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.
Coupling the Hg stocks with soil loss by water erosion to estimate the eventual Hg fluxes with sediment transport
In JRC, we present the first study to couple soil diffuse contamination of an emerging pollutant (mercury) with sediment distribution models at continental scale. In this study, we estimated the Hg stocks in topsoils at 44.8 Gg with a mean density of 103 g ha-1. Then, we coupled the Hg stocks with the pan-European sediment distribution model outputs to estimate the Hg displaced annually by water erosion to at 43 Mg yr-1 (c.a 0.1% of the total Hg stocks). Agricultural lands contributes to more than 85% of those losses. As a follow-up, we used the European Catchment and River database to estimate the total Hg losses in river basins (Fig. 3). In EU and UK, the total Hg losses in river basins is about 6 Mg yr-1 which is 14% of the total displaced Hg as the rest is re-distributed close to the eroded field. The catchments with high Hg concentration and high erosion rates are the ones with extreme Hg annual losses. Therefore, we estimated that 1.5% of the river basins have Hg losses higher 120 mg ha-1 yr-1; all of them are located in the Mediterranean Basin.
The delineation of river basins allows to estimate the Hg losses which potentially can reach the major European Sea outlets (Fig. 4). Summing up the possible Hg losses to sea outlets, we conclude that the Mediterranean Sea gets 2.94 Mg Hg yr-1 which is half of the total Hg losses routed to EU river basins. The Black Sea get c.a. 1.46 Mg Hg yr-1 while the Baltic Sea gets only 0.25 Mg Hg yr-1 and North Sea around 0.55 Mg Hg yr-1. Those are the Hg losses attributed to water erosion in the EU and do not include gullies or landslides.
Taking into account the current policy developments at global scale with the Minamata Convention and the adoption of the Sustainable Development Goals (here we focus on SDG 3.9 and SDG14.1), this study offers some insights in the mercury stocks in EU topsoils and fluxes to the river basins and Sea outlets. In EU policy area, the adopted Zero Pollution Action Plan raises the issues of diffuse soil contamination in EU and envisages the development of relevant indicators to monitor the progress in soil pollution.
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).
Fig. 3. Estimated Hg displaced with water erosion per catchment. The vertical bars show the annual Hg eroded (green) and deposited (orange) per country.
Fig. 4. Estimated Hg losses to river basins and Hg fluxes to sea outlets (Mg yr-1) due to water erosion.
The Hg stock (g ha−1) in European topsoils, the Estimated Hg displaced with water erosion per catchment, and the estimated Hg losses to river basins and Hg fluxes to sea outlets (Mg yr-1) due to water erosion are available for public user.