Establishment of geochemical background and threshold values for 8 potential toxic elements in the Bulgarian soil quality monitoring network.

Establishment of geochemical background and threshold values of potentially toxic elements is essential for soil management activities. Available topsoil data collected in the Bulgarian soil quality monitoring network are explored for the concentrations of the potentially toxic elements (PTEs) of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn. The 'median + 2MAD' and TIF methods for obtaining geochemical threshold values, which differentiate sampling sites with background from those with unusually high element concentrations, are presented. Comparison of the derived national geochemical threshold values with those obtained in European studies outlines the specificity of the Bulgarian PTE background concentrations among the other southern European countries. The threshold values for Cd, Cu, Ni, Pb and Zn are more similar to those derived from the northern European countries since the As and Cr ones are more close to the values derived for southern Europe. The geochemical threshold of Hg is almost one order of magnitude as high as the southern European derived value. The usage of geochemical threshold values for soil risk assessment is suggested by comparison with soil quality guideline values adopted in other countries. The combination of threshold values based on 90th percentile with geochemical mapping identifies the priority areas for further assessment. Regions with more than 2 PTEs (2.26% of the Bulgarian territory) are suggested for detailed (eco)toxicological assessment.