## 3 Results and Discussion  
### PAH Concentrations in the Sediments  
Concentrations of all PAH compounds determined in the sediments of the study area, along with detection limits and recovery factors for two internal standards, are given in Tables S4-S7 of SI. Concentrations of T-PAH and the characteristic groups of PAHs in sediments from 11 stations in the coastal area are shown in Figure 2. T-PAH concentration ranged from 180.71 ug kg\({}^{-1}\) to 2793.18 ug kg\({}^{-1}\). The highest T-PAH concentration was measured in the sediment of the Brac-Split channel near the town of Omis (CS9), while the lowest PAH concentration was measured in the sediment near the island of Pag (CS1). High PAH concentrations in the coastal area are observed at the station near the town of Zadar (CS3). At this station, concentrations of parent-PAH were higher than those of C-PAH, indicating that PAH in this area originates mainly from pyrogenic sources, such as inland and marine traffic, residential heating, etc. C-PAH were the predominant PAH compounds only in the sediment at station CS2, whereas on the rest of stations, unsubstituted PAH were the predominant PAH compounds in the sediments. A high proportion of C-PAH fraction was also observed at stations CS4, CS9, CS10 and CS11. These stations are in the harbor (CS4) and in channel areas with dense marine traffic. Similar results with a dominance of pyrogenic PAH have been reported for sediments from coastal areas in the Mediterranean Sea, the Bay of Mexico and other coastal regions worldwide [2; 26; 27; 28; 29; 30; 31]. The dominance of \begin{table}
\begin{tabular}{c c} \hline
**PAH Compound** & **Toxic Equivalent Factor—TEF** \\ \hline Naphtalene \({}^{*}\) & 0.001 \\ Acenaphtene \({}^{*}\) & 0.001 \\ Acenaphteylene & 0.001 \\ Fluorene & 0.001 \\ Phenanthrene \({}^{*}\) & 0.001 \\ Anthracene & 0.001 \\ Fluoranthene \({}^{*}\) & 0.001 \\ Pyrene & 0.001 \\ Benzo[\(a\)]anthracene \({}^{*}\) & 0.1 \\ Chrysene \({}^{*}\) & 0.001 \\ Benzo[\(b\)]fluoranthene & 0.1 \\ Benzo[\(k\)]fluoranthene \({}^{*}\) & 0.1 \\ Benzo[\(a\)]pyrene \({}^{*}\) & 1 \\ Indeno [1,2,3-cd]pyrene & 0.1 \\ Dibenzo[\(a\),\(h\)]pyrene & 1 \\ Benzo[\(gh\)]perylene \({}^{*}\) & 0.01 \\ \hline \end{tabular}
\end{table} 
Table 1: Toxic equivalent factors of 16 EPA PAHs ( USEPA, 2014). \({}^{*}\) PAH compounds enlisted by EPA as known carcinogenic compounds. C-PAH in the sediment at station CS2 suggests that PAH in this area are mainly derived from unburned petroleum derivatives [10; 32]. A petrogenic source of pollution at this station is also indicated by the relatively high amounts of S-PAH, which may be present in crude petroleum up to 5% [33; 34]. PAH concentrations measured in the sediments of the stations along the mid-Adriatic transect are shown in Figure 3. Concentrations of T-PAH ranged from 42.859 \(\mu\) kg\({}^{-1}\) (TS2) to 963.517 \(\mu\)g kg\({}^{-1}\) (TS6). PAH concentrations tend to decrease as the distance from the coast increases. Contributions of pyrogenic PAH were similar to the contribution of C-PAH at each station, except at TS1, where the concentration of parent-PAH was higher than the concentration of C-PAH fraction. Station TS1 is located near the island of Brac, where forest fires frequently occur in summer, which contributes to high levels of parent-PAH [35]. Similar results have been obtained for open sea regions around the world, where maritime traffic is the most pronounced anthropogenic pressure [2; 27; 29; 36]. Figure 3: Concentration of total PAH (T-PAH), 16 EPA priority PAH (16 EPA PAH), unsubstituted PAH (parent-PAH), methyl-substituted PAH (C-PAH), sulphur-containing-PAH (S-PAH) and oxygen-containing-PAH (O-PAH) at stations along the mid-Adriatic transect. Figure 2: Concentration of total PAH (T-PAH), 16 EPA priority PAH (16 EPA PAH), unsubstituted PAH (parent-PAH), methyl-substituted PAH (C-PAH), sulphur-containing-PAH (S-PAH) and oxygen-containing-PAH (O-PAH) at stations in the coastal area.