Pb, Cu and Zn geochemistry in reclaimed soils (Technosols) of Bulgaria

• Background total contents of heavy metals in Technosols located in coal areas
• Toxic total and available contents of Cu in soils originated from copper mining
• Safe mobile concentrations of Pb, Cu and Zn in alkaline Technosols in coal areas
• Mobile Pb and Cu toxicity is likely to occur in acidic Technosols

The content and availability of Pb, Cu and Zn were determined by the methods of soil extractions with aqua regia, 1 M NH4NO3 and 1 M NH4Ac. Reclaimed soils, which differ in land use, the duration of post reclamation development, geological and geographical characteristics were included in the study. In most pedons trace element total contents are irregular along depth and close to the background values in Bulgarian soils (Pb — 26 μg/g, Cu — 34 μg/g and Zn — 88 μg/g). The copper content greatly varies between the minimum of 8.5 μg/g and the maximum of 500 μg/g. The latter is defined as an intervention concentration by the Bulgarian legislation, and is accepted as a toxic threshold which affects soil functions, the environment and human health.Mobile concentrations of Pb, Cu and Zn are non-toxic assuming 8 μg/g for Cu, 4 μg/g for Pb and 43 μg/g for Zn as acceptable limits of available heavy metals in Technosols with slightly alkaline pH (7.0 - 8.0). The acidic pH (< 6.0) increases the solubility and desorption of zinc and copper, but diversely influences the bioavailability of lead. In acidic Technosols Pb and Cu mobile contents could exceed the Prüeß critical level of 1 μg/g and may provoke independent and joint toxic effects. The lithogenic origin of Cu toxicity which amounted to 47.7 μg available Cu/g soil shows the most serious risk of environmental pollution.Intensive weathering of soil surface layers does not accelerate the trace element bioavailability. Pb, Cu and Zn mobility mostly depends on pH, the content and mineralogy of fine earth fractions (< 0.01 mm) and organic carbon pools in studied soils.