Heavy metal contamination along a soil transect in the vicinity of the iron smelter of Kremikovtzi (Bulgaria)

Based on previous preliminary survey results, elevated heavy metal concentrations of the agricultural soils of the villages of the Kremikovtzi region east of the Bulgarian capital Sofia were attributed to pollution from dusts emitted by the iron smelter in the centre of the region. Additional sampling in a test area indicated that high metal concentrations in this area could also be of geogenic origin. In order to test this hypothesis, we sampled a sequence of 14 soil profiles at increasing distance from the smelter along a 2 km transect through agricultural land outside the metallurgical complex of Kremikovtzi. The transect extended through two different units of soil types: Chromic Luvisols and Alluvial Fluvisols. Samples of the fine earth fraction (<2 mm grain size) from various depths were analysed for 19 elements by X-ray fluorescence spectroscopy (XRF). Maximum concentrations of the suspected main pollutants were approximately 170 mg/kg As, 185 mg/kg Pb, and 350 mg/kg Zn. Concentration profiles over depth were distinctly different between the two soil units for most elements and in comparison to this difference varied little within each unit. Within the two units there were no trends with distance to the smelter. The concentration profiles did not indicate any substantial contribution from atmospheric deposition. The enrichment of elemental concentrations relative to Zr did not provide sufficiently conclusive evidence to distinguish anthropogenic from geogenic metal sources. While the ratios of zinc, copper and arsenic to zirconium were almost constant, chromium, nickel and molybdenum ratios to zirconium varied between 2- and 5-fold (primarily showing a decrease) with depth in the majority of profiles, indicating gradients in the original deposition of the soil-forming parent material rather than an anthropogenic source. The study illustrates how important it can be in identifying the origin of soil pollution not only to look into areal distribution patterns of suspected contaminants, but also to investigate their depth profiles as well as that of other elements and to account for pedological factors and their variability. (C) 2007 Elsevier B.V. All rights reserved.