## 2 Materials and Methods
### Establishing the Geochemical Background  
This step is of great importance in assessing the contamination of sediments by trace elements, as most pollution indices consider the geochemical background, reference, or \"normal\" value, specifically the threshold, which is the actual value used in the calculation. Considering the significant discrepancy in the geochemical background values used in various studies conducted in the Lower Danube (as previously outlined in the introduction), this study employs values extracted from the data provided by the Geochemical Atlas of Romania (scale 1:3,000,000) [60]. This is the first geochemical atlas comprising samples from the entirety of Romania, despite preceding efforts to create atlases for selected regions or for particular objectives. While developing the Atlas, samples of stream water and sediments were collected from locations distributed throughout the country following a grid of 0.5 degrees of longitude and latitude. The samples were obtained from sites located at a considerable distance from areas of anthropogenic activity, such as mines, industrial facilities, human settlements,railways, and major roads. The field team conducted the sampling between 1994 and 1996. The water samples were collected in 125 mL plastic flasks and were preserved with 1 mL of HNO\({}_{3}\) to prevent the adsorption of cations. The water samples were analyzed in the German laboratory for the concentration of 48 elements (Ag, Al, As, B, Ba, Be, Bi, Br, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, I, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr). On-site measurements were conducted to determine the temperature, electrical conductivity, and pH levels. A total of 167 stream sediment samples were collected from minor riverbeds in 0.5 L plastic recipients, dried at 40 \({}^{\circ}\)C, and sieved to obtain fractions of <2 mm and <0.063 mm. The sediment analyses were conducted in the BGR laboratory utilizing X-ray fluorescence spectrometry (XRF) for the determination of major constituents and inductively coupled plasma mass spectrometry (ICP-MS) for the analysis of minor and trace elements. No further information is available on the specific manufacturer and model of the analytical equipment used. The analyzed major constituents were SiO\({}_{2}\), TiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), MnO, MgO, CaO, Na\({}_{2}\)O, K\({}_{2}\)O, P\({}_{2}\)O\({}_{5}\), SO\({}_{3}\), Cl, F, and LOI, and the analyzed minor/trace elements were Ag, As, B, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Ga, Ge, Hf, In, La, Li, Mn, Mo, Nb, Ni, Pb, Rb, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr. This study employs the trace elements (As, Cd, Cr, Cu, Ni, Pb, and Zn) values for stream sediments across the entire Romanian territory, as presented in the Geochemical Atlas of Romania [60], to ascertain the geochemical background at the national level. This approach aligns with the methodology proposed in [31; 32]. Figure 2 illustrates the distribution and range of values for the aforementioned trace elements. The statistical data describing the trace element data sets from the Geochemical Atlas of Romania are provided in Table 1. Among the statistical parameters calculated are the geochemical background and the threshold used as a reference value for the calculation of pollution indices in this study. The threshold was calculated based on the median of measured concentration values [33; 34] from the Geochemical Atlas of Romania:  
\[Threshold=Median+2\ MAD \tag{1}\]  
where \(MAD\) is the median absolute deviation, calculated as the median, \(M_{i}\), of the deviations (in absolute value) between each value, \(x_{i}\), of the series of \(n\) observations and their median, \(M_{j}\)[92]:  
\[MAD=M_{i}\big{(}|x_{i}-M_{j}|\big{)} \tag{2}\]  
The aforementioned thresholds or reference values (in mg/kg) are as follows (Table 1): 11 (As), 0.41 (Cd), 172.5 (Cr), 34 (Cu), 52 (Ni), 29 (Pb), and 111.5 (Zn).