## 3 Results  
### Total Hg and Speciation in Sediment  
The results obtained in surface sediments are summarised in Table 1.  
Total Hg was 3.94 \(\pm\) 2.01 mg kg\({}^{-1}\) (mean \(\pm\) s.d.) and displayed a high degree of variability ranging from 1.30 to 7.35 mg kg\({}^{-1}\) (at TPO5 and FM1, respectively). The spatial distribution showed a clear positive gradient moving eastward (from Marano to Grado basin) and also a significant enrichment in the central sector (TEU4 = 6.66 mg kg\({}^{-1}\)) (Figure 2). These results confirm the evidence reported by [PERSON] et al. [26] (mean = 4.62 mg kg\({}^{-1}\)), where a comprehensive historical dataset (n = 178) was mapped, and, overall, the results of previous studies conducted at different sites of the same lagoon [26; 30; 36; 39; 69; 70; 71]. It should be noted that these results were two orders of magnitude higher than the Hg natural background (0.13 \(\pm\) 0.04 mg kg\({}^{-1}\)) calculated by investigation in deep cores (n = 13; more than 1 m of depth) collected throughout the basin [35]. The threshold set for the good quality of sediment _sensu_ WFD, which is 0.3 mg kg\({}^{-1}\) (deviation permitted = +20%), was exceeded for all the sampling sites. This was also true for the derived effect range low and median (ERL and ERM; 0.15 and 0.71 mg kg\({}^{-1}\), respectively), previously calculated by [PERSON] et al. [50], which have been shown to successfully predict the occurrence of potential toxicity [51]. The calculated geoaccumulation index (I\({}_{\rm geo}\) = log\({}_{2}\) C\({}_{n}\)/1.5 \(\times\) B\({}_{n}\), with C\({}_{n}\) as the concentration of the element considered, B\({}_{n}\) as \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Water** & **Total Hg (mg** & **F1 (\%)** & **F2 (\%)** & **F3 (\%)** & **F4 (\%)** & **F5 (\%)** & **Igeo** & **Sediment** & **Class** \\
**Body** & **kg\({}^{-1}\))** & & & & & & & & **Quality** & **Class** \\ \hline FM1 & 7.38 & 0.39 & 0.04 & 6.85 & 21.65 & 71.08 & 14.8 & EC & 6 \\ FM2 & 5.67 & 0.26 & 0.02 & 4.71 & 40.23 & 54.78 & 12.8 & EC & 6 \\ FM3 & 6.20 & n.d. & n.d. & n.d. & n.d. & n.d. & 13.5 & EC & 6 \\ FM4 & 5.09 & 0.18 & 0.03 & 7.28 & 55.21 & 37.30 & 12.0 & EC & 6 \\ TEU1 & 3.96 & 0.17 & 0.02 & 20.52 & 15.30 & 63.99 & 10.2 & EC & 6 \\ TEU2 & 4.42 & 0.22 & 0.08 & 22.28 & 23.57 & 53.86 & 11.0 & EC & 6 \\ TEU3 & 5.55 & 0.14 & 0.03 & 8.75 & 30.28 & 60.80 & 12.7 & EC & 6 \\ TEU4 & 6.66 & 0.12 & 0.01 & 14.76 & 15.03 & 70.07 & 14.0 & EC & 6 \\ TME1 & 2.77 & 0.30 & 0.05 & 20.21 & 18.07 & 61.36 & 7.5 & EC & 6 \\ TME2 & 1.73 & 0.36 & 0.04 & 10.61 & 33.69 & 55.30 & 4.1 & H to EC & 5 \\ TME3 & 1.91 & 0.21 & 0.04 & 10.87 & 29.81 & 59.06 & 4.8 & H to EC & 5 \\ TME4 & 1.52 & 0.35 & 0.04 & 3.10 & 24.09 & 72.41 & 3.1 & HC & 6 \\ TPO1 & 5.21 & 0.15 & 0.05 & 4.72 & 67.24 & 27.46 & 12.2 & EC & 6 \\ TPO2 & 3.75 & 0.15 & 0.02 & 25.02 & 18.60 & 56.22 & 9.8 & EC & 6 \\ TPO3 & 2.18 & 0.45 & 0.13 & 7.30 & 46.76 & 45.35 & 5.8 & EC & 6 \\ TPO4 & 1.66 & 0.53 & 0.04 & 6.31 & 23.94 & 69.18 & 3.7 & HC & 4 \\ TPO5 & 1.30 & 0.52 & 0.07 & 8.64 & 35.64 & 55.13 & 1.9 & MC & 2 \\ \hline \hline \end{tabular} Note: EC = extremely contaminated; H to EC = heavily contaminated; MC = moderately contaminated.  
\end{table} 
Table 1: Total Hg concentration, chemical fractionation, and calculated I\({}_{\rm geo}\)[49] in surface sediments of the Marano and Grado Lagoon (n.d. = not determined). natural background, and 1.5 as a factor to account for variations due to lithogenic effects), introduced and scaled by [PERSON] [49]. This revealed a high degree of sediment contamination (I\({}_{\rm geo}\) = 1.9-14.8; on average, 9.1 \(\pm\) 4.3), with 76% of sites belonging to class six (extremely contaminated), indicating at least a 100-fold enrichment factor above background values (I\({}_{\rm geo}\) > 6) (Table 1). Despite the natural sources present in the Mediterranean area, mining and anthropogenic activities are responsible for numerous Hg hotspots in coastal sediments, and the MGL with 271 Mg of buried Hg is one of the most polluted [72].