Heavy element contamination in soil and earthworms in the urban area of Siena (Italy)

A biogeochemical study was carried out to assess the influence of vehicular traffic on heavy element contamination in soil and accumulation in earthworms in the urban area of Siena (Italy). Total contents of Cd, Co, Cr, Cu, Ni, Pb, Sb, U e Zn and their distribution in extractable, reducible, oxidable and residual fractions were determined in soil samples collected in urban, peri-urban, green-urban and non-urban sites of Siena municipality. Heavy element concentrations were also measured in Nicodrilus caliginosus (Savigny) earthworm specimens collected in the soil sampling sites. The aim of study was: i) to assess the influence of vehicular traffic on heavy element distribution in soil; ii) to define the heavy element mobility in soil and potential bioavailability in relation to contamination level; iii) to establish the relationship between the heavy element content and fractionation in soil and their uptake by earthworms. Analytical data indicated that vehicular traffic affected the distribution of Cd, Cu, Pb, Sb and Zn in soil, with the highest total contents in urban and peri-urban soils. Based on the enrichment factor, Pb and Sb were the main soil contaminants. The heavy element fractionation in soil highlighted that Cd was the most mobile element being largely in the extractable (bioavailable) fraction. Pb was mainly associated with the reducible and residual fractions, and Co, Cr, Cu, Ni, Sb, U e Zn with the residual fraction. Contamination due to vehicular circulation caused variation in the distribution of the traffic-related heavy elements in soil fractions. Anthropogenic Cd, Cu, Pb, Sb and Zn were accumulated in the extractable, reducible and oxidable soil fractions with difference in partitioning related to the affinity of the element for organic complexation and non-specific and specific adsorption. Concentrations of Cd, Cu, Pb, Sb and Zn in earthworms showed a distribution pattern similar to that in soil, suggesting that soil contamination affected the accumulation of the traffic-related heavy elements in earthworms. The fractionation of these heavy elements in soil ruled their uptake by earthworms. The absorption of Cd, Pb, Sb and Zn by earthworms mostly depended on the extractable fraction, whereas the oxidable soil fraction influenced Cu uptake.