## 3 Results and Discussion
### Pollution Assessment of HMs in Sediments  
Pollution assessments for HMs in sediments have been conducted widely based on several indices, such as the Nemerow pollution index, \(E^{i}\), \(RI\), and \(I_{geo}\)[22, 41, 42]. The Nemerow index was used to evaluate the pollution of HMs in the sediments in the dumping area over different years (Figure 4). In 2012, 2013, and 2016, the Nemerow index exceeded 0.7 but was less than 1. This finding indicated that the sediments in the dumping area had reached light pollution levels in 2012, 2013, and 2016. Sediments in other years were at a clean level. The pollution level in 2016 was slightly higher, which might be related to larger quantity of dumping materials in this year. According to the pollution distribution, the polluted areas were mainly in the northwest (S1 and S2) and central areas (S3) of the dumping area. Additionally, the change in water depth showed that these stations were areas with large dumping quantities, indicating that dumping might have a negative impact on sediments. The \(I_{geo}\) values of As, Cu, Zn, and Pb ranged from 0 to 1 from 2011 to 2016 (Figure 5), indicating that the pollution levels of these four HMs were light pollution. Notably, in 2017, the \(I_{geo}\) values of As, Cu, Zn, and Pb were lower than 0, implying that the pollution levels of these four metals were not polluted in 2017, which may be related to the least dumping amount in this year. In most years, the \(I_{geo}\) values of Hg and Cd were between 1.0 and 2.0, indicating that Hg and Cd had reached a mild contamination level. From 2011 to 2016, the pollution levels of Hg and Cd showed an increasing trend and were higher than those of the other four HMs. Even in 2017, when the dumping amount was the lowest, the pollution levels of Hg and Cd were rated as light pollution, and the pollution from the other four HMs was not observed. Therefore, special attention should be paid to Hg and Cd before dumping dredged materials. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & **TOC** & **Hg** & **As** & **Cu** & **Pb** & **Cd** & **Zn** & **Oils** & **Sulfides** \\ \hline TOC & 1 & & & & & & & & & \\ Hg & 0.139 & 1 & & & & & & & & \\ As & 0.493 & 0.657  & 0.245 & 0.484 & 1 & & & & & \\ Pb & 0.438 & 0.06 & 0.103 & 0.44 & 1 & & & & & \\ Cd & 0.017 & 0.419 & 0.723  & 0.269 & 0.549  & 0.515 [FOOTNOTE:]Footnote : Correlation is significant at the 0.054 [ENDFOOTNOTE] & 0.254 & 1 & & & & \\ Oils & 0.363 & \(-\)0.016 & 0.16 & 0.164 & 0.45 & 0.243 & 0.451 & 1 & \\ Sulfides & 0.213 & 0.017 & \(-\)0.249 & 0.052 & \(-\)0.002 & \({}^{-0.556}\) & \(-\)0.164 & 0.012 & 1 \\ \hline \end{tabular}
\end{table} 
Table 3: Correlation analysis for HMs, TOC, Oils, and sulfides in sediments. Figure 6 shows the integrated potential ecological risk index (\(RI\)) and potential ecological risk index (\(E_{r}^{i}\)) of an individual HM. The \(RI\) values of HMs in the sediments of the  
Figure 4: The Nemerow pollution index of HMs in different years. Figure 5: The \(I_{\rm geo}\) values of HMs in different years. dumping area were higher than 150 in 2011, 2013, 2014, and 2017, suggesting that HMs had a middle ecological risk. The \(RI\) values in 2012 and 2016 were 310 and 388, respectively, indicating that the ecological risk of sediments was relatively high. The \(RI\) values of the dumping area were greater than 150 every year, implying that the potential ecological risk of the dumping area must be paid more attention. For individual metals, Hg and Cd posed more severe ecological risk than the other HMs, with \(E_{r}^{i}\) values higher than 40 in each year from 2011 to 2017, especially in 2016, a high risk level (\(E_{r}^{i}\geq 160\)). The \(E_{r}^{i}\) values of other HMs were less than 40, indicating that the potential ecological risk of other HMs was low. In general, Hg and Cd were the main factors causing the potential ecological risks of sediments in the study area. At present, extensive ecological risk assessments have been conducted on HMs in sediments based on \(RI\)[31; 43].