## 3 Results and Discussion
### Activity Concentration Values of \({}^{238}\)U and \({}^{226}\)Ra  
Table 1 shows the activity concentration results of \({}^{238}\)U and \({}^{226}\)Ra in the 25 sampled soils, whose values were higher than the average value for Spanish soils of 30 Bq kg\({}^{-1}\)[40]. The elevated presence of \({}^{238}\)U and \({}^{226}\)Ra is due to the hydrothermal formation of the study area, which caused a significant increase in uranium radioactive series [41]. This presence is primarily due to the presence of torbernite (Cu(UO\({}_{2}\))\({}_{2}\)(PO\({}_{4}\))\({}_{2}\)-8-12H\({}_{2}\)O) in the area, as described in Section 2.1. Figure 4 shows the activity concentration of \({}^{226}\)Ra as a function of the activity concentration of \({}^{238}\)U. The ratio of \({}^{226}\)Ra/\({}^{238}\)U was obtained from the slope of the linear equation, with a value of 1.237 \(\pm\) 0.065. The obtained ratio showed that U(IV) is oxidized by external agents into UO\({}_{2}\)\({}^{2+}\), which is more soluble than \({}^{226}\)Ra and is thus released into the environment through rain and air [42]. Figure 4: Ratio of \({}^{226}\)Ra concentration (Bq kg\({}^{-1}\)) to \({}^{238}\)U (Bq kg\({}^{-1}\)) (the dotted line corresponds to the confidence intervals of the function). Figure 3: The chemical composition of the soils sampled in this study. The values are presented in Tables S1–S8 of the Supplementary Information. Relationship between the Chemical Composition of Soils and the Activity Concentration of \({}^{238}\)U and \({}^{226}\)Ra  
Figure 5 shows a biplot graph with principal component analysis (PCA) that allows for visualization of the correlation between the activity concentration of \({}^{238}\)U and \({}^{226}\)Ra and the chemical composition and scoring of the different soils based on these variables. The two factors obtained represent 90.4% of the variance, indicating statistical dependence between the different variables. The Kaiser-Meyer-Olkin (KMO) index value was 0.79, which indicates a satisfactory correlation [43]. Some variables were eliminated as they did not exceed the average contribution of the set of variables or 1/(number of variables). The correlation between the different variables is obtained from the cosine of the angle they form with each other. The results showed a correlation between the activity concentrations of \({}^{226}\)Ra and \({}^{238}\)U with Sn, Fe, and Cu. This results indicate that the radionuclides were associated with areas with higher mineral content from the waste heap. Soils 8, 6, 3, 9, 2, and 21 are those that would have the highest content of natural radionuclides, with soil 8 standing out. These soils are located in the waste heap and the mine loading area, which, as verified in a previous study, had the highest radioactive content [23]. The remaining stable elements did not show any correlation with \({}^{238}\)U or \({}^{226}\)Ra. However, coherent correlations are observed, such as those of Si with O (due to SiO\({}_{2}\)) and Na and K, which have the same chemical behavior. Additionally, a correlation was observed between Ca, Sr, and S, possibly due to the presence of sulfates, an aspect that would require further analysis to confirm. However, \({}^{226}\)Ra is not correlated with Ca, Sr, and S, indicating that its presence is more commonly associated with U, its progenitor. Therefore, the correlation between U and Fe is consistent with the findings of other authors such as [PERSON] et al. [44], who found that Fe-rich minerals can reduce heavy metals like U from UO\({}_{2}\)\({}^{2+}\), the most soluble species, to U(IV), which is less soluble. Furthermore, the subsequent formation of Fe oxyhydroxides such as ferrihydrite would favor the release of U under high-temperature conditions, which is consistent with the meteorological conditions of the study area [45,46].