## 3 Results and Discussion
### Trace Elements in Sediments
#### 3.3.1 Nickel  
The mean Ni content of sediment (\(\pm\)SD) from all sites is shown in Table 3. For the graphical depiction (Figure 4), we chose to omit sites 2 and 5 because the metal contents there had no apparent impact on sediment levels in the following system. Moreover, physico-chemical reactions leading to the observed distribution were easier to follow from site 1 onwards along the course of the MIW. The Ni content at sites 1 and 4 was significantly higher than at any other site (Figure 4). The highest content was measured at site 1 (107 \(\pm\) 43 mg/kg), reflecting AMD and the fast (co-)precipitation of Ni from the water \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Site 1** & **Site 2** & **Site 3** & **Site 4** & **Site 5** & **Site 6** & **Site 7** \\ \hline Ti & 179 \(\pm\) 228 & 168 \(\pm\) 53 & 164 \(\pm\) 83.5 & 656 \(\pm\) 187 & 298 \(\pm\) 223 & 342 \(\pm\) 136 & 900 \(\pm\) 587 \\ V & 29.8 \(\pm\) 21.1 & 22.4 \(\pm\) 5.7 & 23.1 \(\pm\) 8.12 & 61.5 \(\pm\) 23.5 & 17.6 \(\pm\) 6.5 & 20.7 \(\pm\) 8.1 & 17.9 \(\pm\) 6.9 \\ Cr & 93.5 \(\pm\) 50.0 & 93.9 \(\pm\) 25.4 & 98.8 \(\pm\) 30.1 & 189 \(\pm\) 68.9 & 68.7 \(\pm\) 29.5 & 77.4 \(\pm\) 24.6 & 65.2 \(\pm\) 23.9 \\ Ni & 107 \(\pm\) 43 & 32.3 \(\pm\) 16.2 & 25.7 \(\pm\) 9.09 & 62.1 \(\pm\) 28.3 & 15.5 \(\pm\) 7.8 & 20.6 \(\pm\) 10.2 & 36.8 \(\pm\) 26.8 \\ Cu & 19.7 \(\pm\) 14.4 & 13.3 \(\pm\) 6.8 & 10.5 \(\pm\) 6.31 & 534 \(\pm\) 12 & 7.07 \(\pm\) 4.01 & 5.67 \(\pm\) 2.87 & 10.2 \(\pm\) 2.7 \\ Zn & 171 \(\pm\) 100 & 63.0 \(\pm\) 45.0 & 50.7 \(\pm\) 28.4 & 56.8 \(\pm\) 30.9 & 26.4 \(\pm\) 10.4 & 28.4 \(\pm\) 13.4 & 57.7 \(\pm\) 16.2 \\ As & 76.2 \(\pm\) 39.7 & 63.8 \(\pm\) 2.13 & 67.5 \(\pm\) 34.5 & 5.14 \(\pm\) 3.50 & 2.76 \(\pm\) 2.07 & 19.9 \(\pm\) 1.29 & 15.4 \(\pm\) 7.7 \\ Rb & 9.00 \(\pm\) 9.96 & 6.77 \(\pm\) 7.92 & 4.17 \(\pm\) 2.85 & 20.2 \(\pm\) 6.49 & 8.74 \(\pm\) 8.43 & 6.69 \(\pm\) 2.02 & 23.2 \(\pm\) 14.4 \\ Sr & 32.4 \(\pm\) 15.6 & 2.28 \(\pm\) 2.78 & 3.35 \(\pm\) 1.49 & 7.85 \(\pm\) 2.31 & 3.53 \(\pm\) 3.10 & 4.00 \(\pm\) 1.42 & 33.7 \(\pm\) 16.8 \\ Ag & 0.082 \(\pm\) 0.052 & 0.069 \(\pm\) 0.035 & 0.047 \(\pm\) 0.051 & 0.057 \(\pm\) 0.048 & 0.031 \(\pm\) 0.044 & 0.018 \(\pm\) 0.012 & 0.020 \(\pm\) 0.015 \\ Cd & 0.098 \(\pm\) 0.054 & 0.021 \(\pm\) 0.008 & 0.019 \(\pm\) 0.012 & 0.036 \(\pm\) 0.023 & 0.011 \(\pm\) 0.006 & 0.021 \(\pm\) 0.040 & 0.024 \(\pm\) 0.027 \\ Au & 0.319 \(\pm\) 0.142 & 0.058 \(\pm\) 0.049 & b.d. & 0.045 \(\pm\) 0.046 & 0.031 \(\pm\) 0.007 & 0.045 \(\pm\) 0.014 & 0.077 \(\pm\) 0.097 \\ Hg & 0.099 \(\pm\) 0.154 & 0.041 \(\pm\) 0.014 & 0.053 \(\pm\) 0.036 & 0.080 \(\pm\) 0.065 & 0.041 \(\pm\) 0.020 & 0.013 \(\pm\) 0.007 & b.d. \\ Pb & 17.6 \(\pm\) 18.49 & 5.60 \(\pm\) 2.08 & 5.50 \(\pm\) 1.15 & 13.7 \(\pm\) 9.0 & 4.85 \(\pm\) 2.71 & 4.65 \(\pm\) 2.13 & 6.43 \(\pm\) 4.65 \\ U & 12.7 \(\pm\) 7.9 & b.d. & b.d. & 3.29 \(\pm\) 2.12 & b.d. & b.d. \\ OM & 15.0 \(\pm\) 2.99 & 1.95 \(\pm\) 1.10 & 1.31 \(\pm\) 1.00 & 5.60 \(\pm\) 2.98 & 1.00 \(\pm\) 0.66 & 1.30 \(\pm\) 0.55 & 1.43 \(\pm\) 0.39 \\ \hline \end{tabular}
\end{table} 
Table 3: Content of trace elements (mg/kg dw) and organic matter content (OM in %) in surface sediment samples from all sampling sites, b.d. : below the limit of detection (LOD), sampling size for each site: n = 13.  
phase as previously discussed. The elevated value at site 4 can be attributed to slower flow velocity and thereby a higher sedimentation rate; the recorded higher OM content at this site might also contribute to capturing Ni in the sediment [47]. Low levels in site 7 point to a depletion of Ni from the system during downstream transportation, especially at site 4, leading to lower levels in the sediment of site 7. Regarding the risk associated with Ni levels, note that no data are available from the Canadian sediment quality guidelines, but the Australian and New Zealand Guidelines introduced a TEL of 15.9 mg/kg and a PEL of 42.8 mg/kg for Ni in sediment [30]. The mean value at sites 1 and 4 exceeded the PEL, and the mean content at sites 2, 3, 6 and 7 was above the TEL.