# A Snapshot on the Occurrence and Risk Assessment of Organic Pollutants in an Urban River
## 3 Results and Discussion
### PAHs and PCBs Presence in Surface Sediments
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **F1** & \multicolumn{2}{c}{**F2**} & \multicolumn{2}{c}{**F3**} \\  & **MEC** & **HQ** & **MEC** & **HQ** & **MEC** & **HQ** \\  & **ng\(\cdot\)g\({}^{-1}\) dw** & & **ng\(\cdot\)g\({}^{-1}\) dw** & & **ng\(\cdot\)g\({}^{-1}\) dw** & \\ \hline
2–3 rings & & & & & \\ Naphtalene (N) & 0.05 & 0.00 & 0.05 & 0.00 & 0.05 & 0.00 \\ Acenaphthylene (Acy) & 0.05 & 0.00 & 0.05 & 0.00 & 0.05 & 0.00 \\ Acenaphthrene (Ace) & 0.05 & 0.00 & 0.05 & 0.00 & 0.05 & 0.00 \\ Fluorene (F) & 2.63 & 0.03 & 1.07 & 0.014 & 0.08 & 0.00 \\ Phenanthrene (P) & 8.08 & 0.019 & 10.13 & 0.024 & 2.95 & 0.00 \\ Anthracene (Ant) & 4.89 & 0.076 & 7.53 & 0.118 & 2.07 & 0.032 \\ \(\Sigma\) & 15.75 & & 18.88 & & 5.25 & \\ \hline
4 rings & & & & & & \\ Fluoranthene (Flu) & 2.48 & 0.165 & 6.28 & 0.418 & 1.31 & 0.087 \\ Pyrene (Pyr) & 6.97 & 0.134 & 9.25 & 0.178 & 0.35 & 0.007 \\ Benz(a)anthracene (BaA) & 2.81 & 0.281 & 9.40 & 0.940 & 1.31 & 0.131 \\ Chrysene (Chr) & 2.72 & 0.045 & 3.18 & 0.053 & 0.49 & 0.008 \\ \(\Sigma\) & 14.98 & & 28.11 & & 3.46 & \\ \hline
5–6 rings & & & & & & \\ Benzo(b + j)fluoranthene (BbiF) & 2.79 & & 2.70 & - & 0.69 & - \\ Benzo(k)fluoranthene (BkF) & 4.77 & 0.954 & 1.80 & 0.359 & 0.49 & 0.099 \\ Benzo(a) pyrene (BaP) & 1.21 & 0.302 & 0.05 & 0.015 & 0.17 & 0.042 \\ Dibenz (a.h)anthracene (DBA) & 0.05 & 0.003 & 0.05 & 0.004 & 0.05 & 0.003 \\ Benzo(ghi)perylene (BphiPer) & 2.21 & 0.737 & 0.05 & 0.017 & 0.05 & 0.017 \\ Indeno(1.2.3-cd) pyrene (IcdP) & 1.79 & 0.138 & 1.28 & 0.099 & 0.23 & 0.018 \\ \(\Sigma\) & 12.82 & & 5.93 & 1.68 & \\ \hline \hline Total \(\geq\) 4 rings & 27.80 & & 34.04 & & 5.14 & \\ \hline Toxic units & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Measured environmental concentrations (MEC) and Hazard Quotient (HQ) for the analyzed PAHs in sediments in F1, F2, and F3. Compound name abbreviations are in brackets. The observed trends in PAHs' concentrations in the Fervenca might be explained based on a multiplicity of factors: from those intrinsic to chemical properties of PAHs to those concerning riverine ecosystem functioning, biota composition (which, as mentioned in the previous section, can decisively influence their biodegradation rates [27; 92] and environmental disturbance such as coarse riverbed clogging and atmospheric deposition rates [20; 22; 24]. The highest concentrations of PAHs found in F1 and F2 might be related to the dominance of HPAHs which, according to [24], have a higher tendency to accumulate in the sediments, since they are less soluble in water than LPAHs (higher log Kow--Table S7). Since HPAHs are released during the incomplete combustion (pyrolysis) of organic material, the main sources of these compounds in Portugal are, according to [101], forest fires, agricultural wastes and industrial waste incineration, and residential wood combustion (RWC). The mentioned authors estimated that the total emissions of PAHs due to RWC in Portugal is about 9 Tm-y\({}^{-1}\) (Braganca district contribution is about 0.3 Tm-y\({}^{-1}\); Figure 3). Indeed, RWC is higher in Portuguese inland rural areas, such as Braganca, where winters are more severely cold. Figure 3 shows that most PAHs emitted from RWC are present in samples from F1 to F3, indicating that sediment contamination might have partially resulted from local and/or regional atmospheric dust deposition [102]. Concerning LPAHs, (petrogenic origin), it is plausible to hypothesize that their main sources might be the rainwater runoff from urbanized areas and roads (F1 and F3 are located near a regional road) discharged directly into the river or into a sewage network. Indeed, it is known that rainwater runoff is one of the most important sources of organic pollution [102; 103; 104]. Concentrations of PCBs' congeners and relative abundance are presented on Table 5 and Figure 4, respectively.