# A Snapshot on the Occurrence and Risk Assessment of Organic Pollutants in an Urban River
## 2 Materials and Methods
### Environmental Risk Assessment  
The Hazard Quotient (HQ) was calculated to evaluate the potential ecological risk. HQ is the ratio between the measured environmental concentration (MEC) and the predicted no-effect concentration (PNEC) [55; 56; 57]. When the measured environmental concentrations were the below LOQ, their values were considered as being LOQ/2 [58]. PNEC value is a ratio between a benchmark concentration selected as the lowest toxicity value from the available experimental data for aquatic organisms from different trophic levels (e.g., algae, crustacean, aquatic insect larvae, fish) and an assessment factor (AF) that varies between 1000 (if there is only acute toxicity data for at least one of the evaluated trophic levels) and 10 (when there is chronic toxicity data for all trophic levels). Therefore, an exhaustive bibliographic search was performed in order to compile a list of published PNEC, derived preferentially from chronic toxicity values, for the compounds evaluated in the present research (Table S3). Reference [59] recommends using PNEC values derived from chronic toxicity, because these compounds generally induce most likely chronic rather than acute toxic effects. Moreover, the lowest value for PNEC for each compound was chosen to maximize the protective threshold. If HQ < 0.1, the risk is negligible; if the 0.1 < HQ < 1, the risk is low; however, a probable adverse effect should be considered; when 1 < HQ < 10, the risk is moderate and, finally, when HQ > 10, the risk is high. Toxic Units (TUs) for each group of compounds were determined by adding the respective HQs [13; 60]. Because of the scarcity or lack of toxicity data for PAHs in sediments, PNEC values for sediments were derived from PNEC values for water using the equilibrium partitioning method expressed as the simplified formula suggested in [61]. The PNEC values used in this approach are presented in Table S4. Complementarily, the classification of PAHs' contamination, proposed by [62], was used. \(\Sigma\)PAHs were categorized into four groups: low (0-100 ng\(\cdot\)g\({}^{-1}\)), moderate (100-1000 ng\(\cdot\)g\({}^{-1}\)), high (1000-5000 ng\(\cdot\)g\({}^{-1}\)), and very high (>5000 ng\(\cdot\)g\({}^{-1}\)). Depending on PAHs composition, it is possible to identify different sources of these compounds by carrying out the diagnostic determination ratios--the ratios of defined pairs of individual compounds [20; 22; 30; 63; 64; 65; 66]. In the present study, the following ratios were determined: F/(F + Pyr), Ant/(Ant + P), BaA/(BaA + Chr), IcdP/(IcdP + BghiPer) and \(\Sigma\)LPAHs (low molecular weight: 2 and 3 ring)/\(\Sigma\)HPAHs (high molecular weight: 4 to 6 ring). To overcome the shortage related to the identified discrepancy among the values of diagnostic ratios [66; 67], the use of the total index calculated was suggested as:  
\[\text{Total Index = An/An + P/0.1 + F/F + Pyr/0.4 + BaA/BaA + Chr/0.2 + IcdP/IcdP + BghiPerP/0.5}\]  
Similarly to PAHs, PCBs' PNECs determination for sediment were derived from PNEC for water (Table S5). However, the available PNEC are considered to be merely indicative values [68] and were only available for PCBs indicators. These PCBs have no or only one chlorine atom at the ortho-position. Mixtures of the non-dioxin-like PCBs are generally assessed on the basis of a chemical analysis of the sum of the seven so-called 'indicator PCBs'; these were selected as suitable representatives for all PCBs because they are predominantly present in biotic and abiotic matrices [69]. So far, there is still no uniform standard available to assess the biological effects of PCBs [26; 70]. Therefore, and also due to the lack of PNEC water values for most of the dioxin-like PCBs, their potential toxicity was also estimated by using the values of the Toxic Equivalence Factors (TEFs). The TEF value for each PCBs was multiplied by its environmental concentration in order to calculate Toxic Equivalents (TEQs) (Table S5). The sum of all TEQs allowed the determination of the total PCB TEQ [71].