## 2 Materials and Methods
### Treatment and Analysis of Vegetable and Soil Samples  
Soil organic carbon (SOC), organic matter (SOM) and pH were measured in the soil of both sites according to the Italian official soil chemical analysis methods described by the Ministerial Decree 13/9/1999 [17]: pH was determined by potentiometric analysis on soil-CaCl\({}_{2}\) suspension, SOC and SOM were measured with the Walkley-Black method [18].  
The aerial part and roots were freeze-dried and ground to obtain a sample as homogeneous as possible. Before proceeding with the analysis, the samples of soil and vegetables were pre-treated mineralizing them with a Milestone MLS-1200 Mega (Milestone, Sorisole, Italy) microwave.  
The digestion procedure for soil samples was performed by adding 5 mL of aqua regia to 0.05 g of soil inside tetrafluoromethoxyl (TFM) vessels, whereas for vegetables samples 0.5 g of epigeal or hypogeal parts were weighed and added with 6 mL of nitric acid and 2 mL of hydrogen peroxide. This mixture is strongly oxidizing and develops a lot of foam; for this reason, the vessels were let to rest under the hood for about thirty minutes before starting the digestion. Three replicates were made for each soil and vegetable sample. Soil and plant extracts, obtained from the mineralization step, were filtered on a Whatman 40 paper filter and then diluted with Highly Purified Water (HPW) to a final volume of 50 mL.  
Twenty elements (Al, As, Ba, Ca, Cd, Ce, Co, Cr, Cu, Fe, K, La, Mg, Mn, Na, Ni, P, Pb, Sr, Ti, V, Zn) in plant samples and twenty-two (with the addition of La and Ce) in soil samples were determined by inductively coupled plasma-optical emission spectrometer (ICP-OES) or, when below the detection limit (LOD) of this instrument, the chemical analyses were conducted by a high-resolution inductively coupled plasma-mass spectrometer (HR-ICP-MS) or a graphite furnace atomic absorption spectrometer (GF-AAS), according to the different matrixes of soil and vegetables samples. In particular, As and Cd were determined by GF-AAS in all soil samples since they presented concentrations below the LOD of ICP-OES. As regards the plant samples, Cu, Ti and Al were below the LOD of ICP-OES and were determined by HR-ICP-MS in the 25% of samples, Cr, Ni and V in the 50% and Pb and Co in the 75% of samples. The other elements were quantified using the ICP-OES. Three instrumental replicates were performed for the determination of each element. The quality of the analytical procedure was verified using two NIST standard reference materials: tomato leaves 1573a for plant samples and San Joaquin soil 2709 for soil samples.  
Models and technical specification of instruments are reported in Table 1.  
Calibration curves were performed using the external standard calibration method for the quantification of all the elements. Standard solutions were prepared by diluting single element concentrated (1000 mg L\({}^{-1}\)) stock solutions (Sigma-Aldrich TraceCERT), with aliquots of sample blanks prepared with the same acid mixtures of samples (matrix matching method) [19].  
The bioconcentration factor for each vegetable was calculated to assess the yield of metals transferred from the soil to the plant as follows [20; 21]:  
BF = metal concentration in aerial part and roots (mg kg\({}^{-1}\) dry weight)/metal concentration in soil (mg kg\({}^{-1}\) dry weight).  
The translocation factor allows evaluating to what extent elements were transferred from the root system to the aerial part of the plant and was calculated for each vegetable through the relationship [20; 21]:  
TF = metal concentration in aerial part (mg kg\({}^{-1}\) dry weight)/metal concentration in roots (mg kg\({}^{-1}\) dry weight).