## 3 Results and Discussion
### Solid Phase
#### 3.1.2. Labile Fraction of Metal(loid)s in Sediments  
The weak extraction did not depict a noticeable difference between sites and depth levels (Table 2), excluding Mn at both sites (35.3% and 28.34% of the total content at VN1 and VN3, respectively), as well as Cr and Ni, in the level 3.5-5 cm at VN3 (19.9% and 14.4% of the total content, respectively) (Figure 2). In general, the labile fraction of metal(loid)s decreased in the following order: Mn > Zn > Cd > Ni > As, Fe, V and Pb (in some levels). Chromium, Cu and Hg were not shown to be associated with the labile fraction (0% of the total content), whereas As, Fe, V and in some levels Pb showed very low mobility (0.04-1.59%), Ni showed a scarce mobility (0.34-4.27%), excluding the level 3.5-5 cm at VN3 (14.4%), and Zn and Cd showed a limited mobility (0.34-13.6% and 4.45-9.96% for Zn and Cd, respectively). Only the labile fraction of Mn was found to be the highest (25.4-37.9% of the total content), especially at VN1. Mineralogy and grain-size, chemical speciation, pH, Eh and organic matter are important factors influencing the accumulation and availability of metal(loid)s in sediments [40] and they affect their remobilisation and partitioning between solid and dissolved phases. It is known that the reduction of Fe and Mn oxy-hydroxides can favor the release of some metal(loid)s previously associated with sediment particles to porewaters [41]. In anoxic conditions, metal(loid)s can also form insoluble sulphides and precipitate in the sediment [42]. In our study, the negative Eh values in sediments and the high H\({}_{2}\)S concentrations in porewaters (up to nearly 1200 \(\upmu\)mol/L, [12]) could have contributed to the formation of stable complexes with sulphides [43], since the mobility of the majority of metal(loid)s was found to be scarce or null. The notable Cd, Ni and Zn partitioning in the labile fraction could be attributed to their bonding onto Mn hydrate complex, as indicated by the high percentage of Mn labile fraction (Table 2). Its reduction might have affected the metal remobilisation and redistribution [41]. A value of unity denotes no enrichment or depletion relative to the local background. The low percentages of the labile fraction, considered as an operational parameter subjected to different non-selective extraction procedures and the presence of various phases [44], seem to limit the role of sediments as a secondary contamination source. In perturbed sediments, the changes of the redox conditions might partially favor the release of metal(loid)s to the porewaters and the water column. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Site**} & **Level** & **Al** & **As** & **Cd** & **Cr** & **Cu** & **Fe** & **Hg** & **Mn** & **Ni** & **Pb** & **V** & **Zn** \\ \cline{2-13}  & **(cm)** & & & & & & & & **Labile Fraction (\%)** & & & & \\ \hline \multirow{8}{*}{**VN1**} & 0–1 & 0.09 & 1.59 & 9.96 & u. f. & u. f. & 0.01 & u. f. & 35.2 & 2.95 & 0.18 & 0.05 & 9.30 \\  & 1–2 & 0.09 & 0.89 & 8.86 & u. f. & u. f. & 0.00 & u. f. & 34.1 & 3.69 & u. f. & 0.03 & 12.2 \\  & \(<\) & LOD & 0.85 & 9.30 & u. f. & u. f. & 0.04 & u. f. & 36.1 & 3.71 & 0.04 & 0.03 & 11.9 \\  & 3.5–5 & 0.02 & 1.33 & 8.63 & u. f. & u. f. & 0.11 & u. f. & 33.2 & 4.27 & 0.12 & 0.03 & 10.6 \\  & 5–7 & 0.03 & 1.42 & 6.77 & u. f. & u. f. & 0.01 & u. f. & 37.9 & 2.32 & u. f. & 0.19 & 4.55 \\ \hline \multirow{8}{*}{**VN3**} & 0–1 & 0.08 & 1.30 & 7.91 & u. f. & u. f. & 0.37 & u. f. & 28.2 & 3.86 & 0.28 & 0.03 & 13.6 \\  & 1–2 & 0.06 & 0.63 & 7.63 & u. f. & u. f. & 0.11 & u. f. & 28.4 & 3.17 & 0.06 & 0.02 & 12.2 \\ \cline{1-1}  & 2-3.5 & 0.10 & 0.51 & 4.45 & u. f. & u. f. & 0.00 & u. f. & 25.4 & 0.34 & u. f. & 0.22 & 0.34 \\ \cline{1-1}  & 3.5–5 & 0.08 & 0.85 & 4.62 & 19.9 & u. f. & 0.08 & u. f. & 29.6 & 14.4 & 0.04 & 0.88 & 1.44 \\ \cline{1-1}  & 5–7 & 0.11 & 0.80 & 4.62 & 0.07 & u. f. & 0.00 & u. f. & 30.1 & 0.43 & u. f. & 1.23 & 0.37 \\ \hline \hline \end{tabular}
\end{table}