## 3 Results and Discussion
### Sediment Pollution Status  
The sediment heavy metal contamination levels in this study were assessed using the contamination factor (CF), geo-accumulation index (I\({}_{\text{geo}}\)), modified degree of contamination (mCd), and pollution load index (PLI). The CF and I\({}_{\text{geo}}\) are often applied to differentiate  
Figure 2: Spatial and seasonal variations in heavy metals (As, Hg, Cr, Cu, Ni, Pb, Zn) and organic matter contents in the sediments of Fengshan River. The horizontal dot lines and dash lines represent the ERL and ERM values, respectively. the metal source between natural terrestrial and anthropogenic pollution in sediments. Both the mCd and the PLI were comparative indices for assessing overall heavy metals contamination level in a simple way. As shown in Figure 3a, As, Hg, Zn, and Cu have CF values greater than 6, which denotes \"very high contamination\". The CF value for Hg shows the highest among all metals and reveals the highest number of sites with CF > 6 among all studied metals. The second-highest occurrence of \"very high contamination\" is As, which appeared at the F2 and F4 both in the dry and wet seasons. Both Zn and Cu have CF > 6 at site F6, which appeared in the wet season for Zn and in each of the wet and dry seasons for Cu. The CF value of \"considerable contamination\" mainly came from As, Hg, Zn, and Cu. Other than the sites showing \"very high contamination\", the CF value for Hg at other sites showed \"considered contamination\". The CF value of \"considered contamination\" for As were shown at sites F1, F3, and F5 both in the dry and wet seasons, while those CF values for Zn were shown at sites F3-F5 and F7 in the dry season and sites F4-F5 in the wet season, and those for Cu were shown at site F7 in the dry season and site F4 and F7 in the wet season. In addition, the CF value with \"considered contamination\" for Cr also had one site (F6) in the dry season and two sites (F6 and F7) in the wet season, and those for Pb only appeared at site F7 in the wet season. In contrast, none of the sites showed \"considered contamination\" in Ni. Overall, the sediment contamination degree of heavy metals is ranked in the following order of Hg > As > Zn > Cu > Pb > Cr > Ni. The geo-accumulation index (I\({}_{\rm geo}\)) results are shown in Figure 3b. The I\({}_{\rm geo}\) classes of the Fengshan River sediments were ranked from \"moderately polluted\" to \"strongly to extremely polluted\" for Hg, \"unpolluted to moderately polluted\" to \"moderately to strongly polluted\"  
Figure 3: (**a**) Contamination factor (CF), (**b**) geo-accumulation index (I\({}_{\rm geo}\)), (**c**) modified degree of contamination (mCd), and (**d**) pollution load index (PLI) for heavy metals (As, Hg, Cr, Cu, Ni, Pb, Zn) in the sediments of Fengshan River. The CF is classified as low (<1), moderate (1–3), considerable (3–6), and very high contamination (>6); the I\({}_{\rm geo}\) is classified as unpolluted (<0), unpolluted to moderately (0–1), moderately (1–2), moderately to strongly (2–3), strongly (3–4), and strongly to extremely polluted (4–5); the mCd is classified as nil to very low (<1.5), low (1.5–2), moderate (2–4), and high degree of contamination (4–8); the PLI is classified as unpolluted (<1), slightly (1–2), moderately (2–3), and strongly polluted (>3). for As and Zn, \"unpolluted\" to \"moderately to strongly polluted\" for Cu, \"unpolluted\" to \"moderately polluted\" for Pb and Cr, and \"unpolluted\" for Ni. Similar to the result of contamination factors (CF), the contamination degree of heavy metal in the Fengshan River based on the Igoo value followed the order of Hg > As > Zn > Cu > Pb > Cr > Ni. The mCd is an overall assessment of the degree of contamination for all studied metals. According to the mCd assessment (Figure 3c), the mCd values classified as \"high degree contamination\" were shown at sites F4 and F6 in both the dry and wet seasons, at site F5 in the dry season and site F7 in the wet season.