAS survey has the advantage of not being limited to agricultural and grassland soils, but samples a broader range of ecosystems (including forests and wetlands) thus providing more a more detailed picture of the distribution of Hg across the European Union. In addiiton, it is the most comprehensive and harmonised soil survey at continental scale. LUCAS topsoil already proved to be able provide detail information allowing the assessment of other potential contaminants, as it was used to analyse the spatial distribution and the factors influencing copper distribution in EU.
Deep Neural Network (DNN) learning models were used to map the European soil Hg distribution (Fig. 1). DNN estimated a median Hg concentration of 38.3 μg kg−1 .10% of the area exceeds the 84.7 μg kg−1 and 209 Hg hotspots (top 1%) have been identified with concentrations >422 μg kg−1. There is a high variation of Hg concentration across the EU, mainly due to impact of natural processes (high soil organic carbon, vegetation, parent material, temperature, soil texture and pH) and some evident high values close to past mining activities and coal combustion sites. High concentrations of mercury have been found close to well-known mining sites like Almaden (Asturias, Spain), Mt. Amiata (Italy), Idrija (Slovenia) and Rudnany (Slovakia). In a more detailed investigation, 42% of the hotspots were associated with well-known mining activities while the rest can be related either to coal combustion industries or local diffuse contamination (Fig. 2).
According to our study, diffuse Hg contamination in European topsoil is not an emerging issue. However, policy actions are needed for managing the existing hotspots and the emissions connected to power production. The mapping effort in the framework of LUCAS can serve as a starting point to guide local and regional
authorities in identifying Hg contamination hotspots in soils.
The result of the Deep Neural Network (DNN) learning models is the Map of topsoil Mercury concentration (μg Kg-1). The spatial interpolated data (as seen in the figure below) are available for public user.
Ballabio, C., Jiskra, M., Osterwalder, S., Borrelli, P., Montanarella, L., Panagos, P. 2021. A spatial assessment of mercury content in the European Union topsoil. Science of the Total Environment.769. Article No: 144755.
Fig.1. Topsoil Hg concentrations (μg kg−1) across 26 EU countries estimated by deep neural network – regression kriging. Colour classes are based on 12.5th percentiles
Fig. 2. Map of kriged residuals including outliers. Overlaid are coal fired power plants in circles with the size of the circle proportional to the power production and Hg mines with the size of the symbols representing their past production (where available).