## 4 Discussion  
In calculating the pollution indices in this study, the threshold values determined based on [60] were used, which are lower than the Romanian legislation for sediments [42] for As, Cd, Cu, Hg, Pb, and Zn but higher in the case of Cr and Ni. Table 6 shows the values for these eight trace elements compared with those in the Romanian legislation for sediments and soils [42,56], other values widely used in the literature for the Danube, or those determined in different parts of Europe and worldwide. Table 6 shows that Cr and Ni determined using [60] are, however, lower than the Dutch intervention values (380 for Cr and 210 for Ni) and lower than the Romanian intervention values for soils (300 for Cr and 150 for Ni) in the case of more sensitive land use categories [56]. It is worth noting that the in-force Romanian legislation [42] has limit values very similar to those of the Dutch standard [57]. Geochemical prospecting works carried out in Romania during the 1970s and 1980s for mineral exploration [106, 107, 108, 109, 110, 111] revealed high contents in Cr and Ni in some areas with mafic and ultramafic rocks in the Western Romanian Carpathians and Southern Carpathians. In the Western Carpathians, the range of these contents is 140-5500 ppm for Cr and 440-2600 ppm for Ni in core drill serpentinites from exploration wells in the Poiana Rusca Mts., and 160-3000 ppm (Cr) and 110-3800 ppm (Ni) in the <1 mm fraction of the pan \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Trace Element** & **As** & **Cd** & **Cr** & **Cu** & **Hg** & **Ni** & **Pb** & **Zn** \\ \hline This study & 11 & 0.41 & 172.5 & 34 & 0.081 & 52 & 29 & 111.5 \\ ROM SV [42] & 29 & 0.8 & 100 & 40 & 0.3 & 35 & 85 & 150 \\ JDS [40] & 10 & 0.25 & 10–50 & 35 & 0.2 & 10 & 25 & 130 \\ Dutch TV [57] & 29 & 0.8 & 100 & 36 & 0.3 & 35 & 85 & 140 \\ Dutch IV [57] & 55 & 12 & 380 & 190 & 10 & 210 & 530 & 720 \\ UCC [24] & 4.8 & 0.09 & 92 & 28 & 0.05 & 47 & 17 & 67 \\ Pre-ind. [23] & 15 & 1 & 90 & 50 & 0.25 & - & 70 & 175 \\ Clark [105] & 5 & 0.5 & 200 & 100 & 0.07 & 80 & 16 & 50 \\ Soils TV [56] & 5 & 1 & 30 & 20 & 0.1 & 20 & 20 & 100 \\ Soils AV [56] & 15 & 3 & 100 & 100 & 1 & 75 & 50 & 300 \\ Soils IV [56] & 25 & 5 & 300 & 200 & 2 & 150 & 100 & 600 \\ FIN Thr. [34] & 5 & 1 & 100 & 100 & 0.5 & 50 & 60 & 200 \\ FIN LGV [34] & 50 & 10 & 200 & 150 & 2 & 100 & 200 & 250 \\ AUS EIL [34] & 20 & 3 & 400 & 100 & 1 & 60 & 600 & 200 \\ AUS HIL [34] & 200 & 40 & 200 & 2000 & 30 & 600 & 600 & 14,000 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Values (in mg/kg sediment) for trace elements according to various sources. ROM SV—Romania’s sediment; JDS—Joint Danube Survey values; TV—Dutch target values for soils; IV—Dutch intervention values for soils; UCC—Upper Continental Crust; Pre-ind.—pre-industrial; Soils TV— soils typical values in RO legislation; Soils AV— soils alert values in RO legislation for more sensitive land use categories; Soils IV— soils intervention values in RO legislation for more sensitive land use categories; FIN Thr.—Finland’s threshold values; FIN LGV—Finland’s lower guideline values; AUS EIL—Australia’s ecological investigation levels; AUS HIL—Australia’s health investigation levels. Figure 7: Statistics of \(Pl_{Nemenova}\) and \(RI\) without mercury. samples [106]. Towards the north, in the Drocea Mts., part of the Apuseni Mts., in ophiolites and other basic igneous rocks [107], the ranges are 2-565 ppm (Cr) and 3-73 ppm (Ni), while towards the southeast, in the Mehedinti Plateau [108], the ranges in the ultrabasic igneous rocks are 1800-2900 ppm (Cr) and 1500-2400 ppm (Ni). The mean values of 2200 ppm (Cr) and 1990 ppm (Ni) for this area are within the ranges of the mean values established in [112], at 1600-2400 ppm for Cr and 1500-2000 ppm for Ni, respectively. The Cr/Ni ratio is >1, with a mean of 1.15 and a range of 0.95-1.46.