## 3 Results and Discussion
### Dissolved Phase  
3.2.1. Metal(loid) Concentration Profiles in Porewaters and Distribution Coefficient (K\({}_{\rm D}\))  
Aluminum, Cd, Cu, Fe, Pb and Zn concentrations in porewaters, at VN3 were higher than those found at VN1 (Figure 3). At site VN1, metal(loid) concentrations varied with depth (excluding Al which remained rather constant) and almost all (As, Cd, Fe, Hg, Mn, Pb and V) showed a maximum in the first level (0-1 cm) and then a decrease with depth, except for As and V, which conversely increased. A similar trend was observed for Hg, Fe, Mn, Cr and Ni. Mercury, Fe and Mn (Figures 3 and 4) showed the maximum concentrations in the first level (0-1 cm) whereas Cr and Ni in the second level (1-2 cm). Similarly, in the porewaters of VN3, the maximum occurred in the first 2 cm. Figure 4: Comparison between vertical profiles of dissolved Fe, Mn and H\({}_{2}\)S [12] in porewaters in the two selected sites, VN1 and VN3. Figure 3: Vertical profiles of metal(loid)s in porewaters at both investigated sites. (*) For a better comparison of the results with those obtained for site VN3, the values of As in VN1 were graphically divided by a factor of 10. According to the early diagenetic sequence [45], the approximate location of Mn reduction should be found above the Fe reduction zone, but this never occurred in any site; the Mn reduction zone is below that of Fe at VN3, whereas they are overlapped at the depth of 0-1 cm at VN1 (Figure 4). However, the marked enrichment of Fe, Mn and H\({}_{2}\)S in the upper levels (0-1 and 1-2 cm) at both sites, where the Fe-Mn oxy-hydroxides and sulphate reduction zones seem to overlap, did suppose the presence of a redox-cline at the top of the sedimentary sequence. Since Fe and Mn concentrations decreased in parallel with Cu, Cr, Ni, Hg, Pb and Zn, whereas the H\({}_{2}\)S concentrations remain high until the last layer (933 umol/L at VN1 and 968 umol/L at VN3), as well as high total sulphur contents in the sedimentary matrix [12], both reactions among H\({}_{2}\)S and metal(loid)s, followed by precipitation, and the formation of insoluble compounds (e.g., sulphides) or complexes with very low solubility [43], could explain these decreasing trends. The vertical profiles of the metal(loid) concentrations were variable in porewaters but relatively constant in sediments (Supplementary Materials Figure S1), and no relationship was identified between the two phases. Factors such as H\({}_{2}\)S concentrations and metal speciation could be important in the partitioning of metal(loid)s between both phases in these strong reductive sediments characterized by low Eh, varying from \(-\)417 to \(-\)439 mV, at VN1, and from \(-\)367 to \(-\)401 mV, at VN3. LogK\({}_{\rm D}\) values, describing the partitioning of metal(loid)s between solid and dissolved phase [41; 46; 47], are rather constant with depth at both sites. Only the logK\({}_{\rm D}\) of Cd, Hg, Pb and Zn slightly vary at VN1 and Al, Cr, Fe and Pb vary at VN3 (Supplementary Materials Table S2). The highest mean values were found for Al (7.55 \(\pm\) 4.15), Fe (6.50 \(\pm\) 0.30), Hg (5.26 \(\pm\) 0.94) and Pb (4.98 \(\pm\) 0.34) at VN1, and Al (6.71 \(\pm\) 0.43), Cd (4.99 \(\pm\) 0.14) and Fe (6.26 \(\pm\) 0.32) at VN3. Since no significant differences were detected between the two sites (Supplementary Materials Table S2), a steady equilibrium between the dissolved and the solid phase was supposed. Comparing our results with previous research, the logK\({}_{\rm D}\) values for Hg (5.26 \(\pm\) 0.94 at VN1 and 4.86 \(\pm\) 0.15 at VN3) are comparable to those found in Taranto's Mar Piccolo (5.4 \(\pm\) 0.2 and 5.3 \(\pm\) 0.1) which is contaminated by Hg and other trace metals of industrial origin [23]. Our values were also found to be similar to those estimated in two marinas (5.30 \(\pm\) 0.88 and 4.95 \(\pm\) 0.88) of the Northern Adriatic [24]. LogK\({}_{\rm D}\) of Cd, Cr, Cu, Fe and Ni were usually found to be the highest; As, Hg, Mn and Pb showed similar values; and Zn was found to be the lowest.