## 3 Results and Discussion  
### Soil Characterization  
The obtained values for organic carbon, organic matter, pH and particle size distribution are reported in Table 1.  
Organic matter content of a soil is of great importance since it can form soluble or insoluble complexes with metals, therefore influencing their tendency to be retained or to migrate throughout it [33]. Organic carbon, which makes up about 60% of the organic substance present in soils, has an essential positive impact on many soil properties. It promotes the aggregation and stability of soil particles, thereby reducing erosion, compaction, crevices and soil crust formation; it binds effectively with numerous substances improving soil fertility and its buffer capacity. Additionally, it improves microbial activity and the availability of nutrients for plants such as nitrogen and phosphorus [34]. pH can influence heavy metal adsorption capacity since these trace elements are very mobile in acidic conditions and their mobility and availability decreases as pH approaches neutrality [33]. Finally, soil texture also plays an important role in the mobility of metals; it reflects the particle size distribution of the soil and thus the content of fine particles like oxides and clay that are important adsorption media for heavy metals in soils [35]. The percentages of sand, silt and clay determined in the investigated soil samples allowed us to classify it as a sandy-loam soil. The texture, the medium-high percentages of organic matter and organic carbon and the pH close to neutrality suggested that the metals were not extremely mobile in the studied soils and were therefore less available. In Table 2, the concentrations of the 22 elements determined in the two soils are reported with the concentration limits imposed by Italian legislation [36], which are different according to the intended use of the land (public green area or industrial area). High concentrations of Pb, Ni and Zn and Co, Cr and Ni were found in the CA and NOB sites, respectively; their values exceeded public green area limits but not industrial area limits. The content of Co, Cr and Ni is high in both sites according to the geogenic nature of these elements in the Turin valley. In fact, Piedmont plains originated during past fluvio-glacial events and are therefore made of sediments, which partly derive from serpentinitiic areas resulting in high background levels of Co, Cr and Ni in soils, as already documented by other researchers [37; 38]. Cu and Zn, which are micronutrients and therefore necessary in low concentration in agricultural lands, are less concentrated in the NOB soil; this confirms that they partially derive from anthropogenic contamination in the CA site. The load of Ba and Pb represents the most significant data--lead concentration in the CA soil is an order of magnitude greater than that of NOB, whereas barium shows a three times greater concentration in CA. This can result from anthropogenic pollution sources such as urban vehicular traffic. In fact, they are typical markers of road dust, since the deposition of Pb on the urban soil usually derives from tetraethyl lead (TEL), used as an antiknock additive in gasoline [39], whereas Ba is added in lubricating oils to prevent smoke and diesel engine abrasion and, \begin{table}
\begin{tabular}{c c c} \hline \hline  & **CA** & **NOB** \\ \hline Organic carbon (\% w/w) & 1.66 & 2.05 \\ Organic matter (\% w/w) & 2.86 & 3.54 \\ pH & 7.10 & 7.30 \\ \hline Particle size distribution & & \\ \% sand & 65.1 & 64.8 \\ \% silt & 28.2 & 28.2 \\ \% clay & 6.78 & 6.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: General characteristics of the soils. as barite, is employed in both the brake pads and in the tires of motor vehicles [40].