Distribution of heavy metals of agricultural soils of central Greece using the modified BCR sequential extraction method

Abstract

The state of heavy metal (Cd, Cu, Ni, Zn, Pb, and Cr) pollution was studied, in 440 texturally different soil profiles in Thessaly, an intensely cultivated region in Central Greece. The study was carried out in 2004 and 2005 on 220 soil samples for each year. Soil samples were classified in three soil orders: Endisols, Alfisols, and Vertisols according to the Soil Taxonomy System. The pseudo-total concentrations of heavy metals were determined by the aqua regia procedure. Heavy metals were also determined after division into four fractions by sequential extraction with (a) acetic acid (exchangeable and specifically adsorbed metals), (b) a reducing agent (bound to Fe/ Mn hydroxides), (c) an oxidizing agent (bound to soil organic matter), and (d) aqua regia (bound to mineral structures, residual). The concentrations of all the metals studied were higher in the topsoil (0-30 cm) and lower in the second soil layer (30-60 cm). Concentrations of 70-82% of Cd, 39-64% of Cu, 41-69% of Ni, 29-51% of Zn, 75-89% of Pb, and 52-87% of Cr were found in the first two fractions. Cd appeared to be the most mobile of the metals studied, while Cu and Zn were found in forms associated with soil organic matter. The chemical partitioning patterns of Pb and Cr indicated that these metals are largely associated with the Fe-Mn hydroxides, while Cr was also found in the residual fractions. Significant correlations between heavy metals fractions and soil physicochemical parameters were obtained and discussed.