## 3 Results and Discussion  
### Spatial and Seasonal Variations in Heavy Metals  
Table 1 shows the concentrations of heavy metals in Fengshan River sediments. The spatial and seasonal variations in trace metals concentrations in the studied sediments are shown in Figure 2. The distribution of heavy metals in the sediments showed large variations with a range of 15.8-352.7, 17.3-175.3, 17.3-64.3, 11.6-72.8, 119-533, 11.2-44.1, and 0.18-1.44 mg/kg for Cr, Cu, Ni, Pb, Zn, As, and Hg, respectively. The average concentrations of those heavy metals (Cr, Cu, Ni, Pb, Zn, As, and Hg) were 118.2, 78.4, 39.1, 33.0, 277.3, 24.9, and 0.6 mg/kg, respectively. In particular, the concentrations of Cr, Cu, Ni, Pb, and Zn displayed a similar trend of spatial variations. During the dry season, Cr, Cu, Ni, Pb, and Zn all showed the highest concentrations at site F6, which were 294.9, 142.6, 57.1, 49.9, and 501 mg/kg, respectively. The second-highest concentration was observed at the river mouth (F7), which was 234.6, 117.7, 39.1, 33.5, and 343 mg/kg for Cr, Cu, Ni, Pb, and Zn, respectively. Those metal concentrations at other stations (F1-F4) were relatively low and gradually increased from upstream to downstream during the dry season. During the wet season, the highest concentration was found at site F6, which was 352.7, 175.3, 64.3, 72.8, 533 mg/kg for Cr, Cu, Ni, Pb, and Zn, respectively. The second-highest concentration was observed at the river mouth (F7) except for Ni, which was 292.6, 148.0, 49.5, and 431 mg/kg for Cr, Cu, Pb, and Zn, respectively. Similarly, other stations also showed an increasing trend from upstream to downstream during the wet season. The mercury concentration in the dry season rose from the upstream to the maximum of 1.44 mg/kg at site F5 and  
then decreased toward site F7, whereas the highest concentration was found at site F6 of 1.30 mg/kg in the wet season. There was no significant variation in the concentration of arsenic along the river in both dry and wet seasons. Overall, the spatial distribution of heavy metals in the Fengshan River sediments showed a relatively high concentration in the lower reaches (F6, F7). The maximum was frequently found at site F6 where the boundary of tidal river reach is located, probably as a result of the flocculation effect [30; 31]. The salinity of river water serves as an important controlling factor for the partitioning of heavy metals to sediments, particularly the flocculation process in the lower reaches and estuary of rivers affected the distribution of heavy metals in the river sediments [31]. The concentration at the river mouth (F7) slightly decreased compared with site F6, probably because of resuspension and transportation of contaminated sediments eastward. The seasonal variation in precipitation plays an important role in the fate of anthropogenic pollutants in the aquatic environment, especially for the river basin with distinct wet and dry seasons, such as the Fengshan River in Kaohsiung City, Taiwan. As shown in Figure 2, most heavy metals showed relatively high concentrations in the sediments during the wet season compared with those collected during the dry season. During the wet season, frequent and heavy rainfall can induce the discharge of pollutants from non-point sources to the Fengshan River through surface runoff. Moreover, the river sediment in the upper reaches may be resuspended by a strong river stream when a heavy rainfall occurs, causing sediments to scour from upstream to downstream and estuaries. Some studies have also shown similar findings that the higher concentration of heavy metals in river sediments during the rainy season resulted from heavy rainfall-induced water erosion and runoff of heavy metals-containing materials from the upper catchment [32; 33]. The comparison of the heavy metals (Cr, Cu, Ni, Pb, Zn, As, and Hg) concentrations in Fengshan River sediments and the sediment quality guidelines are shown in Figure 2 and Table 1. The heavy metal concentrations in the studied sediments were mostly higher