## 3 Results and Discussion
### PCA and Soil Spectral Characteristics
Somehow, these results are comparable with those of other authors [54; 55], who related variations in the slopes within the above spectral regions to those of organic matter content. All the above visual observations are corroborated by the results of correlation analysis (Table 4). Finally (Figure 4), the convexity of the spectra in the SWIR region (1100-2450 nm) tends to decrease, moving from not or low PCB-contaminated to highly PCB-contaminated soils. The visual inspection of mean reflectance spectra (Figure 5) also allows one to assess some evident differences in the intensity of the absorption features in the NIR region, beyond 1300 nm. These differences, among other in the vis-NIR range, become more marked by looking at the second derivative of mean absorbance spectra (Figure 5). In particular, from these spectra, it appears evident that, as the concentration of PCBs8 (and related variables) increases, the depth of absorption bands at around 488 nm and 540 nm, in the visible, and around 1410 nm, 1910 nm, and 2203 nm, decreases. As is known [56; 57], iron-oxides adsorb strongly in the visible region, due to electronic transitions. In particular, in soils, goethite adsorbs mostly at around 480 nm, while hematite adsorbs typically at \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & **R** & & **R** \\ \hline \multirow{3}{*}{PCBs\({}_{18}\) vs.} & Slope 400–550 nm & \(-\)0.023 & & Albedo VIS & 0.031 \\  & Slope 550–600 nm & \(-\)**0.812** & & Albedo NIR & \(-\)**0.461** \\  & Slope 600–800 nm & \(-\)**0.857** & & Albedo SWIR & \(-\)**0.597** \\  & Slope 800–1100 nm & \(-\)**0.888** & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Correlation coefficients (r) among PCBs8 contents, slopes, and albedo in the different spectral regions. Figure 4: Average reflectance (R) spectra of soils from the three clusters (C1, C2, C3) resulting from the application of PCA and Cluster analysis to the investigated soil properties. around 530 nm. Following [58; 59], the intensity of the iron-oxide absorbances in the vis is considerably obscured by the organic matter's optical interference. Being PCB organic compounds, it is reasonable to assume that their presence significantly affects the spectral response of iron-oxides. In the NIR, in particular at longer wavelengths, beyond 1300 nm, many depth absorption features are due to clay minerals [24], due to overtones and combination modes of the fundamental vibrations of functional groups that occur in the mid-infrared region [60]. Kaolinite, Illite, and Smectite, which are by far the most frequent clay minerals in Mediterranean soils [61], are all spectrally active in the short-wave infrared [60]. Kaolinite shows characteristic absorption bands near 2200 and 1400 nm, due, respectively, to Al-OH bend and OH stretch combinations, and to OH stretch vibration [21]. Smectite shows strong absorption bands near 1400, 1900, and 2200 nm; the first one is partly due to an overtone of structural OH stretching in the octahedral layer of this mineral; the absorption at 1400 nm, as that near 1900 nm, is also due to combination vibrations of water bound in the interlayer lattices as hydrated cations and water adsorbed on particle surfaces [62]. Absorption bands near 1400, 1900, and 2200 nm, weaker than those of smectite, are also found in the re  
Figure 5: Average second derivative of absorbance (A = log 1/R) spectra (**a**) and its magnification in two significant ranges of visible (**b**) and near-infrared (**c**) region of soils from the three clusters. flectance spectra of illite; this clay mineral also shows additional absorptions near 2340 nm and 2445 nm [63], which may diagnostically distinguish between illite and smectite. As for iron-oxides, the depth of all clay features is also strongly reduced by the optical interference of PCBs. On the other hand, the absorption increases with increasing PCB content at around 2340. As is known [24], calcium carbonate adsorbs strongly at this wavelength.