## 3 Results and Discussion
### Importance of Wavelengths  
Figure 7 shows the bar plots of regression coefficients, b, versus wavelength derived after PLSR analysis for the best predicted variables (PCBs18, EOX, Penta-CB, and Hepta-CB), with indication of the wavelengths identified as important for the prediction of these variables; i.e., the wavelengths that, for each calibrated model, have both the b-coefficients beyond or below the fixed threshold of \(\pm\) st.dev. and the VIP \(>\) 1. Figure 6: Plots of the measured against the predicted PCBs (**a**), EOX (**b**), Tetra-CB (**c**), Penta-CB (**d**), Hexa-CB (**e**), and Hepta-CB (**f**) contents based on PLSR models. The 1:1 line is indicated on each figure. * = log-transformed values. As can be seen from Figure 7a, the PLSR models for the prediction of PCBs818 has important wavelengths in the visible region of the spectrum, around 470, 540, 620, and 730 nm. The positive coefficients around 470 and 530 nm could be linked to the blue-green colour reflection, while negative coefficients around 620 and 730 nm could be linked to yellow-red colour reflection. [PERSON] et al. [26] recently showed that as the PCBs818 content increases, the reflection on the blue-green spectral domain (540-565 nm) also increases, while that in the yellow-red (565-780 nm) decreases. As previously discussed (Figure 5), the high blue-green reflection associated with PCBs818 is responsible for the reduced absorption of iron oxides in this spectral region. In the NIR, important wavelengths positively correlated with PCBs818 concentrated around 1710, 1920, and 2390 nm. The attribution of these absorptions to PCBs is difficult, considering the lack of specific studies in the literature concerning the application of vis-NIR spectroscopy to the characterisation of PCBs, to refer to. On the other hand, studies have been carried out to identify spectral features occurring in the mid-IR due to fundamental vibrations in the PCB molecules, whose overtones and combination bands are expected to produce weaker absorptions in the NIR, as for other soil constituents [64]. For instance, [65], working on PCBs, attributed the absorptions band around 1590-1600 cm\({}^{-1}\) to  
Figure 7: PLSR coefficient, b, spectra over the vis-NIR wavelength range associated with the PLSR cross-validation models for PCBs8 (**a**), EOX (**b**), Penta-CB (**c**), and Hepta-CB (**d**). Red diamonds indicate the important wavelengths resulting from the combined use of b and the variable importance for projection (VIP). The wavelengths corresponding to the highest absolute values of b for important group wavelengths are indicated. The thresholds used for the PLSR coefficient, b (i.e., b \(\pm\) st.dev. ), are also shown. benzene bending vibration mode, that around 1280 cm\({}^{-1}\) to C=C bridge bending vibration mode, the band around 1030 cm\({}^{-1}\) to the C-H bending in-plane mode, the band around 1000 cm\({}^{-1}\) to the trigonal breathing vibration mode, and those around 1240-1250 cm\({}^{-1}\) and 1140-1200 cm\({}^{-1}\) to vibrations induced by the Cl substituents. With reference to Figure 7a, we could hypothesize the attribution of the absorptions around 1700 and 2390 nm to C-H stretch, and that around 1920 to the C=O stretch. However, this is only a hypothesis and needs to be confirmed through specific and rigorous investigations. It should be observed that clay minerals, in particular, smectite, adsorb near 1910, due to combination vibrations of water bound in the interlayer lattices as hydrated cations and water adsorbed on particle surfaces [66]. As for iron-oxides, the absorption by smectite is also reduced by the optical interference of PCBs (Figure 5). The negative value of the b-coefficient at the wavelengths around 2350 nm indicated that these wavelengths have a negative link to PCBs18. Therefore, the absorption of other components, spectrally active at these wavelengths, in particular, calcium carbonate, is not reduced.