## 3 Results
### Total Hg and Speciation in Sediment
\begin{table}
\begin{tabular}{c c c} \hline
**Fractions** & **Hg Classification** & **Primary Compounds Extracted** \\ \hline F1 & Water-soluble, i.e., salts & HgCl\({}_{2}\) \\ F2 & Weak acid-soluble/”stomach acid”soluble & HgSO\({}_{4}\), HgO \\ F3 & Organo-complexed & Hg-humics, Hg\({}_{2}\)Cl\({}_{2}\), MeHg \\ F4 & Strongly-complexed & mineral lattice bound, Hg\({}_{2}\)Cl\({}_{2}\), Hg0 \\ F5 & Mineral-bound & HgS, m-HgS, HgSe, HgAu \\ \hline \end{tabular}
\end{table} 
Table 3: Primary compounds are released with Bloom’s selective sequential extraction [49]. The percentages obtained for fraction F3 were quite higher (11.4 \(\pm\) 7.0%) and showed a high degree of variability (from 3.1 to 25.0% at TME4 and TPO2, respectively). In detail, TEU1, TEU2, and TPO2, which are sites belonging to the easternmost sector of the lagoon, and TME1, close to the Aussa-Corno and Zellina River mouths, were characterised by percentages up to 20%, probably as a consequence of their enrichment in an organic matter [22]. The F3 fraction includes the organo-chelated forms of Hg (i.e., humic, fulvic, and amino acids, living and dead biota) that have moderate mobility. To a lesser extent, this fraction can be correlated to the presence of MeHg [48]. However, as previously stated by [PERSON] et al. [22], this toxic form in the MGL accounted, on average, for 0.08% of the total Hg pool. This suggests that in the sediments of the MGL, Hg with scarce mobility predominates. F4 includes strongly complexed Hg forms in industrial products such as calomelan (Hg\({}_{2}\)Cl\({}_{2}\)). This was also suggested by [PERSON] et al. [48] as an estimate of the \begin{table}
\begin{tabular}{l c c c c c c} \hline
**Site** & **Hg (mg kg\({}^{-1}\))** & **F1 (\%)** & **F2 (\%)** & **F3 (\%)** & **F4 (\%)** & **F5 (\%)** & **References** \\ \hline Aussa River & 0.82–5.69 & 0.66–5.60 & 0.96–6.05 & 5.59–24.81 & 33.11–90.57 & 0–57.95 & [31] \\ Grado Lagoon & 10.75–13.37 & 0.16–0.20 & 0.01–0.02 & 2.72–4.28 & 43.58–53.31 & 42.24–53.49 & [70] \\ Pialassa Baiona & 14.40–19.10 & 0.20–0.38 & 0–0.02 & 4.77–6.32 & 87.00–87.81 & 5.67–7.83 & [70] \\ Gulf of Trieste & 6.36–13.50 & 0.08–0.18 & 0.04–0.18 & 0.71–1.47 & 32.6–59.7 & 39.3–56.7 & [95] \\ Taranto harbour & 6.61 & 0.30 & 0.00 & 4.20 & 93.10 & 2.30 & [96] \\ Isonzo River mouth & 13.27 & 0.08 & 0.03 & 0.77 & 23.45 & 75.68 & [97] \\ Marano Grado Lagoon & 1.30–7.38 & 0.15–0.53 & 0.01–0.13 & 3.1–25.0 & 15.3–67.6 & 27.5–72.4 & This study \\ \hline \end{tabular}
\end{table} 
Table 4: Speciation of Hg in the sediment of some coastal sites obtained by Bloom speciation. Modified from [PERSON] et al. [95]. Figure 3: Partitioning of Hg is expressed as a percentage in (**a**) sampled sites according to [PERSON]’s procedure and (**b**) box plots representing the average percentage (\(\pm\)s.d.) for each fraction extracted. presence of free elemental Hg, which commonly originates from industrial plant discharges (i.e., CAP) [31; 70; 98; 99]. However, in this work, it is more reasonable to assume that the high percentage found (31.2 \(\pm\) 15.0%) is representative of Hg bound up in amorphous organo-sulphur or crystalline Fe/Mn oxide phases, which are strongly bound species. The F5 (57.1 \(\pm\) 12.3%), was the most abundant fraction, representing the more insoluble species (Kps\(\sim\)10\({}^{-53}\)) [100] such as cinnabar (\(\alpha\)-HgS) and meta-cinnabar (\(\beta\)-HgS). This is consistent with the hypothesis that the main input of Hg in the form of particulate matter can be attributed to the Isonzo River discharge influenced by the contamination of both soils and sediments in the plain of the former mining area of Idrija [22; 101]. As aforementioned, the percentages of the fractions extracted differed significantly (K-W: \(p_{\rm(same)}\) = 4.99 e\({}^{-15}\)). However, a significant correlation was found for F1/F2 (r = 0.05329), F3/F4, and F4/F5 (r = 0.0667 and 0.08975, respectively).