Heavy metal pools in urban soils from city parks of Sofia, Bulgaria

V. G. Kachova1*, I. D. Atanassova2

1Forest Research Institute, Kliment Ohridski Blvd. 132, 1756 Sofia, Bulgaria

2Nikola Poushkarov Institute of Soil Science, Agrotechnology and Plant Protection, 1080 Sofia, Bulgaria

(Manuscript received 4 April 2017; accepted for publication 30 May 2017)

Abstract. With a population ~ 1 300 000 million and an area of 1344 km2, Sofia, Bulgaria is one of the most densely populated cities in Europe with registered contamination of the air, water and soils of the city including heavy metals. The purpose of the study is to analyze the different pools under which heavy metals in urban soils exist. The chosen sites are the largest forest parks in Sofia. Cu, Zn, Pb and Cd in soil of samples were sequentially extracted and separated soluble F1 (70% ethyl alcohol solution), liable F2 (1N CH3C00Na, pH = 7), carbonate bound F3 (1N CH3C00Na + 1N CH3C00H, pH = 3), soil organic matter bound F4 (30% H2O2), amorphous iron oxide bound F5 (0.5 M Na-citrate), total “aqua regia” content and residual fraction F6 (measured as a difference from total). These operationally defined fractions give estimates of available (F1, F2), potentially liable (F3, F4, F5) and pseudo-total contents in soils, respectively. Metals were manly concentrated in the “residual fraction” F6. Organic matter associated is manly Cu and Pb, while carbonates-bound and amorphous iron oxides-bound were Pb and Zn. The mobility of the studied metals decreased in the order: Cd > Zn > Cu ≥ Pb. The soils with high pH and Cation exchenge capacity measured at pH = 8.2 (CEC8.2) show lower amounts of mobile (easily-available) forms of heavy metals.