## 3 Materials and Methods
### Chemical, Physical, and Geotechnical Characterization of Contaminated Soil  
Geotechnical investigations and leaching tests were conducted according to European and Italian standards [59] to obtain information on the physical properties of the soil samples and the levels of contaminants present. Three types of analyses were performed for the chemical-physical characterization of the samples: a geotechnical analysis, a particle size distribution analysis, and leaching tests. A percolation test for soil, along with two batch leaching methods (one with a low liquid/solid (L/S) ratio and one with a high L/S ratio), was used. Geotechnical analyses of the various soil samples revealed that the soil is primarily composed of sand with gravel. Additionally, a slightly condensed thickening was observed, along with moderate humidity and an absence of plasticity. The results of the geotechnical analysis are presented below (see Table 2). The results (Table 2) show significant variability in moisture content between the different samples, with Soil 5 having the highest moisture, followed by Soil 4, both exhibiting relatively high water content. Soil 6, despite having a lower moisture content, shows a \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **Unit** & **SOIL1** & **SOIL2** & **SOIL3** & **SOIL4** & **SOIL5** & **SOIL6** \\ \hline Humidity & \% & 20 & 18 & 13 & 21 & 24 & 14 \\ Wet weight & g & 953.56 & 856.76 & 972.78 & 894.36 & 899.61 & 1273 \\ Dry weight & g & 759.64 & 697.64 & 842.46 & 699.42 & 682.73 & 1092 \\ \hline \end{tabular}
\end{table} 
Table 2: Geotechnical analysis results of soil samples. higher wet weight than the other samples, suggesting a higher density and a more compact soil composition. In contrast, soils with lower moisture content, such as Soil 3, display a more balanced wet-to-dry weight ratio, indicating lower water content and potentially a more permeable structure. These differences suggest varying levels of compaction, porosity, and water retention capacity across the samples. In Figure 5, the particle size distribution of the six analyzed soil samples is presented. The curves of all six samples (Figure 5) are very similar. The intermediate slope of the curves suggests a well-distributed particle size, without a dominant presence of any specific interval. Generally, the soils are coarse-grained, with a predominant sandy component (ranging from 0.1 to 2 mm) and a significant proportion of gravel (greater than 2 mm). Specifically, SOIL2 exhibits a smoother and more continuous curve without interruptions, indicating that the sample is more homogeneous. In contrast, the curves for SOIL4 and SOIL5 show a higher percentage of gravel compared to the other samples. To quantitatively analyze the differences in soil structure, the uniformity and curvature coefficients were calculated. The results are shown in the Table 3 below. Figure 5: Particle size distribution curve: (**a**) SOIL1, (**b**) SOIL2, (**c**) SOIL3, (**d**) SOIL4, (**e**) SOIL5, (**f**) SOIL6. Regarding the uniformity coefficient, all the soils show high values, indicating a very wide distribution. In fact, the soils are well graded, as is evident from the curves. In particular, Soils 1, 2, and 6 exhibit this characteristic. As for the curvature coefficient, there are greater differences between the soils analyzed. Soils 1, 2, and 6 again have high values, exceeding 2, which further confirms the high gradation of these soils. The others, however, have values below 1, indicating finer grain sizes. Overall, all samples display similar curves, suggesting a uniform baseline composition and evenly distributed contamination. However, subtle differences between the curves may indicate localized variations in soil conditions. Table 4 below shows the quantities of contaminants present in the various soil samples analyzed, specifically hexavalent chromium and arsenic, in relation to the acceptable concentration limit values in soil and subsoil, as established by Legislative Decree 152/2006.