## 3 Results and Discussion
### Potential Biological Risk  
The potential biological risk of the Fengshan River sediments for benthic organisms was assessed by the potential ecological index (RI), mean ERM quotient (mERM\({}_{\text{Q}}\)), the sum of the toxic units (\(\Sigma\)TU), and contamination severity index (CSI). As shown in Figure 4a, the sediments in Fengshan River exhibited RI values ranging between 199 and 1233 with an average of 542, showing a considerable ecological risk according to the classification reported by [PERSON] [22]. In particular, sites F4, F5, and F6 showed \"very high ecological risk\" with an average RI value up to 670. The mERM\({}_{\text{Q}}\) values varied from 0.24 to 1.14 with an average of 0.55, indicating that the contamination level of heavy metals in Fengshan River sediments may be considerably toxic (49% probability of being toxic) to benthic organisms (Figure 4b). A relatively high mERM\({}_{\text{Q}}\) value was found at site F6  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Station** & **As** & **Hg** & **Cr** & **Cu** & **Ni** & **Pb** & **Zn** \\ \hline Fengshan River, Taiwan & 11–44 & 0.18–1.44 & 16–353 & 17–175 & 17–64 & 12–73 & 119–533 \\ Keeling River, Taiwan [35] & na & na & na & 3.5–120 & na & 9.3–200 & 41–390 \\ Changjiang River, China [36] & 9.0–34 & 0.01–0.09 & na & 14–37 & 26–34 & 17–42 & 72–131 \\ Yellow River, China [37] & 14–48 & na & 41–128 & 30–102 & na & 26–78 & 90–202 \\ Xijiang River, China [38] & 4.9–60 & 0.01–0.48 & 6–119 & 6–120 & na & 11–155 & 28–479 \\ River Ganges, India [39] & na & na & 1.8–6.4 & 1.0–4.4 & na & 4.3–8.4 & 10–20 \\ Korotoa River, Bangladesh [40] & 2.6–52 & na & 55–183 & 35–118 & 37–163 & 36–83 & na \\ River Soan, Pakistan [41] & na & na & 2.9–19 & 4.8–59 & 19–35 & 7.5–78 & 7.3–189 \\ Nile River Branches, Egypt [42] & na & na & 12–68 & 1.0–3.7 & 15–30 & 30–80 \\ Bormida River, Italy [43] & 9.0–59 & 0.09–0.96 & 63–392 & 16–77 & 28–231 & 13–77 & 48–228 \\ Brisbane River, Australia [44] & 8.9–13 & 1–2 & 82–332 & 20–110 & 20–34 & 25–126 & 142–257 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison among the heavy metal concentrations (mg/kg) of sediments in Fengshan River and those from other regions.  
(mERM\({}_{\rm Q}\) = 1.14 in the wet season and mERM\({}_{\rm Q}\) = 0.97 in the dry season), followed by site F7 (mERM\({}_{\rm Q}\) = 0.82 in the wet season and mERM\({}_{\rm Q}\) = 0.67 in the dry season). It is indicated that the sediments in the lower reaches and the river month were highly toxic (76% probability of being toxic), which may be harmful to the growth of local benthic organisms. Similarly, the \(\Sigma\)TU values ranged between 3.2 and 12.6, with an average of 6.7. The highest \(\Sigma\)TU value was observed at site F6 (\(\Sigma\)TU = 12.6 in the wet season and \(\Sigma\)TU = 10.5 in the dry season), indicating a high risk of potential acute toxicity effects of the river sediments on the benthic community, especially for those in the lower reaches (Figure 4c).  
In addition, the CSI value was calculated by the eigenvalues of 6.387 and loading values of 0.776, 0.910, 0.881, 0.915, 0.819, 0.175, and 0.711 for Cr, Cu, Ni, Pb, Zn, As, and Hg, respectively. The eigenvalues and loading values adopted the VF1 obtained from PCA/FA, which can be interpreted as anthropogenic sources. As shown in Figure 4d, the CSI values ranged between 0.86 and 3.88, with a mean of 1.8. A similar trend to the mean ERM quotient was observed, which showed the highest value at site F6 (CSI = 3.9 in the wet season and CSI = 3.2 in the dry season), followed by site F7 (CSI = 2.7 in the wet season and CSI = 2.2 in the dry season). This indicated moderate to high severity of heavy metal contamination in the lower reaches and the river mouth.