## 2 Materials and Methods
### Establishing the Geochemical Background
The trace element concentrations measured in the Lower Danube Basin in this study were obtained using the same analytical method (ICP-MS) and are reported as \"total metal\" in accordance with the procedures outlined in the Geochemical Atlas of Romania [60], thereby suggesting data compatibility between the analyzed samples and the reference set. The mercury concentration in a sediment sample collected from an excavation at a depth of 3.3 m below the ground surface was used as the reference value for calculating pollution indices, as data for this element were not available in the Geochemical Atlas of Romania [60]. The excavation was situated close to the floodplain sediment sampling profile at Harsova. Five samples were collected from the excavation (Figure 3) at varying depths and were subsequently analyzed in the laboratory for two fractions (i.e., <2 mm and <0.063 mm), together with the other floodplain sediment samples. The mercury concentration at the bottom of the hole was determined to be 0.081 mg/kg for the < 0.063 mm fraction and 0.073 mg/kg for the <2 mm fraction. The value corresponding to the finer fraction,0.081 mg/kg, was selected as the threshold and was considered to be the \"pre-industrial level\" for the purposes of this study. Figure 2: Sample location and concentration distribution for trace elements: (**a**) As, (**b**) Cd, (**c**) Cr, (**d**) Cu, (**e**) Ni, (**f**) Pb, and (**g**) Zn—after [60]. Background map credentials: Esri; USGS; NOAA. Figure 4 illustrates the evolution of the box-plots for the nine sampling locations in the Danube area. The complete set of results can be found in the Supplementary Material, Table S1. The box-plots show the first and third quartiles, the median of the values, the mean, and the outliers for each trace element. The presence of outliers indicates how much some individual concentrations of trace elements are greater than 3/2 of the third quartile. No lower outliers were identified. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Statistical Parameter** & **As** & **Cd** & **Cr** & **Cu** & **Ni** & **Pb** & **Zn** \\ \hline Count & 167 & 167 & 167 & 167 & 167 & 167 & 167 \\ Maximum & 48 & 30.5 & 607 & 177 & 147 & 273 & 3649 \\ Minimum & 1 & 0.05 & 22 & 0.1 & 2 & 3 & 27 \\ Mean & 8.35 & 0.53 & 137.67 & 19.61 & 32.52 & 27.15 & 108.88 \\ Median & 7 & 0.28 & 122 & 18 & 31 & 20 & 79 \\ Mode & 6 & 0.29 & 97 & 0.1 & 19 & 14 & 80 \\ St. dev. & 0.45 & 2.37 & 80.63 & 21.81 & 19.33 & 27.69 & 281.44 \\ MAD & 2 & 0.07 & 28 & 8 & 12 & 6 & 18 \\ Kurtosis & 18.43 & 155.22 & 14.36 & 22.49 & 7.50 & 41.75 & 151.56 \\ Skewness & 3.29 & 12.29 & 3.20 & 3.87 & 1.81 & 5.61 & 12.08 \\ Background & 3–11 & 0.14–0.42 & 66–178 & 2–34 & 7–55 & 8–32 & 43–115 \\ Filtered Count * & 161 & 159 & 160 & 164 & 163 & 153 & 158 \\ Mean * & 7.58 & 0.29 & 125.03 & 17.29 & 30.72 & 21 & 78.2 \\ Median * & 7 & 0.27 & 117.5 & 18 & 30 & 19 & 77.5 \\ MAD * & 2 & 0.07 & 27.5 & 8 & 11 & 5 & 17 \\ Background * & 3–11 & 0.13–0.41 & 62.5–172.5 & 2–34 & 8–52 & 9–29 & 43.5–111.5 \\ Threshold * & 11 & 0.41 & 172.5 & 34 & 52 & 29 & 111.5 \\ \hline \end{tabular}  
* Statistics determined after outlier removal.  
\end{table} 
Table 1: Statistics of the measured concentrations [mg/kg] for 167 stream sediment samples [60]. Figure 3: The excavation at the Härsova floodplain, which provided the sample used to establish the mercury threshold (authors’ photos).