## 3 Results and Discussion  
### Heavy Metal Concentrations in the Sediments  
The mean concentrations of the studied metals (Cr, Cu, Pb, Zn, and Fe) for the sediment samples taken from S1, S2, and S3 are shown in Table 4. The significant differences (\(p\) values \(<\) 0.05) among the samples collected at the three sites are presented in Table S1 (Supplementary Material). Additionally, the estimates of the trace element compositions of the upper continental crust (UCC) were presented as well [43]. LAWA classification [44] was applied to assess the level of contamination of studied sites. The highest values were reached for Cr (705.3 \(\pm\) 12.9 mg/kg) and Fe (13442.4 \(\pm\) 67.10 mg/kg) at S2 (\"Huta\"), which was the nearest sampling site to the slag heap, and it was classified as moderately polluted/heavily polluted with Cr (class II-III). The highest concentrations of Pb, Zn, and Cu were also recorded at S2. This site showed significantly higher concentration levels compared to the other studied sites (\(p\)\(<\) 0.05). The results also revealed moderate contamination (class II) of S2 with Cu and uncontaminated/moderate contamination (class I, II) with Cu at S1 and S3. The Zn content was low, with all sites not contaminated with Zn. The order of the accumulation of the metals is Fe \(>\) Cr \(>\) Cu \(>\) Zn \(>\) Pb at S1 and S2, and Fe \(>\) Cu \(>\) Zn \(>\) Cr \(>\) Pb at S3. Previously, a high concentration of Cr was found in the water bodies in this area [24]. It is not surprising that the concentrations of Cr and Fe were elevated, although Fe is one of the major elements of the upper-continental crust; therefore, its high concentration is not astonishing [43]. The slag heap contains the remains of the ferrochromium alloys, which can easily enter the water bodies. Other toxic metals can also be present as a result of the previous activity of the former ferrochromium plant. The studies of [PERSON] [45] also confirmed the existence of Cr contamination in this area. Based on the groundwater, soil, and dust samples, the authors assessed the possible health risk caused by chromium, especially with Cr(VI), in the former ferrochromium plant area. The metal concentrations in the soil were also assessed in this area [46]. According to the authors, the permissible concentrations of Cu, Zn, Ni, and Cr in the studied samples of soil exceeded the geochemical background level in the uncontaminated soil in Poland. Additionally, in the studies by [PERSON] and [PERSON] [46], it was found that the Cr in the soil of this area exceeded polish regulations (maximum value = 276.49 mg Cr/kg d.m.) which was probably connected (according to authors) with the ongoing works on the heap. The Cu and Zn content was also in excess compared to the geochemical background. There was no exceedance of Pb content in these studies. Studies on the sediments in this area have never been performed before, even though sediments play a key role in the quality of aquatic ecosystems, as they are reservoirs for contaminants. The concentrations of the metals assessed in this study suggest that the bioaccumulation of elements in fish could occur. According to [PERSON] et al. [47], sediments can be long-term sources of contamination to higher trophic levels; therefore, the bioaccumulation of toxicants is highly possible; the level Cr was relatively high compared to the other findings. For example, in a study focused on the heavy metal assessment of sediments conducted in an urban aquaculture pond in the coal-based city of Dhanbad, India, the neighboring traffic roads and industrial and residential areas were found to have high concentrations of Cr, Pb, and Zn, as well as Cd and Mn [48]. The chromium concentrations in the sediments in these studies were 99.71 \(\pm\) 9.71 mg/kg during the premosoon season and 84.4 \(\pm\) 10.49 mg/kg during the postmonsoon season, respectively, which is more than two-seven times lower than in our studies.