## 3 Results and Discussion
### Relationship among Sediment Properties and Heavy Metals  
As shown in Table 3, significantly positive relationships between heavy metals (Cr, Cu, Ni, Pb) and organic matters (OMs) were observed in Fengshan River sediments, showing a correlation coefficient of 0.56-0.65 at a confidence level of 95%. It is indicated that OM is an important factor affecting the sediment heavy metal distribution in Fengshan River. Moreover, the sediment metal concentrations were also well correlated with total nitrogen  
Figure 4: (**a**) Potential ecological risk index (RI), (**b**) mean ERM quotient (mERM\({}_{\rm Q}\)), (**c**) the sum of the toxic units (\(\Sigma\)TU), and (**d**) contamination severity index (CSI) for heavy metals (As, Hg, Cr, Cu, Ni, Pb, Zn) in the sediments of Fengshan River. The RI is classified into low (!150), moderate (150–300), considerable (300–600), and very high ecological risk (?600); the mERM\({}_{\rm Q}\) is classified into 9% (!0.1), 21% (0.1–0.5), and 49% probability of being toxic (0.5–1.5); the CSI is classified into uncontaminated (!0.5), very low (0.5–1), low (1–1,5), low to moderate (1.5–2), moderate (2–2.5), moderate to high (2.5–3), and high severity of contamination (3–4). (TN) and total phosphorus (TP) contents. Due to TN and TP being mainly derived from organic compounds, a good correlation of heavy metals with TN and TP strongly indicated that the organic compounds play an essential role in the spatiotemporal variations in heavy metals in the Fengshan River sediments. However, relatively low correlation coefficient values were observed for heavy metals with grain size (either sand, silt, or clay), indicating that the influence of grain size composition on the sediment heavy metal distribution was less critical than the organic matter. In general, grain size and organic matter serve as two primary factors to affect the variations in heavy metal concentrations in sediments. The grain size effect has been documented in previous studies [45; 46]. Muddy sediments, mainly consisting of aluminosilicate minerals (i.e., clay minerals), have high cation exchange capacity and large specific surface area [47], which are prone to adsorb anthropogenic pollutants (e.g., heavy metals). Consequently, heavy metals are accumulated in the muddy sediments [48]. On the contrary, sandy sediments commonly consist of coarse-grained biogenic carbonates and quartz. The carbonate or quartz sands with relatively low cation exchange capacity and specific surface area are not easy to adsorb heavy metal pollutants compared with clay minerals [49]. On the other hand, high concentrations of heavy metals are often associated with high organic matter content in sediments [17; 50]. Most organic matters have large specific surface areas with negatively-charged sites, leading to the adsorption of anthropogenic heavy metals [51]. [PERSON] et al. [17] reported that the influence of organic matter content on the distribution of heavy metals in sediments was more significant than the effect of sediment grain size, especially for the aquatic environments with rich organic matter (e.g., estuaries and harbors). Furthermore, good relationships among the heavy metals (Hg, Cr, Cu, Ni, Pb, and Zn) in the studied sediments were observed with a correlation coefficient ranging between 0.53 and 0.94 at a confidence level of 95% (\(p<0.05\)). This indicated that those heavy metals were probably derived from similar pollution sources, which may attribute to industrial wastewater discharges from leather processing and metal finishing factories within the river basin. According to the results of PCA/FA, three factors with eigenvalues \(>1\) were extracted from our data set. The corresponding VFs, variable loadings, eigenvalues, and the explained variance are shown in Table 4. The VF1 displays strong positive loadings (\(>\)0.70) on OM, TN, TP, Hg, Cr, Cu, Ni, Pb, and Zn, which accounts for 49% of the total variance.